The dearth of controlled research on seniors stems in part from the fact that the U.S government considers such clinical trials unethical. Based on an idea known as clinical equipoise, scientists can’t test, in a randomized controlled trial, a treatment that the larger medical community already considers to be effective, because doing so would involve denying treatment to half of the participants, potentially putting them at risk. “We’re in a difficult spot,” Shay says—since the CDC already recommends flu shots to seniors, the agency can’t suddenly turn around and ask them to participate in a clinical trial that might deny them the standard of care.
What about kids? In 2010, the U.S. Advisory Committee on Immunization Practices began recommending flu vaccination for all healthy children older than six months, an expansion that they claimed was “supported by evidence that annual influenza vaccination is a safe and effective preventive health action with potential benefit in all age groups.” Yet a July 2012 Cochrane Collaboration systematic review concluded that for kids under the age of two, the currently licensed vaccines “are not significantly more efficacious than placebo.” The review highlighted a single small study conducted on children under two—the only controlled study that has evaluated the efficacy of the shot currently licensed for young kids—which found, overall, that vaccines provided no statistically significant protection over the course of two flu seasons. “One season, the vaccine did something to prevent some symptoms, but in the second, nothing,” says co-author Tom Jefferson, an epidemiologist with the Cochrane group. In kids older than 2, however, flu vaccines do seem to work; according to the Cochrane analysis, the shot reduces the absolute risk that a child will catch the flu by about 3.6 percent, whereas the live (inhaled) vaccine reduces the absolute risk by about 17 percent.
In healthy adults under the age of 65, flu vaccines work, too. A 2010 Cochrane review, also co-authored by Jefferson, estimated that during “good” vaccine years—when the vaccines match the circulating viral strain well, which Jefferson says happens about half the time—the vaccine reduces the relative risk that an adult under 65 will catch the flu by about 75 percent. In absolute terms, however, this means adults have about a four percent chance of catching the flu if they don’t get the vaccine and about a one percent chance if they do. Shay notes that while this estimate is reasonable, some flu seasons are worse than others, so the risk may be higher than 4 percent in some years (and some people) and lower than 4 percent in others. (And of course, the vaccine won’t protect against the nearly 200 viruses that cause flu-like symptoms but aren’t actually the flu.) Although scientists generally believe that the flu vaccine slows the spread of the virus through communities, there are no data showing that this is true, because “those studies are very difficult to do,” Shay explains.
So should people still dutifully line up for their flu shots? Older kids and healthy adults do get some protection from them; just perhaps not as much as they want or expect. But for seniors and toddlers, there may never be a clear answer to this question, particularly because the U.S. government is unlikely to conduct additional clinical trials. On Monday, Osterholm and a group of five other scientists at the University of Minnesota’s Center for Infectious Disease Research and Policy published a report highlighting the need for better alternatives. Although the current options may—for most people—be better than nothing, “we can no longer accept the status quo,” they wrote. “The perception that current vaccines are already highly effective in preventing influenza is a major barrier to pursuing game-changing alternatives.”