Last year's flu vaccine was good, but not perfect

Washington -- Viewing the glass as half-full, Centers for Disease Control and Prevention officials and other infectious disease specialists are pointing to last year's influenza vaccine as a success, even though it wasn't perfectly matched to the flu it was formulated to fight.

Two CDC studies, one on children and the other on adults, found that the vaccine was effective in about half the cases.

The results should persuade more people that an annual flu shot carries important health benefits, said William Schaffner, MD, professor of preventive medicine at Vanderbilt Medical Center in Nashville, Tenn.

"The best protection is achieved by routinely getting your flu shot each autumn and knowing that the protection is going to be either pretty good or great. But at the very least, it will be pretty good, and that's better than having no protection at all," he said.

The CDC determined that the vaccine was effective about 52% of the time at preventing influenza among the Colorado adults studied. The vaccine's effectiveness ranged from 25% to 49% among the children, said Carolyn Bridges, MD, a medical epidemiologist with the CDC's National Immunization Program.

The finding for children was less definitive because laboratory confirmation of the flu was not the end point in that study, whereas in the study on adults, influenza cases were laboratory-confirmed. The researchers looked at children who had received a diagnosis of a respiratory illness, which could include a number of ailments that are not halted by the flu vaccine.

The effectiveness rates were higher than those found in an earlier CDC study. In that research on health care workers, reports of a variety of respiratory symptoms were collected instead of cases of laboratory-confirmed influenza, making the results much less conclusive, said Dr. Bridges.

A moving target

The new studies also provide reassurance that the recommendation to continue to give the vaccine last season was on target, despite the discovery last September that the vaccine was not a perfect match with the circulating virus, said Dr. Bridges.

In years when the vaccine is closely matched to the ever-mutating influenza virus, effectiveness can reach 70% to 90%.

Although last year's less closely matched vaccine didn't achieve these high rates, its use certainly afforded more protection than no flu shot at all, officials said.

The viral strains to be included in the vaccine administered each fall are selected in the previous spring using data that are continually gathered from influenza surveillance performed in more than 80 countries, said Dr. Bridges.

The flu hit the United States in October last year, which is earlier than usual, and it hit hard. The CDC had reports of 142 deaths among children. Because the last flu season was the first in which the CDC asked states to report children's deaths, no comparisons can be drawn.

Now that state and local health departments are reporting influenza-associated deaths among children younger than 18, CDC officials look forward to being able to better track the severity of outbreaks among this population.

Data from a 4-year-old vaccine surveillance network that looks at the hospitalization of children with laboratory-confirmed flu indicate that the last season was more serious than the previous three, said Dr. Bridges.

The flu's early start last fall also prompted a run on vaccine. Many physicians depleted their supplies before all patients could be vaccinated.

For the coming season, word went out to doctors in the spring to place vaccine orders, and manufacturers have begun shipping supplies. However, it's still not too late to place an order, said Dr. Bridges.

Manufacturers have available about 100 million doses, an increase from last year's 86 million doses.

This flu season will also likely see an increase in the number of vaccinations given to young children because the Advisory Committee on Immunization Practices in February confirmed its earlier recommendation that children ages 6 to 23 months, as well as women who will be pregnant during flu season, be immunized.