Health Matters

Flu Fighters: Does That Shot Make a Difference?

Jan. 19, 2015
As flu illness grips the nation, stubborn questions persist about the effectiveness of flu shots. UR Medicine expert Dr. John Treanor explains why it’s important to get a flu shot, even when it seems like it doesn’t work.
woman sick in bed with flu
Health Matters: Some people who got flu shots are still getting the flu. Does it mean the shots don’t work?
Treanor: That’s a really good question, and one we’re actively researching. Flu vaccines aren’t perfectly effective, because flu evolves. Scientists do the best they can to forecast which strains of virus will be the big players in a coming season, but it’s impossible to always guess perfectly. Some years, predictions play out pretty accurately. Other years, they don’t match up as well. There is some evidence that the flu is less severe in people who get the shot, even if it doesn’t completely prevent the flu. 
Although flu shots aren’t perfectly effective for every person every year, they’re our best chance against fighting off and spreading illness.
Health Matters: If I didn’t get a flu shot, but got the flu, am I set for this season? 
Treanor: Even if you’ve had the flu, you should still consider getting vaccinated. Remember that more than one strain of flu can be out there at any given time. So, though you’ve already had one of them, your body may still be susceptible to the others. Today’s vaccines protect against the top three or four strains of flu predicted to circulate in a given season.
Health Matters: I know people who got sick right after they got the flu shot. Can the shot make you sick?
Treanor: The vaccine really can’t make you sick because it doesn’t contain living virus. Each dose holds just enough inactivated (“dead”) virus, rousing your immune system with a dress rehearsal of sorts. Essentially, you practice turning out the antibodies you’ll need in the event of a real infection down the road. That’s why you might feel a low-grade fever, or a little soreness and swelling at the injection site. But it’s certainly not to be confused with a full-blown case of flu.
This is a flu-shot myth that is particularly sticky, partly because when we give the flu shot, there tend to be a host of other respiratory germs circulating. Rhinovirus, which causes the common cold, might easily be confused for a mild case of the flu and accidentally attributed as a flu vaccine side-effect.
Another confounding factor is the lag time between the shot going into your arm and your body being fully protected. It can take a full two weeks to build complete immunity, so you may still be susceptible to flu in that brief time window after vaccination.
Health Matters: It’s almost February. Isn’t it a bit late to roll up my sleeve?
Treanor: No. Within reason, it’s also never too late to get vaccinated. Flu seasons hardly seem as predictable as they used to be. We’ve seen flu subside and then resurge in the spring. If you haven’t gotten a flu shot yet, you may spare yourself a future illness by doing so now.
John Treanor, MD
John Treanor, M.D., professor of Medicine, Microbiology and Immunology, serves as the chief of the Infectious Diseases Division of the Department of Medicine at URMC.