The Most Important Vaccine I’ll Get This Fall

After last year’s eerie lull, flu viruses could be poised to return packing a bigger punch.

A person in a hat sneezing
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On Saturday morning, I finally rolled up my sleeve for the vaccine I’d been waiting for all summer: my annual flu shot, a technological marvel that I opt to receive every fall.

During non-pandemic times, the flu vaccine is a hot autumn commodity that holds a coveted place in the public-health spotlight. As of late, though, the shot’s been eclipsed by the prominence of its COVID-blocking cousins, fueled by debates over boosters and mandates. It’s also been a while since we’ve had to tussle with the flu directly. Thanks to the infection-prevention measures the world took to fight SARS-CoV-2 when the pandemic began, many other respiratory viruses vanished. Last winter, we essentially had “no flu season at all,” Florian Krammer, a virologist at Mount Sinai’s Icahn School of Medicine, told me. The human attention span is short; the flu’s brief sabbatical might have purged it from our minds at an inopportune time.

An absent virus isn’t necessarily an extinct one, and the flu’s return was always going to be a matter of when, not if. And as the weather cools, experts are worried that skipping a season of sickness could come with costs, if we don’t raise our antiflu shields anew. Immune defenses can rust and crumble; flu viruses might return to find a slew of hosts more vulnerable than they were before, especially now that kids are back in classrooms and mask ennui continues to balloon nationwide. “I worry that we are not paying attention,” Hana El Sahly, an infectious-disease physician and vaccine expert at Baylor College of Medicine, told me. Flu shots, then, are particularly valuable this year—perhaps more so than they’ve been in quite some time.

Concerns about resurgent flu aren’t new. Back in February, when I first wrote about the lull in flu cases, experts were already warning that the bugs’ truancies could make them more unpredictable. Flu viruses, already a familiar threat to our immune system, spread less easily than SARS-CoV-2, which made them easier to stamp out with masks, physical distancing, school closures, and international travel bans, even when adherence was spotty. Cases around the globe plummeted. But “no one expected flu to go away forever,” Mary Krauland, an infectious-disease modeler at the University of Pittsburgh, told me.

Now we’re teetering on the edge of the year’s chilly turn as pandemic restrictions wax and wane. Many experts suspect that we might be in for a flu season worse than the previous one, in part because the previous one was so mild. The threshold for an outbreak this year could very well be lower. “I’m probably 60–40: 60 we will have a season, 40 we won’t,” Richard Webby, an influenza expert at St. Jude Children’s Research Hospital, in Tennessee, told me. “My gut feeling is, if it does come back, there’s going to be a little more punch.”

The flu’s absence did have positives. It spared health-care workers, hospitals, and the general population a second winter swell of sickness, atop an already overwhelming pandemic. Our not-flu season also starved the viruses of hosts in which to multiply and shape-shift and persist. Some experts are hopeful that certain lineages might have been squeezed out of existence entirely, or at the very least came close. That could mean that we have fewer flu flavors to contend with, and vaccinate against, in the future, though their disappearance isn’t yet certain.

But the previous season also left our sometimes-forgetful immune cells without an important annual reminder: Flu viruses do, in fact, exist, and can wreak serious havoc on the body. Reasonably good flu-vaccine coverage last winter certainly jogged our bodies’ memories. But without the additional alarms raised by actual illness—which during normal times hits many millions of people in the United States alone—people’s bodies might not be as tuned up as they should be. “You really need the seasonal waves to drive up population-level immunity and prevent large outbreaks,” Helen Chu, a physician and immunologist at the University of Washington, told me.

Infants and young kids might be especially vulnerable this year because a higher number of them than usual may have never met a flu virus. Schools are open again, many without mask requirements, adding risk for both children and those who interact with them. “When it comes to influenza,” El Sahly said, “children are the engines of transmission in the community.” A preview of this pattern already unfolded in the spring and summer with respiratory syncytial virus, another airway-loving pathogen that hits kids particularly hard. Like flu viruses, RSV all but evaporated last winter, but it was able to wriggle its way back into the American population around the start of April, when many COVID-19 restrictions relaxed.

Two recent models from Krauland and her colleagues at the University of Pittsburgh, posted in preprint papers last month, hint at the toll of missing out on our yearly immune boost. Flu cases and hospitalizations, the studies found, could both experience a bump this year, potentially beyond that of typical seasons—an additional burden that the pandemic-battered health-care system can little afford. That’s especially likely if COVID precautions keep falling away, or if we’re hit with an especially contagious flu strain that our bodies don’t recognize well. Worryingly, other experts pointed out, flu viruses and SARS-CoV-2 might even invade some of the same individuals at once, which could fuel very serious bouts of illness among the vulnerable.

These aren’t foregone conclusions, Kyueun Lee, who led one of the studies, told me. Our social behaviors still aren’t back up to their pre-pandemic levels; even intermittent masking, distancing, and the like could put a damper on the flu’s upcoming campaign. In Australia, a nation that countries in the global North usually look to as an epidemic bellwether, flu levels have stayed fairly low, which could bode well for the United States, Ibukun Kalu, a pediatric-infectious-disease physician at Duke University, told me (although she added that the American approach to COVID containment has been “vastly different” from the Australian one). Case counts could end up between last year’s startling low and the pre-pandemic norm.

We also have an extraordinarily powerful, yet underused, tool in our arsenal: an immunity-boosting vaccine. The flu shot typically reaches only about half of the U.S. population, but Lee thinks that ratcheting up that percentage this year is essential, because it could help seal some of the cracks that COVID mitigations left in our antiflu armor. “Getting a flu vaccine this season may be particularly important,” Lynnette Brammer, who leads the CDC’s domestic influenza-surveillance team, wrote in an email. But there’s yet another catch. Normally, surveillance centers stationed around the globe are able to amass many thousands of viral genome sequences to get a good read on which versions of flu viruses are bopping around—which ones might be poised to make a resurgence if given the opportunity. Scientists mine this wealth of data when selecting strains for the yearly shot. But last winter, that genetic wellspring dried up. “It’s hard to pick if you don’t have a clear picture of what’s out there,” Krammer told me.

But there were enough data to make an informed decision, experts reassured me. “The match is always a gamble,” said El Sahly, who was a member of the committee that advised on the FDA’s final vaccine formulation. “Even having high transmission beforehand doesn’t guarantee strain selection is going to be spot-on.” Generally speaking, flu-shot effectiveness against disease tops out at roughly 60 percent. But like most other immunizations, the vaccine is stellar at curbing the severity of symptoms and keeping people out of the hospital; even a somewhat mismatched vaccine could make an enormous dent in the viruses’ impact. “No matter what, it’s going to protect you at least a little bit,” Chu said.

Flu shots are also good at hedging bets. The standard “quadrivalent” formulation contains safe, inactivated representatives from four branches on the flu tree: H1N1 and H3N2, subtypes that belong to the influenza-A family, and B/Victoria and B/Yamagata, lineages in the influenza-B family. Flu-A viruses generally shape-shift more rapidly than their B-list cousins, so those ingredients change more often. The shot I got this weekend contained two updates, compared with last year’s recipe, that will hopefully prepare me better for the flu strains du jour. (Kalu pointed out another perk: We’re still waiting for the official green light on the COVID-19 vaccines for the under-12 crowd, but the flu shot is available now for kids as young as six months old.)

Regardless of how the flu collides with us this winter, my recent vaccine is an insurance policy: Either way, I’m better protected than I was. Getting the shot was also easy. I was able to get an appointment on my first try; the injection itself, which was free and painless, took only a second at my local CVS. (Flu shots and COVID-19 shots, by the way, can be administered at the same time.) It was clearly the awakening my body needed: Within hours, my arm had swelled up a bit; eventually, so did the lymph nodes next to it, likely as they filled with hordes of grumpy, flu-sensitive immune cells, some likely roused from a two-year slumber. I felt a little achy, a little tired. I felt so much better than I had before.