WASHINGTON — A substantial portion of Americans over age 60 may have some immunity to the swine-origin H1N1 influenza virus, a finding that may prove useful whenever a vaccine to the new flu strain becomes available.
A study using stored blood samples found that one-third of people over 60 have antibodies that might protect them from infection with the new virus, said scientists at the Centers for Disease Control and Prevention. If further research is able to better define who has partial immunity, those people might need only one dose of vaccine, not two.
The blood study, published Thursday in CDC's Morbidity and Mortality Weekly Report, gives an immunological explanation for a surprising observation in the swine flu outbreak — that very few old people are getting sick.
Nearly two-thirds of the U.S. cases are in people between 5 and 24 years old. Fewer than 1 percent are in people over age 65, the group most susceptible to typical seasonal outbreaks of influenza. Of those sick enough to be hospitalized, 40 percent have been 19 to 49.
In the study, researchers tested blood collected since 2005 for research on the effectiveness of seasonal flu vaccine. They exposed the blood to samples of the swine flu to see if it contained antibodies that attacked the virus.
Samples from children 6 months to 9 years old contained virtually no antibodies against the swine flu strain. But 6 percent of people ages 18-40, 9 percent of people ages 18-64, and 33 percent of people over age 60 had the antibodies.
When blood samples taken after the same people had received seasonal flu vaccine were tested, the percentage that had active antibodies against the swine flu strain went up in the two older groups. Specifically, for the 18- to 64-year-olds, it increased from 9 percent to 25 percent; and for the over-60 group, from 33 percent to 43 percent.
Overall, the findings suggest that many older people may have been exposed to a flu virus decades ago that bore a distant similarity to the new strain and triggered an immune response. Seasonal flu shots appear to boost that "memory" response a little. A vaccine made from the new strain would be expected to both increase and sharpen the response, perhaps enough that only a single shot would suffice.
CDC deputy director Anne Schuchat said no firm conclusions can be drawn yet.
"Our working hypothesis is that everyone who gets this vaccine is likely to need two doses," she said Thursday. "Perhaps there will be some people where pre-existing immunity will be there and one dose would lead to a 'primed' response."
The virus was first detected last month, and at least 42 countries now have confirmed it in more than 11,000 people. At least 85 people have died from it. For every reported case of swine flu, there may be 20 people sickened with it, Schuchat said — more than 100,000 people in the United States.