A new study has found that many Americans over the age of 60 have some immunity to the H1N1 swine flu virus. Using stored blood samples scientists at CDC have discovered that 1/3 of those over age 60 have antibodies that may help protect them from a new infection. It's likely that this is because they were exposed to a similar strain decades ago. If a vaccine is developed and is in short supply, this information may help scientists decide how to distribute doses of the vaccine. Right now most serious cases seem to be occurring in younger populations, which also have been shown to have fewer antibodies. This Washington Post article explains more.