A new report from the Centers for Disease Control and Prevention found that adults over age 60 may have some protection against swine (H1N1) flu. Researchers studied how blood serum samples react to both the seasonal strain of influenza A H1N1, and the new swine flu virus in a lab setting. Samples from children showed little protection from the newer strain, but those from adults, especially those older than 60, suggested some degree of preexisting immunity to swine flu.

The researchers posit that some adults in this age group may have had exposure to a genetically similar virus many years ago. In 1918, an H1N1 virus appeared and caused that year’s pandemic, explained Daniel Jernigan, M.D., Ph.D., a medical epidemiologist with the CDC. The virus remained in circulation until 1957, when it was replaced by a H2N2 virus, which caused another pandemic. People exposed to H1N1 viruses in the years before 1957, may have developed antibodies that can help defend them against the new swine flu.

The report also found that recent seasonal flu vaccines are unlikely to provide any protection against the swine (H1N1) flu virus. But despite the potential protection from previous exposures, the development of a vaccine for the new H1N1 swine flu virus is needed for optimal protection for all age groups, the CDC report concluded.