How good is the flu vaccine?

People are desperate for protection against pandemic H1N1 influenza. That’s understandable given the media reports of deaths among otherwise healthy children and young adults.

TV news cameras have focused on long lines of people waiting to be vaccinated. Because there has not been enough vaccine to go around, people are even more eager to get their shots.

What no one yet knows, however, is how effective this vaccine will be. Preliminary testing has demonstrated only that the H1N1 shot induces an immune response. We will only learn how well it actually protects people from catching the flu after the season is over.

If you look at the track record for regular influenza vaccines through the years as a model, there may be some disappointment. Most people assume that getting a flu shot provides 100 percent protection. That assumption is not borne out by the data.

In fact, there are questions whether a flu shot prevents serious complications of influenza – pneumonia or death, especially among the most vulnerable. A careful analysis of data from a large U.S. health-maintenance organization showed that previous estimates of flu-vaccine effectiveness were seriously flawed (The Lancet, Aug. 2, 2008). They had not taken the “healthy user effect” into account.

Those seniors who are especially careful about their health are most likely to get their flu shots each year. They are also less likely than sick older people to die from flu or its complications, regardless of vaccination. As a result, the statistics on survival are skewed in favor of immunization.

A new examination of flu-vaccine effectiveness for people over 70 found strong evidence of bias in most studies (Journal of Clinical Epidemiology, July 2009). The term “bias” doesn’t mean deliberate skullduggery, but it does imply that there is a systematic misinterpretation of data.

Even if older, sicker people don’t benefit as much from a flu shot as might be hoped, public health officials assume that younger people are better protected. They usually respond strongly to a vaccine, with their immune systems producing plenty of antibodies to the injected virus.

But even they may not be getting as much benefit as anticipated. Scientists writing in the British Medical Journal (Oct. 28, 2006) found that healthy people under 65 had no fewer days out of work, shorter hospital stays or fewer deaths from flu or its complications when they received the vaccine compared with those who did not.

This year, the real action is with kids, because children appear to be more vulnerable to the H1N1 virus. How well did previous flu shots work for youngsters?

The data on vaccine effectiveness among children leave much to be desired. The Cochrane Collaboration is a highly regarded international group of researchers who evaluate medical evidence. In 2008, the Cochrane Collaboration published a review that found surprisingly little data showing whether flu vaccines work in babies and toddlers under 2 years old (Cochrane Database Systematic Reviews, April 16, 2008). The available evidence suggests “the efficacy of inactivated vaccine was similar to placebo.”

We hope that the new H1N1 vaccine will perform better, but we will only know how well it worked after the fact. In the meantime, following public health guidelines to avoid the flu makes good sense.