FAQs About Immunity and Vaccines

Why do some vaccines last longer than others?

This depends on several things. Two of the main ones are the type of virus and the type of vaccine.

A vaccine teaches your immune system to recognize a virus. But some viruses have more variability than others. This means that the proteins a virus is made out of are a little different from one virus to another. When there are differences, your immune system may have a harder time recognizing a virus.

It’s easier to make an effective and long-lasting vaccine when a virus has low variability. Like for measles, where if you looked at many virus particles you’d see very similar versions of surface proteins.

Other viruses are highly variable. For these, it’s harder to make vaccines. HIV can even change a lot within a single person. And the flu changes from season to season, so each year a new vaccine comes out to target the most recent strains.

Vaccine type matters too. Vaccines that cause a strong immune response, such as whole virus vaccines, tend to last longer. Vaccines made from part of a virus tend to wear off and require future doses.

Many of the highly effective childhood vaccines we have today work well because they have a couple things going for them: they’re for viruses with low variability and made into a vaccine that gets a strong immune response.

If I get a fever after getting a vaccine, does that mean the vaccine has given me a viral infection?

The short answer is it depends. Some vaccines do contain “live” viruses that can infect your cells. These are weaker viruses, though. They replicate slowly, and as long as your immune system is healthy, it should be able to control the infection quickly. And yes, this is a low-level infection that can cause some people to get a fever or body aches, or just feel tired.

Other vaccines—like the ones made from pieces of a virus or a virus that has been “killed”—do not cause an infection. But even a vaccine with no “live” virus can give you a fever—or make you feel achy or tired. These are side effects that happen when your immune system starts to respond to an invader. And activating the immune system is exactly what a vaccine is designed to do! The side effects are signs that the vaccine is working.

To learn more about what vaccines are made of, visit Types of Vaccines.

What are some common and rare vaccine side effects?

Vaccines are among the safest medicines that we use. But, like anything we put in our bodies, they can cause side effects.

Common vaccine side effects tend to be mild, and they go away after a few days. Importantly, they are much safer than having the illness. They include soreness at the injection site, fever, and body aches. These effects are actually good signs that your immune system is working. (But even if you don’t get them, it doesn’t mean it’s not.)

Rarely, people do have more serious reactions. If the vaccine is a shot there’s a chance for infection, just like there is if you get a paper cut or other small injury to your skin. A person may have an allergic reaction to a vaccine ingredient, like eggs or gelatin.

Guillain-Barré syndrome is another very rare, but serious, reaction. It’s an autoimmune response that causes muscle weakness. Some vaccines increase the chance of getting Guillain-Barré syndrome. But most cases are caused by having a viral or bacterial infection. So while the flu vaccine may slightly increase the risk, actually having the flu is a bigger risk for Guillain-Barré (though it’s still small).

Death after a vaccine is extremely rare. When something is so rare, it is hard to know if the vaccine is truly the cause. Additionally, the system where potential vaccine-related deaths are reported (called VAERS) was set up to bring attention to any possible risk. So some severe events are reported as a precaution, even if it’s unclear if a vaccine was the cause.

The chance of a serious reaction varies by vaccine. For some, one person out of every ten thousand may be affected. But it’s usually even less common—around one person out of every million vaccine doses. To put that in perspective, the odds of getting struck by lightning this year are around one in 500,000.

The other thing to keep in mind is that vaccines are given by healthcare professionals. They can take steps to help prevent serious reactions. Like using needles in a way that reduces the chance of infection, or asking about your food allergies before giving you the vaccine. You can also ask them questions about any specific concerns you have.

What are antibodies?

Antibodies are one of the body’s natural defenses against infection. They are proteins, made by specialized immune cells, that circulate in the blood and other fluids. The body makes many types of antibodies, each with a specific target. When an antibody attaches to a virus, it blocks the virus from getting into cells and flags it to be destroyed.

When you get an infection or a vaccine, immune cells start making large amounts of antibodies specific to that infection or vaccine. The body makes fewer of these antibodies over time. But they don’t go away completely. If you are exposed to the infection again, the antibodies can quickly recognize and stop it.


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