Older people may have some kind of immunity to swine flu, US health officials said Thursday, as the number of confirmed and suspected cases of H1N1 virus rose again around the country.
A study by the US Centers for Disease Control and Prevention (CDC) found that more than 64 percent of US infections have occurred among patients between the ages of five and 24, with just one percent of flu victims aged 65 or older.
One possible explanation is that "older adults might have been in contact a long time ago with a virus related to the one that we see now," said Anne Schuchat, interim deputy director for science and public health programs at the US Centers for Disease Control and Prevention.
"Adults might have some degree of pre-existing ... antibodies to the H1N1 virus, especially older adults over 60 or 65," she added.
"The presence of pre-existing antibodies may be due to previous exposure to (influenza) infection or vaccination."
Health authorities around the world have been surprised that with the number of cases of swine flu now topping 11,000, with 85 deaths, many of those infected have been younger people.
The CDC's study suggests children and teens may be particularly vulnerable to contracting the disease -- a worrying prospect as officials brace for the possible return in a few months' time of a more virulent strain of the H1N1 virus.
Schuchat added the flu outbreak was still "far from over" in the United States, where the tally of confirmed or probable US cases is now 5,764.
"The virus continues to circulate in the US," Schuchat said, adding there have been "localized outbreaks" in various states.
US health officials also remain concerned at delays in beginning production of a vaccine that could prove effective against swine flu.
This is especially urgent since the current batch of season flu vaccine does not provide protection against the H1N1 virus.
"We remain hopeful that we'll have vaccine viruses to send to manufacturers by the end of May," the CDC official said.
But Schuchat added that there are "many steps involved, and it will still be several months before a vaccine against this new virus will be available."
Schuchat offered a scrap of upbeat news, noting that flu-related visits to doctors and hospitals were on the wane across the United States, in what she considers a "good sign."