Why the flu vaccine may not save that many lives after all— at least among senior citizens
Mike Jackson '98 is an epidemiologist with the Centers for Disease Control and Prevention in Atlanta. For his 2007 doctoral dissertation from the University of Washington, he studied the efficacy of the influenza vaccine among the elderly, closely examining the medical records of more than 3,500 people aged sixty-five to ninety-four. His findings, which radically depart from conventional wisdom, were published in The Lancet late last summer. They prompted international headlines, including extensive coverage in the New York Times.
Cornell Alumni Magazine: Should the average person get a flu shot?
Mike Jackson: Yes. The vaccine is safe; the risk of complications is vanishingly small. For most healthy adults, the flu isn't very dangerous, but it's certainly unpleasant—you feel like you've been hit by a truck for a few days.
CAM: Why is the vaccination particularly recommended for seniors?
MJ: They're the ones most likely to be hospitalized or die. The groups at highest risk of complications from influenza are seniors and children under two.
CAM: How is the flu different from other diseases?
MJ: The interesting thing about influenza is that it's seasonal. In the U.S., the virus circulates only during the winter. It shows up around November or December and spreads for two or three months, then disappears and goes into circulation in the southern hemisphere. Also, influenza is unique because you need a new vaccine every year, since the virus mutates so rapidly.
CAM: Why is there a flu season, anyway?
MJ: Nobody really knows. It's probably a combination of things. A temperature decrease and a humidity change make it easier for the virus to survive in the air longer. It could be because of being crowded indoors, or because there's less UV light coming from the sun in the winter so the virus isn't killed as quickly.
CAM: What got you interested in studying whether the flu vaccine really protects the elderly?
MJ: My PhD adviser had been reading the literature and noticed that a number of prior studies quoted implausibly large benefits for flu vaccines in seniors. It was on the order of vaccines preventing half of deaths from all causes: strokes, car accidents, heart attacks, everything. This is clearly impossible, because the flu doesn't cause half of all deaths.
CAM: What was wrong with those studies?
MJ: They looked at seniors who got vaccinated and those who didn't, and how many died the following influenza season. And because it's hard to know which senior actually died of flu, they looked at deaths from any cause. But the vaccine can only prevent deaths from influenza, which is a small fraction—maybe 5 percent in the average year—so you'd expect to see maybe a 5 percent difference in the death rate. These studies were reporting much higher benefits, but there was no way the difference was due to the vaccine; it had to be something else. It was clear that these studies were biased—that seniors who get vaccinated are somehow different from seniors who don't.
CAM: How are they different?
MJ: Those who choose to be vaccinated are healthier. Seniors who are in pretty good shape—they're mobile, taking care of themselves—are the ones who can go out in the fall and get a flu vaccine. Seniors who are more frail and less functional or can't walk as well, they're less likely to get one.
CAM: So the difference in their survival rates isn't so much about who got a flu shot, but the fact that the ones who did tended to be healthier in the first place? MJ: That's right. The classic double-blind study, which was done in the early Eighties, showed the flu vaccine reduces the risk of influenza infection in seniors. But it focused on seniors who were younger and didn't have a lot of chronic health problems. The question is, once you use the vaccine in the general population of seniors, many of whom are over eighty and have heart and lung disease, how effective is it in practice?
CAM: Why might the vaccine be less effective in older, frailer people?
MJ: As people get into their later years, there's evidence that their immune systems become weaker. The way a vaccine works is that you present your body with pieces of the virus and your body reacts to it—so if you really encounter influenza your body can react more quickly. If your immune system is not working as well, then when you're exposed to the actual influenza your body can't mount a quick response.
CAM: What sort of policy recommendations might come out of this?
MJ: This was the largest case study of its kind, but it's still comparatively small, so it's not something worthy to shape policy on. We need other studies to confirm it, and home in more accurately on how well the vaccine works on seniors before we make those decisions.
CAM: Did this teach you any broader lessons about the scientific process?
MJ: You can have a bunch of studies that get similar results—and they're all wrong. We could literally point to twenty papers that came to the same conclusion: that the flu vaccine reduces the risk of death in seniors by half. Just because it's commonly reported doesn't mean it's right.
— Beth Saulnier