[An influenza outbreak of type A and type B influenza viruses]

Results of investigations carried out during an outbreak of influenza occurred in January 1981 in a School of Milan are reported and discussed. A total of 9 type A (H1N1) influenza virus strains, antigenically intermediate between A/USSR/90/77 and A/Brazil/11/78 variants, and 2 type B influenza virus strains, antigenically intermediate between B/Hong Kong/8/73 and B/Singapore/222/79 variants, have been isolated. In one case both epidemic strains have been simultaneously detected. From the results of type-specific complement-fixing antibody titration it appears that an influenza virus infection occurred in 64% of the 300 subjects serologically examined. In 43% of the cases type A strain was involved, in 12% type B strain and in 9% both serotypes. Judging from the data of school absenteeism it seems that infections by type A virus were more severe than those by type B virus. Finally, because a group of pupils attending the school had been vaccinated in December 1978, it has been demonstrated that the rate of type A presumptive infections was correlated to the hemagglutination-inhibiting antibody titers attained after vaccination.