Problem: Influenza is a common viral disease. Because it's so common (in any one year, somewhere between 5 percent and 20 percent of Americans will get the flu) and because people tend to call any illness with fever, sore throat, vomiting, or diarrhea a "flu," it is often taken casually—more a fact of life than a cause for anxiety. Many of these misnamed infections are pretty minor, but true influenza is often quite a serious disease, leading to more than 200,000 annual hospitalizations in the United States and about 36,000 deaths every year. Unfortunately, catching the flu doesn't guarantee immunity—the virus's unstable genetic makeup changes frequently, and the immunity stimulated by an infection or a shot probably won't be helpful in the next year if even a minor change occurs and a new strain emerges. And sometimes those new strains are exceptionally dangerous. The "Spanish flu," the worst of these varieties, appeared in 1918 and is thought to have killed somewhere between 50 million and 100 million people worldwide—between 2.5 percent and 5 percent of the world's population—during a two-year period.
The vaccine: In most years, the circulating influenza virus is particularly bad for infants and the elderly. People with chronic illnesses like asthma, heart disease, or diabetes are also at higher risk for severe illness and death. So, the first flu-immunization campaigns focused on the elderly and on patients with significant chronic diseases. (The vaccine is not approved for use in children younger than 6 months.) The recommendations broadened as it became clear that children in the 6-months to 2-year age range were also at high risk for complications of flu and hospitalization, expanding first to include children up to age 3, then age 6, and, this year, 18.
There are several advantages to broadening the range of people to be given flu shots. Because influenza carries substantial risk of requiring hospitalization, giving more people shots will certainly decrease the annual U.S. hospitalization rate and death rate, even though the vaccine isn't perfect—about 25 percent of the time, it fails partly or completely. There is also a secondary benefit: People who have been immunized are much less likely to pass the disease on to others who are unprotected or incompletely protected, a phenomenon called "herd immunity." That's the main reason for the push to immunize all children through age 18—those hacking and spewing youngsters are influenza's version of Typhoid Mary.
New study: If we're going to give annual flu shots to children from 6 months to 19 years old, the parents of babies, pregnant women, people 50 years and older, and everyone else with a chronic disease, maybe everybody should just get it. Starting in 2000, the Canadian province of Ontario offered free flu shots to everyone older than 6 months. Fortunately for science, though perhaps not for public health, the other Canadian provinces continued to offer the flu vaccine just as we do in the United States—targeted to specific populations, like the very old, the young, and people with chronic disease. This "experiment in nature" gave Canadian public-health researchers a unique opportunity to compare the benefits of universal influenza immunization with targeted policies.
Results: It's slightly dicey business to compare data from Ontario with those of the provinces that didn't enact a universal-immunization policy. Researchers can't be sure that the differences in the rate of immunization actually caused the differences in influenza diagnoses, hospitalization rates, or excess deaths. In fact, during the seven-year period under study, there was improvement in flu-vaccination rates in all the provinces. As a result, influenza statistics everywhere in Canada improved—but they improved a lot more in Ontario than in the rest of Canada. Influenza-associated deaths dropped by 57 percent in the rest of Canada, but they fell by 74 percent in Ontario. Every other statistic about influenza in Canada—flu-related cases seen in emergency rooms, doctors' offices, or hospital admissions—showed exactly the same pattern: Things are significantly better in Ontario. We do need to be a bit skeptical—that difference might, indeed, be due to some other environmental, economic, or educational difference between Ontario and the rest of Canada. But this evidence is the best we have today, and it's probably good enough to serve as a basis for changes in public-health policy. The only Ontario patients who didn't get significant benefit from flu shots were the elderly. As other studies have also shown, it seems as if it's simply harder to give the elderly good protection against flu using our standard methods of immunization, and there is active ongoing research to develop new, more potent vaccines. Meanwhile, we keep giving older people the present vaccine, hoping that at least some will benefit.
Conclusion: The next question is, Can it be done? Judging by the results in Ontario, the answer seems to be "yes" but only by providing newer sites for immunization, more easily available to the public—for instance, schools, stores, airports, train stations, even election polling sites. Should we do it? People afraid of needles are going to hate me for saying this, but yes, I think so. Extrapolating from the Canadian results, I think it is very likely that a policy of universal influenza immunization will lower hospitalization and death rates and even be economically advantageous. But, of course, it's easy for me to say: As a health care provider, I've already gotten my shot this year.