So you’re debating about when to get your flu shot, wondering how long the protection lasts.
Inasmuch as the Centers for Disease Control and Prevention recommends getting a shot as soon as the vaccine is available, which this year was mid-to-late August, I wondered whether a shot before Labor Day would wear off by the time the flu arrived. The great part about this job is I have experts on speed dial (or speed bytes). Here’s what Dr. John Treanor, chief of Infectious Disease at the University of Rochester Medical Center as well as professor of medicine, and of microbiology and immunology, had to say about how long it takes for the shot to kick in and how long the protection lasts.
Here’s the unedited email response:
How long does it take for the vaccination to become effective? Usually, when we talk about the vaccine being effective, we are talking about the vaccine preventing the flu. It would be very difficult to know with certainty exactly when you become protected from flu with vaccine. You would have to do a complicated study with lots of subjects where you compared the rate of flu between vaccinated and unvaccinated subjects every day from the day of vaccination and see when the two rates started to diverge, or something like that. That study has never been done. If you think that the flu vaccine protects against influenza by stimulating the body to make antibodies against the flu, then you might be interested in how long does it take before I can measure an increase in the amount of antibody against the flu in your blood after vaccination. That answer is about 7 to 10 days. So, to be on the safe side, we usually say that you are protected about 10 or maybe 14 days after vaccination. If you were the kind of person who was so at risk of the flu that we thought you needed antiviral prophylaxis, we would give that to you for the first 14 days after vaccination, and then stop.
How long does the protection last? Again, there is no positively known answer to that question. The level of antibody in the blood does drop over time, but there were some observations made in studies done many years ago that suggested that the protection from vaccine might last for more than one season if the virus didn’t change. On the other hand, there have been severe outbreaks in nursing homes late in the spring that suggested that maybe the vaccine protection doesn’t even last an entire season. So nobody really knows, but since we almost always change the vaccine composition anyway, the simplist strategy is just to administer the vaccine every year. All things considered, it’s more important to get vaccinated than to time it to the exactly optimum time, so we tell physicians that if the patient is in the office and the vaccine is available, it should be given, even if it seems a little early, like sometimes happens in late August.
Thanks, Doc, on behalf of all the readers.