WASHINGTON (Reuters) - The seasonal flu vaccine provides virtually no protection against the new H1N1 influenza strain, federal health researchers confirmed on Thursday.
Their study using stored blood samples supports an intriguing theory that people over the age of 60 have some immunity to the new H1N1 strain, perhaps because it resembles an older version of seasonal flu.
The findings are important as U.S. and global health officials grapple with the question of whether to offer a vaccine against the new swine flu virus, which has infected more than 11,000 people globally and killed at least 85.
They also illustrate the need for a so-called universal influenza vaccine -- one that can protect people from a range of strains of the constantly changing virus.
"We would love to have an influenza vaccine that did not have to be reformulated each year," Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention said at a news briefing.
The researchers at the CDC, Food and Drug Administration, National Institute of Allergy and Infectious Diseases, along with, universities and vaccine makers, looked at blood samples given after people were vaccinated for seasonal flu.
Seasonal flu vaccines carry an H1N1 component, an H3N2 strain and an influenza B strain. They tested these samples against both seasonal flu viruses and the new H1N1 strain.
Blood from children showed no immune system protection from the new strain, they wrote in the CDC's weekly report on death and disease.
"Results among adults suggest that some degree of pre-existing immunity to the novel H1N1 strains exists, especially among adults aged over 60 years," the researchers added.
One possible explanation is that older adults were either infected with or vaccinated against a much older strain that more closely resembles the new H1N1 strain.
Schuchat said it is odd that genetic analysis shows the new swine flu virus is very different from any other H1N1 virus seen previously. There may be an aspect to flu viruses that the human immune system recognizes that does not show up in the genes, she said.
"We are wondering whether there were viruses around in the 1930s, 1940s, 1950s that were immunologically similar," she said.
The H1N1 family of flu viruses was first documented when it caused the 1918 influenza pandemic. H1N1 viruses have been circulating among humans and pigs on and off since then but with frequent mutations and shifts.
Because of these changes, the seasonal flu vaccine must be reformulated every year.
Schuchat said the tests done for the study cannot show whether the immunity seen in the blood samples was enough to protect people from infection with the new strain.
On Thursday the CDC reported 5,764 confirmed U.S. cases of the new flu and nine deaths. Schuchat said it is likely more than 100,000 people have been infected.
An unusually high proportion of those being infected and hospitalized with serious disease are young adults, teenagers and older children. Seasonal flu, in contrast, is usually far more severe in very young children, people over the age of 65 and people with chronic disease.
"We think this virus is initially amplifying among teenagers in schools and college students coming back from spring break and mixing," Schuchat said. It may move into other age groups later, she said.
Editing by Bill Trott