WASHINGTON — A substantial portion of elderly 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 question 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 federal Centers for Disease Control and Prevention announced Thursday that 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. If further research is able to better define who has partial immunity, those people might need only one dose of vaccine, not two.
As of Thursday, the United States had 5,764 confirmed cases and 9 deaths in 48 states. Epidemiologists believe, however, that more than 100,000 people have been infected since the new virus came to public attention a month ago.
Worldwide, 41 countries have reported 11,034 cases and 85 deaths — numbers that are almost certainly also an undercount.
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 ill enough to be hospitalized, 40 percent have been 19 to 49.
The CDC on Thursday said it knows of nine U.S. deaths associated with the outbreak, but news reports suggest there have been at least 10.
Swine flu cases seem to be decreasing in some parts of the country. The Southeast, Midwest and Southwest all have shown signs of decreased flu cases, Dr. Anne Schuchat said. But New England, New York and New Jersey seem to have more swine flu activity, she said.
The Associated Press contributed to this report.
