 The virus was found to be a novel strain of influenza for which extant vaccines against seasonal flu provided little protection . A study at the U.S. Centers for Disease Control and Prevention published in May 2009 found that children had no preexisting immunity to the new strain but that adults , particularly those older than 60 , had some degree of immunity . Children showed no cross @-@ reactive antibody reaction to the new strain , adults aged 18 to 60 had 6 – 9 % , and older adults 33 % . While it has been thought that these findings suggest the partial immunity in older adults may be due to previous exposure to similar seasonal influenza viruses , a November 2009 study of a rural unvaccinated population in China found only a 0 @.@ 3 % cross @-@ reactive antibody reaction to the H1N1 strain , suggesting that previous vaccinations for seasonal flu and not exposure may have resulted in the immunity found in the older U.S. population . 
