When is the optimal time for a flu shot?
—Emily
This question comes up every year in the fall. Should we get the flu shot as soon as it’s available, in August? Or is it better to wait until November or December, when people really start getting sick?
I’m answering this now because a new paper was just released with the answer (spoiler: it’s October, but don’t leave until you hear why!). The paper is by friends-of-ParentData Anupam (Bapu) Jena and Chris Worsham. They are the authors of the book Random Acts of Medicine and absolute masters at using clever approaches to data to find causal effects even without explicit randomization.
In prior work, Bapu and Chris showed that when kids get flu vaccines, it protects them from the flu and also protects their older relatives. In this paper, they ask what month you should vaccinate to get the maximum form of protection.
An obvious way to start answering this question would be to look at when kids get their flu vaccine and then whether or not they get the flu. There is a problem with that, though. Parents who are eager to get the vaccine for their children — and so are first in line — may also do other things to protect them from the flu. Or they may want to be first in line because they know their child is vulnerable to the flu. In technical terms, we would say the timing of the flu vaccine is endogenous: it’s possibly related directly to flu risk, so we can’t just compare kids with different vaccine timing.
Instead, what this research does is take advantage of the fact that a lot of kids are vaccinated for the flu at their well-child visit. Access is still a significant barrier to flu vaccines, so it makes a big difference if you show up at the doctor at the time that they have the flu vaccine. Well-child visits tend to be timed with birthdays. This means that kids who are born in October are, on average, more likely to get a flu vaccine in October than kids born in August or December (or April, for that matter). Flu vaccines are generally available from August onward, so this paper compares flu in kids with August birthdays (likely to be vaccinated in August) to those with September birthdays (September vaccination), and so on through December.
What it finds is that incidence of the flu over the whole season is lowest for kids with October birthdays, implying that October is the best vaccination month. This is intuitively sensible: a vaccination in October is late enough that you retain good antibody protection through the full flu season, but early enough that you don’t risk getting the flu before you get the vaccine.
A final note: These effects are significant statistically, but we should say that all flu vaccines — regardless of timing — protect against the flu. So if August or December are your only options, get the shot then! But if you get to choose, choose October.
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