Push to double up on Covid booster and flu shot may have a downside, experts caution

“I really believe this is why God gave us two arms — one for the flu shot and the other one for the Covid shot.” — Ashish Jha, White House Covid coordinator, Sept. 6, 2022

As the promotional push to get people vaccinated with one of the updated, bivalent Covid vaccines heats up, federal health authorities are urging Americans to consider getting their Covid shot and their flu shot at the same time. And with concern about a fall wave of Covid paramount in this effort, the messaging is stressing the importance of doing this sooner rather than later.

Earlier this week, Anthony Fauci, President Biden’s chief medical adviser, put it bluntly: “Get your updated Covid-19 shot as soon as you are eligible.” For many people over the age of 12, that would be right now.


There’s just one problem with the advice. It’s still early to get a flu shot.

The protection generated by influenza vaccines erodes pretty quickly over the course of a flu season. A vaccine dose given in early September may offer limited protection if the flu season doesn’t peak until February or even March, as it did during the unusually late 2021-2022 season.


“If you start now, I am not a big fan of it,” Florian Krammer, an influenza expert at Mount Sinai School of Medicine in New York, told STAT. “I understand why this is promoted, but from an immunological point of view it doesn’t make much sense.”

A number of studies have shown that the benefit of a flu shot wanes substantially over the course of a flu season — exacerbating effectiveness problems that are frequently seen when some of the strains in the vaccine aren’t well matched to the strains making people sick.

Work done by researchers from the Kaiser Permanente Vaccine Study Center and the Harvard School of Public Health estimated vaccine effectiveness declined by about 18% for every 28-day period after vaccination. A study done by scientists at the Centers for Disease Control and Prevention and elsewhere showed that the vaccine’s protection against flu that is severe enough to trigger hospitalization decreases by between 8% and 9% per month after vaccination. In older adults, who are more likely to get seriously ill from flu, the decline happened at a rate of about 10% to 11% per month.

“You’ve got about four months of pretty solid protection,” said Emily Martin, an associate professor of epidemiology who specializes in flu at the University of Michigan School of Public Health. Martin was an author on the latter study.

If you ask someone who researches flu and flu vaccines, they will likely quietly — or in some cases, not so quietly — advise you to wait at least until the end of October to get a flu shot, though they’ll attach the caveat that if you start to hear about flu activity picking up where you live, you should fast-forward your plans.

“I’ll follow very carefully the activity in the community,” said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy. “If it starts to pick up, I’ll move immediately. Otherwise I’m counting on sometime in late October, early November.”

Martin gets her flu shot every October two weeks before a scientific meeting she attends, unless she hears something that makes her rethink that timing. “I’m right across the street from a dorm,” she said. “As soon as I start seeing those flu numbers go up, I’ll go get it.”

Krammer waits longer. “I usually get the flu shot at the end of November, because typically we see a rise in cases in the middle of December, and by then my antibody response should have kicked in, and be at its prime for a December-January season, and there might still be enough for a late season,” he said.

The CDC’s official advice states that “September and October are generally good times to be vaccinated against flu,” though the agency’s website adds that “ideally, everyone should be vaccinated by the end of October.”

The “everyone” thing is one of the sticking points when trying to time flu vaccinations. While waiting would generally make more sense on an individual basis, the goal most years is to get flu shots into tens of millions of arms over a period of a few weeks. That’s a big task for the health care system when it’s just flu, but this fall, vaccine deliverers are trying to get Covid boosters and flu shots into as many arms as possible.

Noel Brewer, a professor of health behavior at the University of North Carolina, said he thinks moving to this joint-administration approach is the right idea. Covid vaccine delivery has been overly complicated, he said, requiring people to keep track of too many things. How many shots they’ve had. When they last had a shot. When they are next eligible for a shot.

People have been tuning out, he said — an assertion bolstered by the booster uptake rate. The percentage of eligible people who got a second Covid booster is lower than the percentage of people who got a first booster, which is lower than the percentage of people who got a primary series of two shots.

Making things simple and pairing Covid shots with another health intervention makes it easier for people, Brewer said — even if the combination benefits one of the interventions more than the other.

“I don’t think any of this is free. I don’t think any of it’s easy,” he said. “There isn’t a single best option that we can do that doesn’t have some consequences. We just have to take the less onerous consequence.”

Ed Belongia, director of the Center for Clinical Epidemiology and Population Health at Wisconsin’s Marshfield Clinic Research Institute, said in a case like this, public health authorities need to think in terms of what works best for the collective, not each individual in it.

Some people get flu shots every year; they are motivated to take advantage of this preventive tool. Some of them may well decide to book a flu shot appointment for later in the fall rather than get their flu shot when they get a Covid booster. Belongia is among them. He’d rather wait a while and he’s planning on getting a high-dose flu shot, which will probably have more side effects than the standard vaccine. While getting flu and Covid shots during the same health care visit is safe, Covid shots can carry quite a kick. Belongia would prefer to get the two vaccines separately.

But other people are unlikely to book two health appointments to get two different shots, authorities realize from experience with other health interventions. Given that reality, what’s ideal may have to give way to what’s pragmatic.

“There’s a tradeoff, right, between starting too early and having waning, versus missing opportunities to vaccinate,” said Belongia. “In terms of trying to optimize timing, you lose people who don’t get vaccinated at all. Trying to balance those is difficult because you don’t know when the flu season is going to begin each year.”

That is the other key sticking point with advising people on when to get a flu shot. Ideally, getting vaccinated shortly before flu season starts to take off would make sense. Estimating when flu season is going to take off — well, good luck with that.

“Trying to predict flu seasons is in many cases a lot harder than predicting the stock market,” Osterholm said.

Influenza is notoriously variable. In many years, activity only really comes to a head in January and February, but in 2021-2022, the week of peak activity was the week between Christmas and New Year’s. That said, there was a long tail to the 2021-2022 season, with cases remaining at atypical levels in June.

Figuring out what flu is going to do — when it’s going to hit, which subtype will be dominant, whether the season will be tough or mild — is even harder in the pandemic era. Covid wiped a lot of pathogens off the table for a time. There was no flu season to speak of in 2020-2021, and while the 2021-2022 season was unusually extended, it was mild.

Some of the bugs that stopped circulating in the early part of the pandemic have come back with a vengeance, and scientists worry that the same will be true when flu activity returns to pre-pandemic levels. In a recent interview with Bloomberg News, Fauci warned the country might be facing a “pretty bad flu season.”

Flu experts think that’s possible, but not guaranteed.

“If I had to predict, I do think, whether this year or in future years, sooner or later the immunity debt is going to catch up with us, particularly younger folks,” Martin said. “You’ve got entire birth cohorts of children growing into the population that haven’t had that regular introduction to viruses that they normally would have had between the years 0 and 3.”

Little kids play a pivotal role in spreading flu around communities. “They’re little bioreactors, right?” said Krammer.

People often look to the southern hemisphere to try to predict the coming winter’s flu activity, though what happens north of the equator doesn’t alway match what happened south of it.

This year Australia had a very active flu season, akin to pre-pandemic seasons, Kanta Subbarao, director of the World Health Organization’s Collaborating Center for Reference and Research on Influenza in Melbourne, Australia, said via email. Though most of the activity was caused by the influenza A virus H3N2 — a virus that is typically very hard on older adults — children were most affected this year, Subbarao said.

Elsewhere in the southern hemisphere, the flu season was not particularly active, said Osterholm, who suggested it’s not yet clear what we’re facing, influenza-wise.

“All I can do is say based on a typical year, 1) you don’t want to get the vaccine too early and 2) we could see everything from a mild to a severe flu season and everything in between and we don’t know,” he said. “And the best way to be prepared for that is to try to time your vaccination to what a typical winter season incidence looks like. And that means to me, mid-October to early November, and of course, always being open to change if we see unusual early activity.”

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Source: STAT