Differing virulence of H1N1 and H3N2 influenza strains

Sequential influenza A/Texas/77 (H3N2) and A/USSR/77 (H1N1) epidemics occurred during the winter of 1977-1978 in two populations under viral surveillance for influenza. In college students who reported to the Vanderbilt student health service, roughly equivalent amounts of typical influenzal disease were documented by virus isolation and total health service visits with both strains. However, considering that the college population was fully susceptible to the first introduction of H1N1 virus in 20 years and partially immune to H3N2 viruses through repeated exposure, A/USSR appeared to be a relatively less pathogenic strain. Stronger proof of this was seen in a closely monitored group of 200 largely seronegative infants and young children less than 4 years of age enrolled in an experimental vaccine clinic. In this young population, A/USSR caused no recognizable illness and A/Texas caused typical febrile respiratory disease. Thus, two influenza strains circulating in close temporal association differed in virulence when observed in carefully monitored susceptible populations.