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Can vaccines move from person to person?

Can vaccines move from person to person?


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Can someone who has been given a vaccine "infect" another person with the virus used in that vaccine? For example, let's say Bill is vaccinated against tuberculosis. If Frank is exposed to Bill, is there a chance he could "catch" the tuberculosis vaccine, and become immune?


Most immunizations are not transmissible. A few are. To be transmissible, an immunization would have to be a live attenuated vaccine (i.e., a vaccine that can reproduce in the host, but doesn't cause disease), AND have the necessary reproduction rate and virulence factors that would allow for person to person transmission. There are many live attenuated vaccines in use today, but only one that is transmissible, Oral Poliovirus Vaccine, or OPV. OPV is currently not in use in the US, but is used in the developing world, and can circulate in communities. Some would argue that this is a good thing, as it further disseminates immune protection to the population, but it is a little problematic, because it is not without risk, including reversion to a virulent type that can cause Vaccine associated Paralytic Poliomyelitis.

On editing your question, I'm reminded that your example was of TB. I would note that BCG, the vaccination against Tuberculosis, may be a particular case, and I'm not as familiar with it except as a complicating factor in interpreting the results of a TB test in U.S. immigrants. BCG is a live attenuated strain of Mycobacterium bovis, can (rarely) produce disseminated disease itself, and the mycobacterium itself can be isolated in the blood of individuals some time after vaccination. There is also a possible report of vertical transmission of the mycobacterium (from mother to child), but these results use novel methods, and I'm unfamiliar with this group's work. Whether this would lead to immunity of the child is definitely an open question. BCG even when administered in the standard way (as in intradermal inoculation) isn't particularly good at producing immunity, though it is good at preventing meningitis and miliary TB in young children. I'm not aware of any documented case of horizontal person-to-person transmission, but wouldn't declare it impossible.


Can You Mix the COVID-19 Vaccines? Moderna and Pfizer Second Dose Explained

Three different COVID-19 vaccines are authorized for use in the U.S., with two of these requiring the administration of two doses. But can you mix and match the vaccines?

The Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines are all being administered in the United States, but whereas the Johnson & Johnson vaccine only requires one shot for immunization, the Pfizer-BioNTech and Moderna vaccines each require two doses, administered several weeks apart.

This has raised the question of whether or not it's safe or effective to receive one dose of the Pfizer-BioNTech vaccine followed by one dose of the Moderna vaccine, or vice versa.

Scientists are investigating the idea, but in the meantime they've told healthcare providers to follow current vaccine guidelines and to not mix and match COVID-19 vaccines.

"COVID-19 vaccines are not interchangeable," the U.S. Centers for Disease Control (CDC) says.

"The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product," it continues, adding that "every effort should be made to determine which vaccine product was received as the first dose to ensure completion of the vaccine series with the same product."

If the same vaccine is temporarily unavailable when a second shot is due, the CDC says "it is preferable to delay the 2nd dose (up to 6 weeks) to receive the same product than to receive a mixed series using a different product."

However, in "exceptional situations in which the vaccine product given for the first dose cannot be determined or is no longer available," the CDC says that "any available mRNA COVID-19 vaccine may be administered at a minimum interval of 28 days between doses to complete the mRNA COVID-19 vaccination series."

Both the Pfizer-BioNTech and Moderna COVID-19 vaccines are mRNA vaccines.

The CDC also says that a single dose of the Johnson & Johnson vaccine can only be administered to somebody who has already received a shot of either the Pfizer-BioNTech or Moderna vaccine "in limited, exceptional situations where a patient received the first dose of an mRNA COVID-19 vaccine but is unable to complete the series with either the same or different mRNA COVID-19 vaccine."

In this case, the Johnson & Johnson shot may only be administered at a minimum interval of 28 days from the initial mRNA COVID-19 vaccine dose, and patients should be considered to have received a valid, single-dose Johnson & Johnson vaccination &mdash not a mixed vaccination series.

There was a great deal of controversy when health officials in England opened the door to mix-and-match vaccinations earlier this year, despite a lack of supporting evidence that the vaccinations would still be effective.

"There is no evidence on the interchangeability of the COVID-19 vaccines although studies are underway. Therefore, every effort should be made to determine which vaccine the individual received and to complete with the same vaccine," Public Health England's (PHE) guidance reads.

However it adds that it is "reasonable" to administer a dose of a different product to individuals who don't know which vaccine they've already received, or if the same vaccine is not available.

"In these circumstances, as the vaccines are based on the spike protein, it is likely the second dose will help to boost the response to the first dose," PHE says.

The U.S. Food and Drug Administration (FDA) supports the CDC's stance.

"We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19," the FDA says.

It describes them as "reasonable questions to consider and evaluate in clinical trials," but says that "suggesting changes to the FDA-authorized dosing or schedules of these vaccines is premature."

It adds: "Without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19."

The Infectious Diseases Society of America further backs this up, saying, "As always, our approach against this pandemic must be founded in science, leadership, funding, collaboration and cooperation."

Furthermore, in February, Dr. Anthony Fauci told the LA Times, "I wouldn't make any changes unless you've got good data. I don't think you mix and match without results showing it's very effective and safe."

In the U.K., the University of Oxford is leading Com-Cov, a study called that aims to evaluate the feasibility of getting vaccinated using two different vaccines.

The trial will take 13 months, and involves the Pfizer-BioNTech and Oxford-AstraZeneca vaccines.


Vaccinated? Here’s How Your Life May Change After Getting the COVID-19 Vaccine

As the COVID-19 vaccine rollout gains steam across the country, an increasing number of people are finding themselves with more protection from the SARS-CoV-2 virus. But just because you’ve been fortunate enough to get a vaccine doesn’t mean you can return to your pre-pandemic lifestyle. At least not yet.

The COVID-19 vaccines are very effective for individuals, but they are most effective when everyone has gotten one. Your new vaccine will protect you from serious disease and will very likely keep you from getting COVID-19 at all. But you could still have a mild or asymptomatic infection and that could still set off a chain of infections and complications in others.

Eventually, we will get to “herd immunity,” which occurs once a high percentage of the population is immune, either from having had COVID-19 or getting the vaccine. In that situation, cases are low, and they stay low because there aren’t enough susceptible people who can pass the virus around.

Until then, you’ll need to continue taking precautions (like wearing a mask and keeping distance) in certain situations. Over the coming months, expect things to look more and more like “normal,” but it won’t happen overnight and there may be bumps in the road along the way.

Very good, but not perfect

Vaccines make you immune, not invincible. The COVID-19 vaccines that are currently available in the U.S. provide remarkable protection against the SARS-CoV-2 virus. But, like anything, they’re not 100% effective. Current data show the mRNA vaccines are about 94% to 95% effective. That means there’s still a chance people who’ve been fully vaccinated could contract COVID-19. While we know people who’ve been fully vaccinated won’t get as sick as they would have without a vaccine, you could still pick up a mild or asymptomatic case of COVID-19. If that happens, you’ll be able to spread virus to people who haven’t been vaccinated yet.

As the vaccine roll out continues and more and more people are vaccinated, the risk of catching a mild case or spreading it to someone else will decrease. We will also learn more about the strengths and weaknesses of the different vaccines in different situations.

Full protection takes time

While the vaccines provide a high-degree of (but not total) protection from COVID-19, they also take time to reach their full level of protection. Both the Pfizer-BioNTech and Moderna vaccines require two doses, which need to be given in specific increments. The first dose offers good protection, but you won’t get the vaccine’s full and durable protection until two weeks after your second dose. The Johnson & Johnson vaccine only requires one dose, but it also needs time to reach its full protective power. Don’t consider yourself fully protected until two weeks have passed from your final vaccine (Pfizer and Moderna) or your only vaccine (Johnson & Johnson).

Since they’re so new, we don’t have data to say how long the vaccines’ protection lasts. But based on what we know about coronaviruses and mRNA vaccines, we think you may need a booster in the future we don’t yet know when or how often.

Waiting for herd immunity

If we want to see post-COVID times come faster, we need two things: more vaccines and more people who are vaccinated.

Diseases need people to spread. So the more people who are vaccinated, the safer everyone is. While things are trending in the right direction, we know restrictions shouldn’t get lifted until we have lower rates of COVID-19 in our communities. And that won’t happen until a lot more people have access to, and are able to get, the vaccine.

What you can do after you’re fully vaccinated

Having a vaccine for yourself is great, and it certainly gives you peace of mind, even if you still have to take precautions.

The latest CDC guidance says people who are fully vaccinated can spend time unmasked and inside with others who are also fully vaccinated. The national’s top public health agency also says that it’s safe, in certain instances, for fully vaccinated people to spend unmasked time inside with those who are unvaccinated — provided those who are unvaccinated are from the same household and have no high-risk health conditions. (If there’s unvaccinated people from more than one household, or an unvaccinated person has high-risk conditions, everyone should continue to wear a mask. And it’s best to meet outside.)

If you live with or care for people who can’t yet get vaccinated, you should still keep your unmasked contacts limited so you don’t bring home what may be a mild case for you, but one that turns into a serious illness for them.

As cases decline in your area and vaccination rates increase, close contact with others gets safer and safer, but the new variants could still bring another surge. We are nearing the end of this marathon, and now isn’t the time to quit early. Cases are decreasing in most parts of the country, but they are still high. The mix of high case counts and growing vaccination levels makes an epidemiological soup that is primed to breed new variants that are resistant to the vaccines. We definitely don’t need that hurdle at the end of the race, so please keep taking care to protect yourself and your loved ones.

Just like you did at the start of the pandemic, make sure you stay on top of the rules and guidance in your community and talk candidly to family and friends about their and your risk tolerance as we slowly make our way out of this mess.


Asymptomatic infections can still transmit the virus

If the virus enters cells and begins replicating but never causes disease, that’s an asymptomatic infection. With presymptomatic infections, on the other hand, a person goes on to develop symptoms and is especially contagious in the days before symptoms appear, says Natalie Dean, an assistant professor of biostatistics at the University of Florida in Gainesville.

“We know from contact tracing data unrelated to vaccines that people who never develop symptoms tend to be less infectious,” Dean says.

Morrison adds that asymptomatic people probably have an excellent initial immune response to slow down how quickly the virus can copy itself, “but not enough that viral replication is completely shut off,” she says. “That’s why they could still shed virus but we’re not seeing any disease symptoms.”

Supporting that idea is the fact that the severity of COVID-19 disease tends to correlate with the total number of viruses in the body, called viral load, Kindrachuk says. Early research showed that people with lower viral loads transmit less virus, further suggesting that asymptomatic infections are less contagious than symptomatic ones. But less is not zero: People with asymptomatic infections still have replicating viruses in their system that they can transmit to others.

When the vaccines were authorized, experts did not yet know whether the shots could prevent infections entirely or whether vaccinated people could develop an asymptomatic—but still contagious—infection.


No, other people’s Covid vaccines can’t disrupt your menstrual cycle.

In recent weeks, people who oppose Covid vaccinations have spread a claim that is not only false but defies the rules of biology: that being near someone who has received a vaccine can disrupt a woman’s menstrual cycle or cause a miscarriage.

The idea, promoted on social media by accounts with hundreds of thousands of followers, is that vaccinated people might shed vaccine material, affecting people around them as though it were secondhand smoke. This month, a private school in Florida told employees that if they got vaccinated, they could not interact with students because “we have at least three women with menstrual cycles impacted after having spent time with a vaccinated person.”

In reality, it is impossible to experience any effects from being near a vaccinated person, because none of the vaccine ingredients are capable of leaving the body they were injected into.

The vaccines currently authorized for use in the United States instruct your cells to make a version of the spike protein found on the coronavirus, so your immune system can learn to recognize it. Different vaccines use different vehicles to deliver the instructions — for Moderna and Pfizer, messenger RNA, or mRNA for Johnson & Johnson, an adenovirus genetically modified to be inactive and harmless — but the instructions are similar.

“It’s not like it’s a piece of the virus or it does things that the virus does — it’s just a protein that’s the same shape,” said Emily Martin, an infectious disease epidemiologist at the University of Michigan School of Public Health. “Transferring anything from the vaccine from one person to another is not possible. It’s just not biologically possible.”

Microorganisms spread from person to person by replicating. The vaccine ingredients and the protein can’t replicate, which means they can’t spread. They don’t even spread through your own body, much less to anybody else’s.

“They’re injected into your arm, and that’s where they stay,” Jennifer Nuzzo, an epidemiologist at Johns Hopkins, said of the vaccines. “mRNA is taken up by your muscle cells near the site of injection, the cells use it to make that protein, the immune system learns about the spike protein and gets rid of those cells. It’s not something that circulates.”

It’s also not something that sticks around. Messenger RNA is extremely fragile, which is one reason we’ve never had an mRNA-based vaccine before: It took a long time for scientists to figure out how to keep it intact for even the brief period needed to deliver its instructions. It disintegrates within a couple days of vaccination.

Vaccinated people can’t shed anything because “there’s nothing to be shedding,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center and a member of President Biden’s transition advisory team on the coronavirus. “The people who shed virus are people who have Covid. So if you want to prevent yourself or others from shedding virus, the best way to do that is to get vaccinated so you don’t get Covid.”

This brings us to the reports of women having abnormal periods after being near vaccinated people. Because one person’s vaccine can’t affect anybody else, it is impossible for these two events to be connected. Many things, like stress and infections, can disrupt menstrual cycles.

The shedding claims are “a conspiracy that has been created to weaken trust in a series of vaccines that have been demonstrated in clinical trials to be safe and effective,” Dr. Christopher M. Zahn, vice president of practice activities at the American College of Obstetricians and Gynecologists, said in a statement. “Such conspiracies and false narratives are dangerous and have nothing to do with science.”

Some women have expressed a related concern that getting vaccinated themselves could affect their menstrual cycles. Unlike secondhand effects, this is theoretically possible, and research is ongoing — but anecdotal reports could be explained by other factors, and no study has found a connection between the vaccine and menstrual changes.

“There’s no evidence that the vaccine affects your menstrual cycle in any way,” Dr. Gounder said. “That’s like saying just because I got vaccinated today, we’re going to have a full moon tonight.”


No, the COVID-19 vaccine will not change your DNA

Kate Langlois received her first dose of the COVID-19 vaccine at a Muskegon County walk-in clinic on Saturday, March 27. "I'm ready to get back to life and see people without this thing on," she said pointing to her mask. (Photo by Rose White | MLive)

The COVID-19 vaccine won’t change your DNA.

None of the three vaccines between Pfizer, Moderna or Johnson & Johnson actually enter the nuclei in a person’s cell, according to the CDC, meaning none of them actually interact with DNA or a genome.

“The Pfizer-BioNTech and Moderna vaccines are mRNA vaccines, which teach our cells how to make a protein that triggers an immune response,” according to the CDC. “The mRNA from a COVID-19 vaccine never enters the nucleus of the cell, which is where our DNA is kept. This means the mRNA cannot affect or interact with our DNA in any way.”

Regarding the Johnson & Johnson shot, the material it delivers to a person’s cells “does not integrate into a person’s DNA,” the CDC states. The Johnson & Johnson vaccine was temporarily halted in Michigan following guidance from federal regulators after six people nationwide reported rare, but serious blood clots.

Infectious disease and biology experts however said none of the vaccines access or change DNA, refuting a series of conspiracy theories circling around social media.

The concern over DNA alteration was perhaps most prominently voiced in an April 8 article in The Defender, a publication run by the anti-vaccination group Children’s Health Defense. The post cited a preprinted research paper from Harvard and MIT scientists that asserts that mRNA from the virus can “very rarely” persist in an individual’s body tissue even after infection.

Richard Young, a co-author on the paper and an MIT professor of biology, told MLive it’s “terrible” his team’s research is being used in anti-vax circles, since his team’s findings only address the COVID-19 virus and not any of the vaccines.

“It is possible that the (COVID-19) virus might integrate on a rare instance into a human genome into tissue culture itself,” Young said. “But the vaccine is just a tiny piece of spike protein in an mRNA molecule. So when the vaccine mRNA goes into the cell, it only goes into the cytoplasm where it can be made into proteins by ribosomes. So it doesn’t even go into the nucleus.”

Spike proteins, according to the CDC, trigger our immune system cells to recognize the COVID-19 virus and begin producing antibodies to fight the infection.

Young said he and his colleagues’ research should be seen as more reason to avoid natural COVID-19 infection, not to avoid the vaccine. Compared to the virus, the vaccine carries less than 1% of the molecules used to replicate viral mRNA that can lead to “very rare” genetic alteration, Young said.

“If you were weighing a concern, I’d be very concerned about being infected with the virus,” he said, “because the virus is giving some people ‘long COVID,’ whereas the vaccine doesn’t seem to be hurting anyone.”

While the Johnson and Johnson vaccine works differently than its counterparts, it accomplishes the same goal of creating proteins to catalyze the creation of antibodies, said Dr. Anthony Ognjan, infection disease doctor with MacLaren Macomb hospital.

“It’s called a viral vector vaccine,” he said. “Similar to AstraZeneca, what it does is it takes the virus and creates a kind of infection in people, but not really. it attaches spike proteins to the virus, the viruses are naturally taken up by the cells and then the cells process automatically an immune reaction.”

The bottom-line: the COVID-19 vaccines “does not get incorporated into human DNA,” Ognjan said. Vaccines that treat herpes are examples of ones that can alter DNA, but the COVID-19 shots don’t follow the same method.

The genetic alteration concern picks up on a fear some people have about how changed DNA leaves some individuals susceptible to cancer down the road, Ognjan said. While altered DNA does carry those risks, that fear is being conflated with the COVID-19 vaccine in a frustrating way, he said.

“People who don’t understand the science, anti-vaxxers and pseudoscientists are taking advantage of people’s naivety and not understanding the basic science of what’s going on,” he said. “You see that stuff get scattered over the internet, and it drives me crazy.”


Who are the FDA career professionals evaluating EUAs for vaccines?

The FDA staff are career scientists and physicians who have globally recognized expertise in the complexity of vaccine development and in evaluating the safety and effectiveness of all vaccines intended to prevent infectious diseases. These FDA professionals are committed to decision-making based on scientifically driven evaluation of data. FDA staff are like your family - they are fathers, mothers, daughters, sons, sisters, brothers and more. They and their families are also directly impacted by the work that they do, and are exactly who you want making these important public health decisions for the United States.


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Q: I had COVID-19, so why must I bother with getting both doses of a vaccine?

A: Being infected with the virus does stimulate your immune system, and it does provide some protection.

But It’s not clear how long the protection will last. And it’s not clear how effective that protection will be. It’s not a controlled study. One person might have been infected by just a few viral particles. Another person might have had a much greater dose. We don’t yet know the impact of these differences on the strength and duration of immunity. It’s not the same as injecting someone with a defined dose of the vaccine at a defined time point.

Immunity from natural infection just isn’t as good.

Q: With all the variants of the virus circulating, will two shots be enough?

A: Vaccine makers may have a financial incentive for this answer. But, Pfizer CEO Albert Bourla said in a CNBC interview Thursday, a likely scenario is that there will be a need for a third dose, somewhere between six and 12 months. It is also possible that COVID-19 vaccinations will be annual, just like flu shots.


Guidance for what Canadians can, and can't do after vaccines coming 'shortly': Hajdu

OTTAWA -- Canadians will “shortly” receive federal guidance around what they can, and can’t do safely after their first and second COVID-19 shots, according to Health Minister Patty Hajdu.

Facing questions about why Canada has yet to offer any formal guidance to people who have been vaccinated about what degree of risk they have in certain circumstances in the way the United States has, Hajdu said it’s in the works.

“We are working with provinces and territories to understand their own epidemiology… It’s the percentage of Canadians that are vaccinated, and it's the extent of disease that's being transmitted in communities. We will have guidance out for Canadians very shortly about what they can do with one dose or two doses of the vaccine,” said Hajdu in an interview with Evan Solomon, host of CTV’s Question Period.

In early April, the U.S. Centres for Disease Control and Prevention (CDC) issued interim public health recommendations for Americans who are fully vaccinated, outlining what health measures they should still take while offering new freedoms.

For example, fully vaccinated Americans have been told they can resume domestic travel without taking COVID-19 tests, no longer need to self-isolate after arriving back from an international destination, and can visit with other fully vaccinated people indoors without masks or physical distancing.

On Tuesday, the CDC took their guidance a step further, stating that fully vaccinated Americans no longer need to wear masks outdoors unless in a large crowd of people.

Earlier this week in response to questions from CTVNews.ca about why the Public Health Agency of Canada has yet to follow suit, spokesperson Anna Maddison said that for now all people “must stay the course,” and keep following the complete suite of suggested public health measures.

Questions around whether Canada has metrics or goalposts on which it will base any decisions around easing public health measures have been asked by MPs for months, with little clarity in the responses from federal officials.

Hajdu said that because fully vaccinated people can still become sick with COVID-19 if there's a high degree of spread in their community, the federal health agency is needing to be cautious about what recommendations they issue.

While first doses of the three two-shot COVID-19 vaccines in use in Canada offer some degree of efficacy after one dose, it isn’t enough that Canadians should be letting their guards down while they wait to receive their second doses.

“Canada's approach is much aligned with the U.K. approach which is really to make sure that we vaccinate as many people as possible but that first dose,” Hajdu said, referencing the widely adopted strategy of prolonging the time between the first and second doses of two-shot COVID-19 vaccines to up to four months. The United Kingdom is offering its citizens shots three months apart.

“We will have guidance out shortly for Canadians. And it's very important that we all continue to follow those public health measures until we are certain that our communities are safe,” Hajdu said.

In a small step towards providing a target rate of vaccination for when public health measures may begin to be lifted, as part of April 23 modelling Chief Public Health Officer Dr. Theresa Tam projected that should 75 per cent of adults have their first dose, and 20 per cent have their second, restrictions could be lifted without maxing out hospital capacity.

NO DOMESTIC VACCINE PASSPORTS

Hajdu also said that while work continues to establish what the international standard will be when it comes to vaccine passports to travel, domestic vaccine passports aren’t going to be something the federal government pursues.

That said, Hajdu noted that there could still be circumstances where proof of vaccination will be required.

“There's no intention to impose a domestic vaccination passport at the federal level, but I will remind people that certain settings will require vaccination as they always do. So, for example schools require certain childhood immunizations. Some universities and colleges may require vaccination. There might be requirements for certain workplaces, and those are all, as you know determined that local and provincial levels,” Hajdu said.

One of the key outstanding aspects of issuing vaccine certification may be the patchwork in ways people across Canada are receiving their COVID-19 vaccines and whether at some point down the line a more uniform vaccine card could be issued.

Hajdu said during a G7 health ministers meeting this week about international vaccine passports that the consensus was that there should be “some sort of common way to be able to quickly credential people’s certification of vaccination.”

“We know there are a lot of different kinds of vaccines around the world, and we want obviously Canadians to be able to participate in international travel, so I can reassure Canadians that no matter what those requirements will be we'll have Canadians ready when the time is right to travel.”

Federal Health Minister Patty Hajdu prepares to receive a first dose of COVID-19 vaccine from Brian Gray, Director of Oak Medical Arts – Mountdale Pharmacy, in Thunder Bay, Ont., Friday, April 23, 2021. THE CANADIAN PRESS/David Jackson-Pool


How Vaccines Work

A vaccine works by training the immune system to recognize and combat pathogens, either viruses or bacteria. To do this, certain molecules from the pathogen must be introduced into the body to trigger an immune response.

These molecules are called antigens, and they are present on all viruses and bacteria. By injecting these antigens into the body, the immune system can safely learn to recognize them as hostile invaders, produce antibodies, and remember them for the future. If the bacteria or virus reappears, the immune system will recognize the antigens immediately and attack aggressively well before the pathogen can spread and cause sickness.

The Herd Immunity Imperative

Vaccines don't just work on an individual level, they protect entire populations. Once enough people are immunized, opportunities for an outbreak of disease become so low even people who aren't immunized benefit. Essentially, a bacteria or virus simply won't have enough eligible hosts to establish a foothold and will eventually die out entirely. This phenomenon is called "herd immunity" or "community immunity," and it has allowed once-devastating diseases to be eliminated entirely, without needing to vaccinate every individual.

This is critical because there will always be a percentage of the population that cannot be vaccinated, including infants, young children, the elderly, people with severe allergies, pregnant women, or people with compromised immune systems. Thanks to herd immunity, these people are kept safe because diseases are never given a chance to spread through a population.

Public health officials and scientists continue to study herd immunity and identify key thresholds, but one telling example is the country of Gambia, where a vaccination rate of just 70% of the population was enough to eliminate Hib disease entirely.

However, if too many people forgo vaccinations, herd immunity can break down, opening up the population to the risk of outbreaks. That is why many officials and doctors consider widespread immunization a public health imperative and blame recent disease outbreaks on a lack of vaccination.

For example, in 1997, prominent medical journal The Lancet published research claiming to have found a link between the measles vaccine and autism. As a result, in following years the parents of over a million British children decided not to vaccinate their kids. The research has since been thoroughly debunked, but the number of measles cases has skyrocketed, from just several dozen a year in 1997 to over 2,000 cases in 2011. Similar outbreaks have occurred throughout the United States, involving both measles and whooping cough, with doctors and officials blaming low rates of vaccination.

Types of Vaccines

The key to vaccines is injecting the antigens into the body without causing the person to get sick at the same time. Scientists have developed several ways of doing this, and each approach makes for a different type of vaccine.

Live Attenuated Vaccines: For these types of vaccines, a weaker, asymptomatic form of the virus or bacteria is introduced into the body. Because it is weakened, the pathogen will not spread and cause sickness, but the immune system will still learn to recognize its antigens and know to fight in the future.

  • Advantages: Because these vaccines introduce actual live pathogens into the body, it is an excellent simulation for the immune system. So live attenuated vaccines can result in lifelong immunity with just one or two doses.
  • Disadvantages: Because they contain living pathogens, live attenuated vaccines are not given to people with weakened immune systems, such as people undergoing chemotherapy or HIV treatment, as there is a risk the pathogen could get stronger and cause sickness. Additionally, these vaccines must be refrigerated at all times so the weakened pathogen doesn't die.
  • Specific Vaccines:
    • Measles
    • Mumps
    • Rubella (MMR combined vaccine)
    • Varicella (chickenpox)
    • Influenza (nasal spray)
    • Rotavirus

    Inactivated Vaccines: For these vaccines, the specific virus or bacteria is killed with heat or chemicals, and its dead cells are introduced into the body. Even though the pathogen is dead, the immune system can still learn from its antigens how to fight live versions of it in the future.

    • Advantages: These vaccines can be freeze dried and easily stored because there is no risk of killing the pathogen as there is with live attenuated vaccines. They are also safer, without the risk of the virus or bacteria mutating back into its disease-causing form.
    • Disadvantages: Because the virus or bacteria is dead, it's not as accurate a simulation of the real thing as a live attenuated virus. Therefore, it often takes several doses and "booster shots" to train the body to defend itself.
    • Specific Vaccines:
      • Polio (IPV)
      • Hepatitis A
      • Rabies

      Subunit/conjugate Vaccines: For some diseases, scientists are able to isolate a specific protein or carbohydrate from the pathogen that, when injected into the body, can train the immune system to react without provoking sickness.

      • Advantages: With these vaccines, the chance of an adverse reaction in the patient is much lower, because only a part or the original pathogen is injected into the body instead of the whole thing.
      • Disadvantages: Identifying the best antigens in the pathogen for training the immune system and then separating them is not always possible. Only certain vaccines can be produced in this way.
      • Specific Vaccines:
        • Hepatitis B
        • Influenza
        • Haemophilus Influenzae Type B (Hib)
        • Pertussis (part of DTaP combined immunization)
        • Pneumococcal
        • Human Papillomavirus (HPV)
        • Meningococcal

        Toxoid Vaccines: Some bacterial diseases damage the body by secreting harmful chemicals or toxins. For these bacteria, scientists are able to "deactivate" some of the toxins using a mixture of formaldehyde and water. These dead toxins are then safely injected into the body. The immune system learns well enough from the dead toxins to fight off living toxins, should they ever make an appearance.

        Conjugate Vaccines: Some bacteria, like those of Hib disease, possess an outer coating of sugar molecules that camouflage their antigens and fool young immune systems. To get around this problem, scientists can link an antigen from another recognizable pathogen to the sugary coating of the camouflaged bacteria. As a result, the body's immune system learns to recognize the sugary camouflage itself as harmful and immediately attacks it and its carrier if it enters the body.

        DNA Vaccines: Still in experimental stages, DNA vaccines would dispense with all unnecessary parts of a bacterium or virus and instead contain just an injection of a few parts of the pathogen's DNA. These DNA strands would instruct the immune system to produce antigens for combating the pathogen all by itself. As a result, these vaccines would be very efficient immune system trainers. They are also cheap and easy to produce.

        Recombinant Vector Vaccines: These experimental vaccines are similar to DNA vaccines in that they introduce DNA from a harmful pathogen into the body, triggering the immune system to produce antigens and train itself to identify and combat the disease. The difference is that these vaccines use an attenuated, or weakened, virus or bacterium as a ride, or vector, for the DNA. In essence, scientists are able to take a harmless pathogen, dress it in the DNA of a more dangerous disease, and train the body to recognize and fight both effectively.