Influenza Activity --- United States, 2000--01 Season

This report summarizes influenza activity in the United States during October 1, 2000--March 10, 2001 (1)*. Influenza activity increased in December and January and peaked at the end of January. The most frequently isolated viruses were influenza A (H1N1); however, influenza B viruses have been co-circulating and appear to be increasing.

During October 1, 2000--March 10, 2001, the World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories tested 64,840 specimens for influenza, and 8386 (13%) were positive. Of these, 4885 (58%) were influenza type A and 3501 (42%) were influenza type B. Of the 4885 influenza A viruses identified, 1826 (37%) were subtyped: 1746 (96%) were A (H1N1) and 80 (4%) were A (H3N2). The percentage of specimens positive for influenza infections, an indicator of influenza activity, peaked at 24% during the week ending January 27, 2001. For the week ending March 10, 6% of tested specimens were positive for influenza (Figure 1).

CDC antigenically characterized 436 influenza viruses received from U.S. laboratories since October 1. Of the 259 influenza A (H1N1) isolates characterized, 246 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000--01 influenza vaccine, and 13 (5%) were similar to A/Bayern/07/95. Although A/Bayern-like viruses are antigenically distinct from A/New Caledonia-like viruses, the A/New Caledonia/20/99 vaccine strain produces high titers of antibody that cross-react with A/Bayern/07/95-like viruses (2). Of the 16 influenza A (H3N2) characterized viruses, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 161 influenza B viruses characterized, 29 (18%) were similar to the vaccine strain B/Beijing/184/93, and 132 (82%) were more closely related antigenically to the B/Sichuan/379/99 reference strain than to the current vaccine strain. The B/Sichuan virus exhibited cross-reactivity with the vaccine strain.

During October 1--March 10, the percentage of patient visits to U.S. sentinel physicians for influenza-like illness (ILI)^† peaked at 4.1% during the week ending January 27. During that week, the percentage of patient visits for ILI was elevated above baseline levels (0--3%) in six of nine surveillance regions. For the week ending March 10, 1.6% of patient visits to U.S. sentinel physicians were the result of ILI.

As reported by state and territorial epidemiologists, influenza activity^§ peaked during the weeks ending February 3 and 10, 2001, when 38 states reported regional or widespread influenza activity. For the week ending March 10, one state reported widespread activity, 12 states reported regional activity, 35 states reported sporadic activity, one state reported no activity, and one state did not report.

For the week ending March 10, the 122 Cities Mortality Reporting System attributed 8.0% of recorded deaths to pneumonia and influenza (P&I). This percentage was below the epidemic threshold^¶ of 8.7% for this week. The percentage of P&I deaths remained below the epidemic threshold each week since October 1.

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. WHO collaborating laboratories. National Respiratory and Enteric Virus Surveillance System laboratories. Sentinel Physicians Influenza Surveillance System. Surveillance Systems Br, Div of Public Health Surveillance and Informatics, Epidemiology Program Office; WHO Collaborating Center for Reference and Research on Influenza, Influenza Br and Respiratory and Enteric Virus Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

Influenza activity during the 2000--01 season was moderate and lower than the previous three seasons. Three surveillance system components (i.e., WHO/NREVSS laboratories, U.S. sentinel physicians, and state and territorial epidemiologists' reports) indicated that activity peaked during late January and early February. The predominant influenza strain circulating this season has been influenza A (H1N1); however, the proportion of influenza B virus isolates has been increasing. During the weeks ending February 24, March 3, and 10, 70% of isolates nationwide were influenza B, and during those weeks influenza B viruses predominated (range: 61%--93%) in eight of nine surveillance regions.

Influenza activity as reported by WHO/NREVSS laboratories and U.S. sentinel physicians peaked during the week ending January 27, when 24% of specimens tested were positive for influenza and 4.1% of visits to U.S. sentinel physicians were the result of ILI. During the previous three seasons, the peak percentage of specimens testing positive for influenza ranged from 28% to 32% and the timing of the peak varied from as early as mid-to-late December during the 1999--2000 season to as late as the middle of February during the 1998--99 season. The peak percentage of patient visits to sentinel physicians for ILI ranged from 4.9% in late December of the 1997--98 season to 5.6% during early February of the 1999--2000 season.

As reported by state and territorial epidemiologists, influenza activity peaked during the weeks ending February 3 and 10, when 38 states reported regional or widespread influenza activity. This peak was lower than those reported during the 1997--98, 1998--99, and 1999--2000 seasons, when 46, 43, and 44 states reported regional or widespread influenza activity, respectively. Similar to the laboratory and sentinel physician data, the peak number of states reporting regional or widespread activity during the 1999--2000 season occurred earlier (mid-January) than this season and either of the previous two seasons.

As reported by the 122 Cities Mortality Reporting System, the percentage of total deaths that resulted from P&I remained below the epidemic threshold each week since October 1. During the previous three seasons, the percentage of deaths attributed to P&I was above epidemic threshold for 10 consecutive weeks each season.

Influenza A (H1N1) viruses, the predominant strain this year, last circulated widely in the United States during the 1995--96 and 1988--89 seasons. Influenza A (H1N1) viruses circulated during 1918--1957, then disappeared for 20 years. The influenza A (H1N1) virus that reappeared in 1977 was antigenically and genetically similar to strains isolated in 1950 and 1951. Since their reappearance in 1977, influenza A (H1N1) viruses have had less impact on persons born during or before the mid-1950s than on those born after that time probably because immunity developed during the 1940s and 1950s (3).

CDC collects and reports U.S. influenza surveillance data during October--May. This information is updated weekly and is available through CDC's voice information system, telephone (888) 232-3228, the fax information system, telephone (888) 232-3299 (request document number 361100), or on the World-Wide Web, http://www.cdc.gov/ncidod/diseases/flu/weekly.htm.

References

1. CDC. Influenza activity---Unites States, 1999--2000 season. MMWR 1999;48:1039--42.
2. CDC. Influenza activity---United States and worldwide, April--October 2000. MMWR 2000;49:1006--8.
3. Noble GR. Epidemiologic and clinical aspects of influenza. In: Beare AS, ed. Basic and applied influenza research. Boca Raton, Florida: CRC Press, 1982:11--50.

* The four components of the influenza surveillance system have been described (1). Data reported as of March 15, 2001.

^† Temperature of >100.0 F (>37.8 C) and either cough or sore throat in the absence of a known cause.

^§ Levels of influenza activity are 1) no activity; 2) sporadic---sporadically occurring ILI or culture-confirmed influenza with no outbreaks detected; 3) regional---outbreaks of ILI or culture-confirmed influenza in counties with a combined population of <50% of the state's population; and 4) widespread---outbreaks of ILI or culture-confirmed influenza in counties with a combined population of >50% of the state's population.

^¶ The epidemic threshold is 1.654 standard deviations above the seasonal baseline. The expected seasonal baseline is projected using a robust regression procedure in which a periodic regression model is applied to observed percentages of deaths from P&I since 1983.

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