Original Contributions LABORATORY-BASED SURVEILLANCE OF INFLUENZA VIRUS IN THE UNITED STATES DURING THE WINTER OF 1977-1978 L PERIODS OF PREVALENCE OF HIM AND H3N2 INFLUENZA A STRAINS, THEIR RELATIVE RATES OF ISOLATION IN DIFFERENT AGE GROUPS, AND DETECTI

Laboratory-based surveillance of influenza virus In the United States during the winter of 1977-1978.1. Periods of prevalence of H1N1 and H3N2 Influenza A strains, their relative rates of isolation In different age groups, and detection of antigenlc variants. Am J Epidemiol 110:449-461,1979. Influenza A (H3N2) viruses were isolated from outbreaks and epidemics of disease during the period December 1977 to March 1978. For the last two months of this period, H1N1 strains of Influenza A were also responsible for epidemics. In some regions (e.g., Hawaii) co-circulation of H1N1 and H3N2 strains occurred, whereas In other regions (e.g., Wisconsin) isolation of H3N2 strains had almost ceased prior to Isolation of H1N1 strains. Few influenza B Isolates were reported. Analysis of the ages of patients from whom speci-mens were submitted for Influenza virus Isolation confirmed that, whereas H3N2 strains were isolated from persons of all ages, greater than 97 per cent of H1N1 Isolates in six states analyzed were recovered from patients <26 years old, although specimens were tested from older persons who were il