The inside scoop on flu vaccines: Addressing myths about the flu shot

Every autumn, we start to hear the buzz about getting a flu shot. As an infectious disease specialist and hospital epidemiologist at UF Health, that’s also when I hear a lot of questions about the flu shot. There is a lot of misinformation about flu vaccination, so I will try to demystify it by going through the two most common misconceptions that I hear.

Here they are:

“The last time I got the flu shot, it gave me the flu.”

This statement is a good example of what we call in medicine “true, true, unrelated.” It means that it may be true that you got the flu shot, and it may be true that you got the flu, but those two events are not related to each other. The simple fact is that the flu shot contains only killed virus. There is no live virus in it that could begin to multiply to cause the flu. So how could you develop flu soon after getting the flu shot? Let’s say you were vaccinated against flu on Dec. 10, and then on Dec. 12, you develop fever, cough, runny nose and body aches. You are tested for flu, and the test result is positive. The reason you got the flu is that you were already infected with the flu virus when you got the flu shot. You were going to get sick no matter what! It takes about 14 days to develop a full immune response to the shot, so you were not yet protected on day two after vaccination. Also, the “incubation period” for flu is five days. That means that you could have been infected with flu as early as Dec. 8. Here’s a really interesting fact: A person can transmit the flu to another person a full 24 hours before developing any of their own symptoms. So, you might have no idea from whom you contracted the flu because he or she may have appeared perfectly healthy.

“I heard that this season’s flu shot doesn’t work.”

This statement is a good example of over-extrapolation. It stems from “vaccine efficacy” or “VE” studies aimed at determining how many flu infections are prevented by vaccination. These studies do not provide information about the severity of illness in vaccinated people who do come down with the flu. Many studies have shown very important benefits of flu vaccination, such as reducing hospitalizations, reducing ICU stays, protecting babies from flu after birth when their moms are vaccinated in pregnancy and reducing cardiac events in people with heart conditions, to name a few. Returning to the VE studies, it is important to know that there is a separate VE for each flu strain, of which there are four in total. Each flu shot contains either three or four flu strains: So, even if the VE is lower for one of the strains, it is usually a lot higher for the other ones. This is the case for this year’s flu VE.

I hope I’ve convinced you that the flu shot is a very important tool you can use to maintain good health and protect against severe illness. The protections it offers include a decreased chance of getting the flu, a more mild course if you do get infected and a decreased chance of transmitting the virus to others. Finally, it is still not too late to get vaccinated, so GET VACCINATED!

At UF Health, we’re committed to helping keep our community healthy and safe. We appreciate everyone’s help in sharing information — and reducing misinformation — about flu this season. Please visit us on the web to learn more about flu prevention and our visitor guidelines this flu season.

Photo by Kelly Sikkema on Unsplash