INFLUENZA

Groups of embryonated eggs infected with the PR8 strain of influenza virus A were incubated at 34 degrees , 37.5 degrees , and 40 degrees C. At frequent intervals, for periods ranging up to 96 hours, pooled allantoic fluids were tested simultaneously for infectivity and hemagglutination. After about 12 hours of virus growth, fluids often showed infectivity titres greater than 10(-5), but were incapable of causing hemagglutination. At later time intervals, marked disagreement between the two tests for viral activity was noted at all temperatures, but most strikingly at 40 degrees C. Hemagglutination titres were highest and best sustained in eggs incubated at 34 degrees C., while incubation at 37.5 degrees C. resulted in the highest and best sustained infectivity titres. Hemagglutination titre determinations do not reflect accurately the rate of influenza virus multiplication. Possible reasons for the lack of correspondence between hemagglutination and infectivity are discussed.