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How Effective Are Flu Shots?

Posting Date: 10/24/2005

People are freaked out about the flu. Public health officials have been fretting about bird flu and the possibility that it could turn into a worldwide pandemic.

There is not much anybody can do about avian flu. No vaccine has been developed, and there is not enough antiviral flu medicine to treat everyone. No one even knows yet whether the bird flu will actually morph into a real danger or if it will just remain a threatening prospect.

In the meantime, public health pundits are urging high-risk Americans to sign up for their annual flu shots. Last year, shortages of flu vaccine created panic as people waited in long lines, only to have supplies run out.

This year experts do not expect a shortfall of flu vaccine. But the heavy emphasis on immunization does not fit well with research on the efficacy of flu vaccine, especially for older, sicker people.

Although policy makers pay homage to the concept of evidence-based medicine, the scientific basis for influenza immunization is not that impressive. Two years ago, for example, the flu shot protected nearly half the healthy adults who got it. But only 38 percent of those at high risk because of chronic illness were protected by the immunization. This was due in part to the fact that the vaccine that year was not a close match for the dominant strain of influenza.

But even in years when the vaccine matches the various types of flu virus circulating in the community, immunization for the elderly may not be as effective as most people think. A study of mortality records for the past three decades shows that death rates from the flu among older people has not changed much during that time. Over the same period, there was a dramatic increase in the rate of flu vaccination, from 15 percent to 65 percent. The investigators concluded that flu shots do not protect this vulnerable population particularly well (Archives of Internal Medicine, Feb. 14, 2005).