Should you get the flu shot and a Covid booster at the same time? New data sheds light.

An annual flu shot for all. An annual Covid shot for some (Or many? Or most? Or all? We don’t know just yet.)

That seems to be where we are headed.

While there’s a reasonable debate about whether everyone needs an annual Covid booster (rather than only people with increased risks), let’s skip that for now. The rest of this article assumes patients who need both shots this fall.

The big question. Co-administration or not?

One of the most common questions I hear: Is it okay to get both the flu shot and a Covid booster at the same time? The concern is that if both vaccines are given at the same appointment, the body’s immune response to one or both might be blunted, leading to lower effectiveness.

Last year, the CDC said it was fine to get both at once. This was based on research that looked at immune responses when one of the doses in the primary Covid vaccine series and an annual flu shot were given together.

Safety: (It’s fine).

The upshot was that doing this—what we call “co-administration”—is safe, albeit with the possibility of somewhat higher rates of non-dangerous side effects. What the CDC didn’t say—but I will—is that side effects might be a bit higher with a co-administration strategy, but it terms of overall hours or days with any side effects (such as body aches, fever, or fatigue), the “twofer” might cause less “momentary misery” for many.

Hypothetically, let’s say that the Covid and the flu shot each have a 15% chance of causing 12 hours of side effects, but that when given at the same time, 25% of people have some side effects. If someone receives the Covid shot today and the flu shot two weeks apart, the odds of side effects from at least one of them is 27.75% (The math: 85% of recipients don’t have side effects from the Covid shot. Of that 85%, another 85% will have no side effects from the flu shot two weeks later; that means 72.25% won’t have any side effects and the remaining 27.75% will. That’s higher than the 25% of recipients who had the shots co-administered).

So, in my view, it’s probably a wash. That said, there might be a signal suggesting that co-administration recipients have symptoms lasting a bit longer and which are a bit more intense. But, the differences are not big and some might prefer to have side effects once rather than twice.

Effectiveness (It’s good).

The big question for scientists is whether Covid and seasonal flu shots work as well when co-administered. Prior research suggests that immune responses are basically equal—with either no difference or a small but likely clinically unimportant decrease in antibody levels. But, those studies were done looking at the Covid vaccine primary series, not boosters. These days, virtually all adults getting Covid shots are getting boosters.

Good news, then. A study released this morning in JAMA Network Open shows that recipients who had a seasonal flu shot and a Covid booster (in this case, the bivalent booster) at the same appointment had Covid antibody levels that were “not substantially inferior” to those who got one at a time. Researchers in Israel found that antibody levels were indeed 16% lower in the co-administration recipients, but the margin of error given the sample size rendered this finding not distinguishable from statistical noise. On top of that, both levels are way, way above any reasonable threshold for a “good immune response.” So, even if the finding were “statistically real,” it probably does not matter in any clinical sense. Either way, the vaccines triggered an ample response in antibodies, both of which would be adequate to stimulate the broad immune repertoire we need to stave off severe disease.

One caveat: we don’t know whether co-administration affects the immune response to the flu shot. (This new study only looked at Covid antibodies.) But prior experience tells us that there should not cause a problem in that direction either.

Timing (Early is not always better).

Okay, so you’re going to get one or both shots. When should you get them? Increasingly, my colleagues are coming around on the idea that sooner is not better in all cases.

The old idea: You never know when flu or Covid waves will happen. So, the sooner you get a shot, the more days you are protected.

The newer idea: Actually, December-early March is when we expect to see far higher levels of viruses in circulation. Vaccine effectiveness does wane over time, so why waste your window of best protection in September or October when cases are likely to be far lower?

I’ve always been in the latter camp. I think more experts are leaning in this direction, as long as people don’t end up forgetting to get their shots entirely!

Of course, there can be early unexpected peaks of these viruses. Fortunately, very high-risk people can get a Covid booster more often than once. So getting the Covid booster soon (i.e., later this month once the new booster comes out) and in a few months might be a good strategy for people with highly compromised immune systems. Those updated boosters should be rolled out in the coming couple of weeks, so my blanket advice is for people to wait for that. But there are always exceptions which hinge on individual circumstances (which your doctor and you can sort out).

What about repeat dosing of flu vaccines for high-risk adults?

Can adults with immunecompromise get two doses of the flu shot, spaced out similarly to how some at-risk patients are doing Covid boosters? This approach has not been routinely applied to influenza vaccines, and the CDC does not comment on this in its guidance.

My guess?

More than one dose, given a few months apart, would be useful for at least some people. (Prior studies did not space the doses enough to address waning; they were designed to see if two shots up front packed a bigger punch). I believe researchers should re-examine this for people on chemotherapy or other profound immune suppression like those taking anti-rejection medications for organ transplants; some patients in these groups are likely to benefit from more than one dose. This should be studied immediately!

Bottom-line (Co-administration is fine, some nuance is possible).

The CDC is going to be pushing co-administration of Covid and flu shots as its primary public health strategy for those who need both. That makes great sense. Any small gain from getting one vaccine at a time would quickly be offset if even a small number of people didn’t end up returning for the other shot. Life happens and people get busy—or sick! So, if there’s even a chance that you might not be able to get back for that second appointment, it’s best to get both shots at once. But for those who can (and prefer it), it might make sense to space the shots. Higher-risk individuals in particular might choose this approach, especially those with immune systems that aren’t quite as strong. In these cases, the possibility of a somewhat better immune response to one-at-a-time vaccine receipt might carry some meaning.

Do you get your shots all at once? Or do you space them? Please share your experience in the comments!