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Like many health care workers, I am required to receive my annual flu shot by the end of October. Every year, I wait until the last possible day to get vaccinated.
That’s because a substantial body of research shows that flu shot effectiveness wanes markedly over time, just as that of Covid-19 shots do, particularly in high-risk individuals. After just a few weeks, antibodies produced in response to flu vaccinations reach their highest levels. I don’t want my antibodies idling in September and October, only to be dropping right as the dependable winter influenza peak approaches.
And yet, tens of millions of Americans heeded public health messaging to get vaccinated as soon as possible — starting as early as September. By now a quarter of seniors in the United States have received their annual flu shots. Their bodies are brimming with new vaccine-induced antibodies, ready to fend off any flu virus they encounter.
The trouble is there’s no battle for them to wage.
In the United States, seasonal flu levels are still reliably low at this time of year. Levels can start to increase in November, but in most years, they really take off in December, or even later. In the first week of October last year, weekly flu-related hospitalizations were over 44 times lower than they were at their eventual peak, which occurred, as expected, in late December.
Why doesn’t the Centers for Disease Control and Prevention recommend people wait until late October or even early November to receive their annual flu shots?
Epidemiologists for the C.D.C. are chasing the wrong outcome. They seem focused on one metric: vaccine uptake, the percent of the population vaccinated against the flu every season. They should instead focus on a different one: the percent of severe illnesses averted.
Those two goals may conflict with each other. Public messaging encouraging people to get vaccinated early is likely to improve uptake but may lead to less overall protection when it is most needed in the winter.
One model that takes into account how vaccine effectiveness decreases over time suggests that in years in which flu cases peak in January or February, starting the annual vaccine campaign in October rather than September could reduce flu-related hospitalizations by three or four percent. That might not seem like much, but in some years it might mean up to 10,000 fewer hospitalizations.
The problem is trying to get everyone vaccinated in a short period. It’s possible that what would be gained in increased effectiveness would be negated by fewer people getting the shot. That’s why the C.D.C. has so far opted not to delay its annual flu shot campaign.
There’s a solution.
The C.D.C. should start vaccinations in October instead of September, which, with a concerted effort, should be more than enough time to run a successful vaccine campaign. The National Institutes of Health and other researchers should also begin studies to assess the effectiveness of a two-dose flu vaccine for high-risk populations, like seniors and the immunocompromised, which would better protect them throughout the flu season. The Covid-19 vaccine experience taught us that for high-risk individuals, boosting even just a few months apart was linked to decreased hospitalizations associated with Covid-19 in those populations, and it’s likely that flu shots work similarly.
Scientists have tested two-dose versions of the flu vaccine in the past, but they tried the wrong regimens. Rather than testing two doses several months apart (for example, in October and January), studies have assessed second doses given just a few weeks after the first ones, when antibodies were already very high and the additional shot was unlikely to provide much of a bump. However, a second dose given a few months later, after antibodies have fallen, might provide more protection and prevent many more hospitalizations. We need scientists to study whether this approach works in the real world.
Will it be hard to convince people to take two shots when many don’t even take one? While vaccine hesitancy is a problem, seniors and other high-risk people consistently show more willingness to get vaccinated, and these populations stand to benefit most.
New data from the recently concluded flu season in the Southern Hemisphere show that this year’s flu shot was 35 percent effective at preventing hospitalizations. But that figure is an average. Effectiveness also varied by age and other risk factors, and likely differed based on when in the season people got their shots. It was 31 percent effective in older people, but 59 percent effective for young adults with medical comorbidities, another important target group.
What does this mean for people who have already gotten their annual flu vaccines? Not all is lost. Many people, especially those with strong immune responses to the vaccine, are likely to still have lingering immunity that lasts well into the winter season. And for those who dependably receive their flu shots every year — including older people who by far have the highest rates of annual flu vaccination — there may be some cumulative benefits from regular shots. That may be from long-term bolstering of the immune system, which could help to prevent or at least minimize the severity of infections that exacerbate chronic diseases.
For those of us who have yet to receive our flu vaccine, our delay may work in our favor, provided we still get them in the next month or so. Remember, vaccines work only if they’re in our bodies.
Jeremy Faust is a doctor in the emergency medicine department at Brigham and Women’s Hospital.
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