I recently received the following question, which contains some common misconceptions about vaccines so I thought I would answer here instead of just in a private e-mail. The e-mailer writes:

Hello, I’m a new listener to the show. A few times now I’ve heard the subject of vaccinations come up on the show and I’m not sure I understand.

I fully understand that the “anti-vac” folks are wrong, and that vaccinations are very helpful in eradicating disease. I also understand that a certain portion of the population needs to be immune to a disease in order to protect the rest of the population from it. But I don’t understand – why does everyone need to get them, every year, and isn’t it possible that doing so weakens our natural immune system? If my body can fight off the flu quite easily, then why get the shot? Won’t the net effect (my body killing the virus) be the same?

Is there something I’m missing?

The e-mailer refers initially to herd immunity – once a certain percentage of the population is immune to infection (for whatever reason) there will not be enough susceptible hosts for the bacteria or virus to spread, and therefore there will likely not be an outbreak. The exact percentage of the population that needs to be immune varies depending on the infecting agent, but around 90% is typical. This means that for most vaccine-preventable diseases we would need vaccine rates to be above 90% in order to achieve herd immunity (some people may be immune because of prior infection, but this number is low for diseases for which there has been a vaccine available for decades).

Further, the 90% figure applies to local populations, not the country as a whole. So if there are pockets of low vaccine compliance then herd immunity will be compromised and outbreaks can occur. We are seeing this happen now with mumps, measles, and pertussis in communities with an large anti-vaccine sentiment.

The e-mailer next asks, “But I don’t understand – why does everyone need to get them, every year?” I assume they are referring to the flu vaccine. The reason for this is simple – the flu virus comes in many strains. Every year a few different strains are dominant, and the CDC and other organizations have to make their best guess as to which strains are likely to come around. They essentially look at the rest of the world to see what strains are making the rounds, and then try to anticipate which strains will hit their region.

This is a difficult guessing game, and sometimes they get it right and the flu vaccine is very effective, while other years the strains change between the time they start vaccine production and flu season, and the flu vaccine is less effective.

Strains are largely defined by the HN system – this refers to two glycoproteins: hemagglutinin and neuraminidase. So H1N1 (swine flu), for example, has the first variation of both hemagglutinin and neuraminidase, while the bird flu is H5N1.

Flu vaccines have activity against specific strains. Usually they include three strains out of the many possibilities. In the last two years H1N1 has been incorporated into the flu vaccine. There is also evidence that after getting the flu vaccine for many years a person can develop fairly broad resistance to many flu strains, and they are less susceptible to the flu in general.

The holy grail of flu vaccines, however, is to develop a universal flu vaccine, one that targets structures on the flu virus that are conserved or common across all strains, instead of targeting structures that are specific to one strain. Researchers are working on the universal flu vaccine, and recently announced progress in this area. It is possible that we could have a universal flu vaccine by 2016. Until then, it is recommended that you get the flu vaccine every year. (Now would be a good time if you have not gotten your vaccine yet this year.)

The last question is this, “isn’t it possible that doing so weakens our natural immune system?” This is a common claim of the anti-vaccine movement (the e-mailer is clearly not anti-vaccine, but you can see how misinformation spreads). Vaccines do not weaken the immune system, they strengthen it. The concept behind vaccines is that they expose the immune system to proteins, killed viruses, parts of viruses or bacteria, or attenuated viruses or bacteria (which are alive but not virulent). The immune system will react as if there is an infection, making antibodies against the structures, including the memory cells that will be able to react the next time the immune system sees the same proteins or structures. Therefore, when the host is exposed to the infecting organism they can mount a much more vigorous immune response and fight off the invader before a clinical infection occurs.

The immune system is therefore strengthened, not weakened. I have never been able to figure out what the reason is to believe vaccines can weaken the immune system. It seems that some people may believe the vaccines fight off infection without using the immune system, as is suggested by the e-mailer’s reference to the “natural immune system,” as if the vaccine represents an artificial immune system. Vaccine works through the immune system – it does not replace it.

There is a real question about the strength and duration of the immune response from vaccines vs a natural infection. Infections do often produce greater and longer lasting immunity from vaccines, because the immune response is often in proportion to the duration of exposure, and an infection can produce more exposure than a vaccine. But scientists track the immune response to vaccine by measuring antibody levels. For many vaccine types sufficient antibody levels last for decades or even a lifetime, so even when they are not as long as the infection they are still robust and long lasting. For others booster shots are required.

In any case this is all better than getting the actual infection. The e-mailer’s suggestion that we can easily fight off infections (again, I assume they mean the flu) is not accurate. Most healthy individuals will weather the flu just fine, but will be miserable for a week or so. For me avoiding a week of the flu is worth getting a shot. But also the flu causes hundreds of thousands of hospitalizations in the US every year, and from 3-49 thousand deaths (it varies greatly from year to year). Also, there are many people in the community who are not healthy, or cannot get vaccinated, so getting the vaccine can help them as well.

The flu vaccine, and vaccines in general, are a safe and effective public health measure. There are some common misconceptions, however, that need to be cleared up on a regular basis. Further, there is a dedicated campaign of ideologically motivated misinformation about vaccines, requiring an equal campaign of public education about the true state of the science.