Natural and 'in vitro' selected antigenic variants of influenza A virus (H2N2).

We provide data on the prevalence of SRH antibody to influenza A/Singapore/1/57 (H2N2). Approximately 10.3% of sera had antibody to the influenza A (H2N2) subtype virus in comparison to the 36.9% of positive sera to a representative influenza A (H3N2) and 31.5% to influenza A (H1N1) viruses. The percentage of subjects with antibody constantly decreased from the older to the younger age groups. Persons born after 1968 were essentially seronegative, whereas subjects born before 1900, and in the decade 1950-1959, showed the highest antibody levels to influenza A (H2N2) viruses. These age groups also appeared to have 'protective' levels of anti-HA antibody to influenza A (H2N2) virus. An antigenic variant of A/Singapore/1/57 virus was selected in the laboratory using a monoclonal antibody to HA. Serological comparison of the new in vitro variant with the parental virus and two naturally occurring viruses, namely A/England/12/64 and Tokyo/3/67, showed that certain human sera were able to distinguish the variant, indicating a restricted antibody repertoire in these adult and children's sera, providing an explanation of how such variants could actually arise in nature.