Centers for Disease Control and Prevention

Influenza Summary Update
(for the week ending January 22, 2000--Week 3)

The following information may be quoted:

Synopsis: During week 3 (January 16 through January 22), 21% of specimens tested by WHO and NREVSS laboratories for influenza were positive. State and territorial epidemiologists from 28 states reported widespread influenza activity, and 13 from other states reported regional influenza activity. The proportion of patient visits to sentinel physicians for influenza-like illness was within baseline levels of 0% to 3% in the United States overall and in 5 of 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 11.0%. This percentage is above the epidemic threshold for week 3 and is unusually high.

During the current season, the overall national percentage of respiratory specimens positive for influenza appears to have peaked at 34% during week 51. During the past 3 years, the peak percentages of respiratory specimens testing positive for influenza viruses have ranged from 28% to 34%. For this season, the percentage of patient visits for influenza-like illness appears to have peaked at 6% during week 52. During the past 3 years, the peak percentages for such visits have ranged between 5% and 7%. So far, the proportion of deaths attributed to pneumonia and influenza (P&I) has not begun to decline. During the previous 3 years, P&I mortality levels have peaked between 8.8% and 9.1%. The current season's P&I figures must be interpreted with caution because important changes have taken place in this year's case definition that may be contributing to higher estimates of P&I mortality than in previous years.

U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratory Reports*: During week 3, WHO and NREVSS laboratories tested 1,941 specimens for influenza viruses and 410 (21%) were positive. One hundred were influenza A(H3N2) viruses, 1 was an influenza A(H1N1) virus, 304 were unsubtyped influenza A viruses, and 5 were influenza type B. In the South Atlantic region, 34% of specimens tested over the past 3 weeks (weeks 1 through 3) were positive for influenza. In the 8 other regions, the percentage of specimens testing positive for influenza ranged from 15% to 27% during the past 3 weeks.

Since October 3, WHO and NREVSS laboratories have tested a total of 46,669 respiratory specimens for influenza viruses, and 8,735 (19%) were positive. Of the positive results, 8,706 (99.7%) were influenza type A and 29 (0.3%) were influenza type B. Of the 8,706 influenza A viruses, 2,083 (24%) have been subtyped and 2,076 (99.7%) were A(H3N2) and 7 (0.3%) were A(H1N1).

Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 246 influenza viruses received from U.S. laboratories since October 1. Of the 239 influenza A(H3N2) viruses tested, 214 (90%) were similar to the vaccine strain A/Sydney/05/97 and 25 (10%) showed somewhat reduced titers to ferret antisera produced against A/Sydney/05/97. All 4 of the influenza B viruses antigenically characterized were similar to B/Beijing/184/93, which is represented in the current vaccine by B/Yamanashi/166/98. Of the 3 influenza A(H1N1) viruses antigenically characterized, 2 were similar to A/Beijing/262/95, the H1N1 component of the current vaccine, while 1 was more closely related to the antigenic variant A/New Caledonia/20/99.

Pneumonia and Influenza Mortality*: During week 3, the proportion of deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 11.0%. This percentage is above the epidemic threshold of 7.4% for week 3 and is unusually high. The percentage of pneumonia and influenza deaths has exceeded threshold values for this time of year for 17 of the past 18 weeks. Whether this increase in the percentage of pneumonia and influenza deaths is due to influenza activity, respiratory illness due to some other pathogen, or reporting changes in the 122 Cities Mortality Reporting System is unknown. However, because these changes include a revision of the reporting case definition, the current increase in pneumonia and influenza mortality should be interpreted with caution.

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Influenza Activity as Assessed by State and Territorial Epidemiologists**: During week 3, influenza activity was reported as widespread in 27 states (Alabama, Arizona, Arkansas, Colorado, Delaware, Georgia, Idaho, Indiana, Iowa, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, Tennessee, Texas, Utah, Vermont, Virginia, and Wisconsin). Fourteen states (Connecticut, Florida, Hawaii, Illinois, Kansas, Kentucky, Maine, Maryland, Mississippi, Montana, Oregon, South Dakota, Washington, and West Virginia) reported regional influenza activity. Seven states reported sporadic influenza activity and 2 states did not report.

Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 3, 3% of patient visits to U.S. sentinel physicians were due to influenza-like illness (ILI). The percentage of ILI was above the baseline levels of 0% to 3% in 4 of the 9 surveillance regions (East South Central, Pacific, South Atlantic, and West South Central) and ranged from 4% to 10%.

*Reporting is incomplete for this week, so numbers and percentages may change as more reports are received.

**Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.

Report prepared: January 28, 2000

National Center for Infectious Diseases

Centers for Disease Control and Prevention
Atlanta, GA

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