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Current Trends Late Season Influenza Type B Virus Activity -- United States

Despite a decline in levels of overall influenza activity, reports of influenza B virus activity increased during April and May. Influenza B outbreaks in April or May have been reported in California, New Mexico, North Dakota, Oregon, and Wisconsin, including two nursing home outbreaks in Napa and Solano counties, California. These, and two earlier nursing home outbreaks reported from Maine (1) and New York, bring to four the number of influenza B outbreaks in nursing homes reported this season. Other outbreaks in the above states occurred predominately among children.

One outbreak affecting a younger population occurred on the Fort Totten reservation in North Dakota beginning mid-April and continuing for 2 weeks. Absentee rates in the reservation school (grades K-6) peaked at 42%, compared with a normal absentee rate of 10%. All 16 teachers and the school principal also reported influenza-like illness during the outbreak. Records from the reservation clinic showed that patient visits had increased 49% during the outbreak and that almost all the increase was associated with influenza-like illness. During the outbreak, 52% of clinic visits were from patients 9 years of age or younger, and 40% of these involved influenza-like illness. Influenza type B virus was isolated from specimens collected from children with typical influenza during the outbreak.

Since April, 14 states have identified their first influenza type B isolates of the season; 13 states had identified type B isolates before April. Reported by W Freeman, MD, Ft. Totten Reservation, J Pearson, DrPH, Acting State Epidemiologist, North Dakota State Dept of Health; Respective state epidemiologists and laboratory directors; Div of Surveillance and Epidemiologic Studies, Epidemiology Program Office, Statistical Svcs Activity, WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC, D Streitz, North Dakota State Dept. of Health.

Editorial Note

Editorial Note: The report of late influenza B activity, including several outbreaks, emphasizes the importance of maintaining surveillance programs that include laboratory diagnosis for respiratory virus outbreaks outside the fall and winter months. For example, confirmation of occasional outbreaks throughout the summer months would provide one indicator that the implicated virus strain is likely to have a considerable impact in the next winter season. "Early warning" has occurred in 2 recent years--with isolation of influenza B/Singapore/222/79-like strains during summer 1979 and influenza A/Bangkok/1/79-like strains during summer 1980. Presently available information, however, does not permit any reliable prediction that influenza type B, rather than type A, will be responsible for most influenza infections in the United States next winter. As in other years, monitoring of influenza activity during the next few months in countries with tropical climates (where influenza tends to be endemic) and in the Southern Hemisphere, will also be important in improving knowledge of the virus strain likely to appear in the United States next winter.


  1. CDC. Update: influenza activity--United States, MMWR 1983;32:136-41.

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