Surveillance for influenza--United States, 1994-95, 1995-96, and 1996-97 seasons.

PROBLEM/CONDITION Influenza epidemics occur nearly every year during the winter months and are responsible for substantial morbidity and mortality in the United States, including an average of approximately 114,000 hospitalizations and 20,000 deaths per year. REPORTING PERIOD This report summarizes U.S. influenza surveillance data from October 1994 through May 1997, from both active and passive surveillance systems. DESCRIPTION OF SYSTEM During the period covered, CDC received weekly reports from October through May from a) state and territorial epidemiologists on estimates of local influenza activity, b) approximately 140 sentinel physicians on their total number of patient visits and the number of cases of influenza-like illness (ILI), and c) approximately 70 World Health Organization (WHO) collaborating laboratories in the United States on weekly influenza virus isolations. WHO collaborating laboratories also submitted influenza isolates to CDC for antigenic analysis. Throughout the year, vital statistics offices in 121 cities reported deaths related to pneumonia and influenza (P&I) weekly, providing a measure of the impact of influenza on mortality. RESULTS During the 1994-95 influenza season, 25 state epidemiologists reported regional or widespread activity at the peak of the season. Cases of ILI reported by sentinel physicians exceeded baseline levels for 4 weeks, peaking at 5%. Influenza A(H3N2) was the most frequently isolated influenza virus type/subtype. The longest period of sustained excess mortality was 5 consecutive weeks, when the percentage of deaths attributed to P&I exceeded the epidemic threshold, peaking at 7.6%. During the 1995-96 season, 33 state epidemiologists reported regional or widespread activity at the peak of the season. ILI cases exceeded baseline levels for 5 weeks, peaking at 7%. Influenza A(H1N1) viruses predominated, although influenza A(H3N2) and influenza B viruses also were identified throughout the United States. P&I mortality exceeded the epidemic threshold for 6 consecutive weeks, peaking at 8.2%. The 1996-97 season was the most severe of the three seasons summarized in this report. Thirty-nine state epidemiologists reported regional or widespread activity at the peak of the season. ILI reports exceeded baseline levels for 5 consecutive weeks, peaking at 7%. The proportion of respiratory specimens positive for influenza peaked at 34%, with influenza A(H3N2) viruses predominating. Influenza B viruses were identified throughout the United States, but only one influenza A(H1N1) virus isolate was reported overall. The proportion of deaths attributed to P&I exceeded the epidemic threshold for 10 consecutive weeks, peaking at 9.1%. INTERPRETATION Influenza A(H1N1), A(H3N2), and B viruses circulated during 1994-1997. Local surveillance data are important because of geographic and temporal differences in the circulation of influenza types/subtypes. PUBLIC HEALTH ACTIONS CDC conducts active national surveillance annually from October through May for influenza to detect the emergence and spread of influenza virus variants and monitor the impact of influenza-related morbidity and mortality. Surveillance data are provided weekly throughout the influenza season to public health officials, WHO, and health-care providers and can be used to guide prevention and control activities, vaccine strain selection, and patient care.