Every year around this time, 120 million Americans roll up their sleeves to get their annual flu shots. Since 2010, the U.S. Centers for Disease Control and Prevention has recommended yearly jabs for every healthy American over the age of six months. The goal is to curb the spread of infection and minimize the risk for potentially dangerous complications such as pneumonia, particularly among the elderly and the very young. But science on the vaccine’s efficacy is scant among those two vulnerable groups. And although healthy adults do get some protection, it may not be as robust as they expect.
One oft-cited claim, based on several large meta-analyses published more than a decade ago, is that seasonal flu shots cut the risk of winter death among older people by half. But the research behind that claim has been largely debunked. A 2005 study published in the Archives of Internal Medicine noted that influenza only causes about 5 percent of all excess winter deaths among the elderly—which works out to one death from flu per 1,000 older people each season—so it’s impossible for the shot to prevent half of all their winter deaths. The following year, a study reported that as vaccine coverage increased among the elderly in Italy in the late 1980s, there was no corresponding drop in excess deaths. In another 2006 paper, Lisa Jackson, an infectious disease epidemiologist at the Group Health Research Institute in Seattle, and her colleagues showed that although vaccinated seniors were 44 percent less likely to die during flu season than unvaccinated seniors were, the vaccinated ones were also 61 percent less likely to die before flu season even started. “Naturally, you would not expect the vaccine to work before the thing it protects against is going around,” says Lone Simonsen, a research professor in global health at George Washington University and a co-author of the 2005 study in the Archives of Internal Medicine.
Researchers now attribute these odd findings to a “healthy user” effect. People who don’t get vaccinated often “are the most frail or [those] whose health has gone down dramatically in the last few months,” explains CDC epidemiologist David Shay. People who choose to get flu shots, in other words, are already healthier and therefore the least likely to die.
So how much does the vaccine truly help older people? In January 2012, Michael Osterholm, an epidemiologist at the University of Minnesota’s Center for Infectious Disease Research and Policy, and his colleagues published a meta-analysis in The Lancet Infectious Diseases that analyzed the results of all randomized controlled clinical trials conducted between 1967 and 2011 on the effects of flu shots. It found that there have been no clinical trials evaluating the effects of the traditional flu vaccine in the elderly. The only vaccine shown to protect against infection or death in older adults, it said, is the live-attenuated vaccine—an inhalable vaccine that contains a live, modified version of the virus—which is not approved in the U.S. for adults over age 50.
The traditional vaccine may not work so well in older people because of an idea known as immune senescence, which posits that as people age, their immune systems weaken, resulting in poor vaccine response, especially to inactivated strains. Although the U.S. Food and Drug Administration licensed a high-dose vaccine for seniors in 2009 that could theoretically overcome this problem, no studies have yet been published on how effective it is. “The higher dose produces a higher level of antibodies, but we don’t really know what that correlates to,” says Jackson. A 2010 systematic review published by the Cochrane Collaboration, an independent, nonprofit organization that promotes evidence-based medicine, concluded that “until such time as the role of vaccines for preventing influenza in the elderly is clarified, more comprehensive and effective strategies for the control of acute respiratory infections should be implemented.”