A substantial portion of older Americans may have some immunity to the swine-origin H1N1 influenza virus, a finding that may prove useful when and if a vaccine to the new flu strain becomes available.
The questions of whom to target with a swine flu vaccine and how to stretch the supply if it is limited are among the most important issues facing public health officials over the next four months.
Scientists at the Centers for Disease Control and Prevention announced yesterday that a study using stored blood samples found that one-third of people older than 60 have antibodies that might protect them from infection with the new virus. If further research is able to better define who has partial immunity, those people might need only one dose of vaccine, not two.
"Our working hypothesis is that everyone who gets this vaccine is likely to need two doses," Anne Schuchat, CDC's deputy director for science and public health, said yesterday. She added, however, that the new study suggests "perhaps there will be some people where preexisting immunity will be there, and one dose would lead to a 'primed' response. That is definitely . . . something we're interested in."
If a swine flu vaccine is produced, about 2 billion doses would be ready by next fall, the World Health Organization estimates. Public health authorities presumably would recommend it for people at greatest risk for severe illness and death.
The blood study, published yesterday 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. Less than 1 percent are in people older than 65, those most susceptible to typical seasonal outbreaks of influenza. Of the people ill 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 whether 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. However, 6 percent of people 18 to 40, 9 percent of people 18 to 64 and 33 percent of people older than 60 had the antibodies.
When blood samples taken after the same people had received seasonal flu vaccine were tested, the percentage with active antibodies against the swine flu strain increased in the two older groups. Specifically, for the 18-to-64-year-olds, it increased from 9 to 25 percent; and for the older-than-60 group, from 33 to 43 percent.
Overall, the findings suggest that many older people may have been exposed to a flu virus decades ago that bore a 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 a single shot would suffice. However, people whose immune systems have never encountered a flu strain even remotely like the new one would almost certainly need two shots to gain protection.
Schuchat, the CDC epidemiologist, said no firm conclusions can be drawn yet.
It is not known whether the "cross-reactive" antibodies found in the study will prove to be protective against illness, or whether the 79 children and 280 adults whose blood samples were tested are representative of the population at large. Studies to answer those questions are underway.