Abstract
An outbreak of influenza caused by the A(H1N1) subtype in military recruits in February 1978 afforded an opportunity to study the association of ABO blood groups with influenza morbidity and serological response. Fifty-eight per cent of 336 recruits became clinically ill. There was no differential distribution of clinical influenza by blood group. However, seroconversion to a titre of greater than or equal to 20 was significantly and appreciably higher in groups A and B than O and AB. Also, among those with serologically confirmed clinical influenza, the occurrence was significantly higher in groups A and B than groups O and AB.