BioEd Online

Antibodies against some seasonal flu strains from prior years may be active against the new H1N1 swine flu currently circulating the globe, a recent study reports. The findings suggest an explanation for why swine flu appears to infect the young more often than the elderly, who are normally more susceptible to seasonal flu viruses.

The study, published today in the Morbidity and Mortality Weekly Report, analyzed blood samples taken from 359 participants in flu vaccine studies conducted from 2005 to 2009. 33% of the samples from people over 60 years old had antibodies that reacted with the swine flu virus, as compared to 6%-9% of the samples from people aged 18–64 years, and none of the samples taken from children¹.

The results match the apparent current epidemiology of swine flu infection, says Anne Schuchat, interim deputy director for the Science and Public Health Program at the Centers for Disease Control and Prevention (CDC) in Atlanta. Most cases of swine flu have occurred in people who are under 60 years old, and only 1% of confirmed swine flu infections in the United States were in patients over the age of 65.

Nevertheless, the results should be interpreted with caution, Schuchat urged in a press briefing today. Researchers have shown that the antibodies react with the virus in test-tube assays, but they have not yet shown that the antibodies can fend off the virus in animals or people. "Whether this particular assay will pan out over time as predictive of clinical protection, we can't say," Schuchat said.

Cross-protection

For the study, CDC researchers used two tests to determine whether antibodies in the blood were active against the swine flu. One test studied the impact of the antibodies on growth of the virus in cell culture, and the other assayed the ability of the antibodies to inhibit an important viral protein called hemagglutinin.

The results indicated that vaccination against recent H1N1 seasonal flu viruses did not generate antibodies that react with the swine flu virus. But exposure to older seasonal flu viruses or vaccines — perhaps dating back to the 1950s or earlier — may have yielded cross-reactive antibodies in some older study participants.

Although the results have not yet been confirmed clinically, the researchers used standard techniques that are often used in preclinical studies of flu vaccines, says Ralph Tripp, a viral immunologist at the University of Georgia in Athens. "The reality is, there does appear to be a substantial level of cross-reactivity in those adults aged 60 years and older," he says.

Genetic analysis of the new swine flu virus has shown that it differs dramatically from previous seasonal viruses. But Tripp notes that H1N1 viruses were circulating in swine during the 1940s and 1950s and could have mixed with seasonal flu viruses during that time. Exposure to these viruses could have launched an antibody response that continues to protect individuals against today's swine flu.

Meanwhile, Schuchat notes that it is possible for immune responses to the current swine flu and past seasonal viruses to overlap, despite the genetic dissimilarity. "We don't have a particular virus that we're thinking about," she says, "but we're wondering if there might have been some viruses around in the '30s, '40s and '50s that might be immunologically similar to the one we're seeing now."