During a pandemic, influenza vaccines that rely on neutralizing antibodies to protect against matched viruses might not be available early enough. Much broader (heterosubtypic) immune protection is seen in animals. Do humans also have cross-subtype immunity? To investigate this issue, archival records from the Cleveland Family Study, which was conducted before and during the 1957 pandemic (during which a shift from subtype H1N1 to H2N2 occurred), were analyzed. Only 5.6% of the adults who had had symptomatic influenza A in earlier study years developed influenza during the pandemic, despite living in households with participants who had influenza. In contrast, 55.2% of the children who had had symptomatic influenza A contracted it again. These findings suggest an impact of accumulated heterosubtypic immunity during a pandemic. Such immunity, as well as its implications for vaccination, should be further investigated