How Effective Is the Flu Shot?

My patients often ask me how good the flu shot is. Sometimes they just tell me. "It's only 62% effective, Doc." That's true, as far as it goes, but what does it really mean? This season influenza hit early, giving epidemiologists a unique opportunity to give us an early estimate of how "good" this year's flu shot is. The number "62%" refers to a very specific kind of effectiveness. At the beginning of the season, the CDC sought out patients with symptoms such as cough, sore throat, flu-like symptoms. They then tested the patients for flu, and asked them if they had received a flu vaccine.

Efficacy here means that of the sample of people who came to the doctor with a flu-like illness, 62% of those who had been vaccinated did not have influenza. That's one way to measure the effect of a vaccination.

But it's not the only way, or even necessarily the most important. How many people who received the vaccine and got the flu had a milder case? How many hospitalizations were prevented? How many deaths were prevented?

There was a bit of a brouhaha recently when The Lancet published a study of flu vaccine efficacy. The results were widely mischaracterized especially by anti-vaccination websites, but according to the study's authors:

"Our data strongly support that there can be a moderate level of protection from influenza vaccines," he said. "As an intervention goes, that's still an important level of protection."

As for the sparse evidence for benefits in people over 65 in particular, he observed that a study sponsored by Kaiser Permanente showed an 8% reduction in hospitalization in older people who were vaccinated, adding, "That would seem to be a good return on investment."

Kelley, his coauthor, commented, "For seniors, there's a reduction [in flu risk with vaccination] and it's greater than the reduction from a lot of other interventions in that age-group."

The preponderance of the evidence points toward influenza vaccination as a beneficial public health measure. There is still much we need to learn about its utility in certain groups, especially the elderly, and its cost effectiveness. But based on available evidence, current vaccination recommendations seem appropriate.

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Peter Lipson, Contributor