Catching flu's drift: vaccines fight unexpected influenza.

Vaccination can prevent three of every four flu infections, even when the vaccines are imperfectly tailored to block the common wintertime pathogens, a new study shows. That finding is reassuring, researchers say, because it's difficult to anticipate how the flu virus will evolve each year during the time from the start of vaccine manufacture to deployment.

The flu reinvents itself from year to year through a process in which previously rare variants of the virus supplant once-dominant ones. Since a vaccine trains the immune system to spot viruses by their unique surface proteins, a vaccine may work poorly if an unexpected variant dominates the flu season.

Past research has shown that injected and inhaled vaccines perform comparably, with efficacy as high as 90 percent, when the vaccine is on the mark. But in about 3 years out of every 10, designers guess incorrectly which variants of flu will predominate, says Arnold Monto of the University of Michigan in Ann Arbor.

In one such year, the new study found that a conventional flu shot called Fluzone was more effective than a newer, inhaled vaccine called FluMist.

Monto and his colleagues conducted the study during the 2004-2005 flu season, when the dominant flu variant wasn't among those targeted by that year's vaccines.

In late 2004, the researchers injected the Fluzone vaccine into the shoulders of about 500 healthy adults in Michigan. They gave a similar group of volunteers the intranasal FluMist spray. About 200 people received a placebo that resembled either the shot or the spray.

During the subsequent flu season, which began in January 2005, about 4 dozen of the participants developed influenza.

Compared with people who had received a placebo, those who got the spray vaccine developed flu about half as often, and those who got the injected vaccine fell ill only one-quarter as often, the researchers report in the Dec. 14 New England Journal of Medicine.

"We have two effective vaccines," Monto concludes. The shot's efficacy of 75 percent is surprisingly high, given that it wasn't designed to target that year's dominant flu strain, adds Monto, who has been a paid consultant for MedImmune, the maker of the inhaled vaccine.

In the study, both type A and type B influenza viruses caused illness. The type A group includes the most common agents of influenza and the targets of flu vaccines.

The two types of vaccine performed similarly against influenza type A, Keiji Fukuda and Marie Paule Kieny of the World Health Organization in Geneva note in the journal issue. However, the injected vaccine performed twice as well as the intranasal spray against the type B strains.

Type B strains are becoming increasingly common, Monto says.