Like many health care workers, I am required to receive my annual flu shot by the end of October. Every year8k8, 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.

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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.

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