Q. Does the flu vaccine become less effective over the course of the flu season? If yes, should I delay getting my flu shot until later in the season?
A. Yes, immunity to the flu virus may wane over the course of the flu season, so you don’t want to get your flu shot too early, such as in the summer months. But you also don’t want to get it too late, once flu season starts. For most people, sometime in October is best.
In 1990, the Immunization Practices Advisory Committee, the branch of the Centers for Disease Control and Prevention that issues vaccine recommendations, warned that, in individuals 65 years of age and older, “it is particularly important to avoid administering vaccine too far in advance of the influenza season because antibody levels begin declining within a few months.”
Since then, the question of waning immunity has been the subject of several studies, but results have been inconsistent.
The C.D.C. examined data from four flu seasons and concluded that vaccine effectiveness can decline by as much as 6 to 11 percent per month. And a European study reported that effectiveness declined from 53 percent to 12 percent beginning three months after receipt of the 2011-12 flu vaccine.
But other studies found that immunity waned for some, but not all, strains of influenza, or that immunity waned only in children and adults 65 years of age and older. Some found no significant waning at all.
One explanation for such conflicting results is that many of these studies have been observational, which are inherently more susceptible to bias and hidden methodological problems that can skew results than randomized placebo-controlled trials.
An example of such skewing is the so-called “leaky vaccine effect.” Because unvaccinated individuals are vulnerable to flu, they tend to get sick early in the flu season. As a result, there is a greater proportion of people who have been vaccinated who are susceptible to getting sick late in the flu season. These and other factors can complicate the statistical models upon which these studies are based.
In weighing the evidence and its limitations, the best approach is to follow the recommendations of the C.D.C.: “Balancing considerations regarding the unpredictability of timing of onset of the influenza season and concerns that vaccine-induced immunity might wane over the course of a season, it is recommended that vaccination should be offered by the end of October.”
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