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Centers for Disease Control and Prevention

Influenza Summary Update
(for the week ending April 1, 2000--Week 13)

The following information may be quoted:

Synopsis: During week 13 (March 26 through April 1), 2 % of specimens tested by WHO and NREVSS laboratories for influenza were positive. No state or territorial epidemiologist reported widespread or 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 all 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 7,9 %. This percentage is above the epidemic threshold for week 13.

During the current season, the overall national percentage of respiratory specimens positive for influenza peaked at 33 % during week 51. During the previous 3 years (1996-97, 1997-98, and 1998-99), the peak percentages of respiratory specimens testing positive for influenza viruses ranged from 28 % to 34 %. For this season, the percentage of overall patient visits for influenza-like illness peaked at 6 % during week 52. During the previous 3 years, the peak percentages for such visits ranged between 5 % and 7 %. The proportion of deaths attributed to pneumonia and influenza (P&I) peaked at 11,2 % during week 3. During the previous 3 years, P&I mortality levels 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 13, WHO and NREVSS laboratories tested 403 specimens for influenza viruses and 8 (2 %) were positive. Four were influenza A(H1N1) viruses, 3 were unsubtyped influenza A viruses, and 1 was an influenza B virus. Over the past 3 weeks (weeks 11 through 13) the percentage of specimens testing positive for influenza was below 5 % in all 9 surveillance regions (range 0 % to 4 %).

Since October 3, WHO and NREVSS laboratories have tested a total of 84 433 respiratory specimens for influenza viruses, and 13 527 (16 %) were positive. Of the positive results, 13 472 (99,6 %) were influenza type A and 55 (0,4 %) were influenza type B. Of the 13 472 influenza A viruses, 3 689 (27 %) have been subtyped and 3 591 (97 %) were A(H3N2) and 98 (3 %) were A(H1N1). Of the viruses that have been subtyped during March, influenza A(H1N1) viruses have been identified more frequently than influenza A(H3N2) viruses. Influenza A(H1N1) viruses have been reported by WHO and NREVSS laboratories from all but the New England region.

Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 508 influenza viruses received from U.S. laboratories since October 1. Of the 447 influenza A(H3N2) viruses tested, 431 (96 %) were similar to the vaccine strain A/Sydney/05/97 and 16 (4 %) showed somewhat reduced titers to ferret antisera produced against A/Sydney/05/97. All 17 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 44 influenza A(H1N1) viruses antigenically characterized, 1 was similar to A/Beijing/262/95, the H1N1 component of the current vaccine, 16 were similar to A/Bayern/07/95, and 27 were more closely related to the antigenic variant A/New Caledonia/20/99. A/Bayern/07/95-like viruses are antigenically distinct from the A/Beijing/262/95-like viruses; however, the A/Beijing/262/95 vaccine strain produces high titers of antibody that cross-react with A/Bayern/07/95-like viruses.

Pneumonia and Influenza Mortality*: During week 13, the proportion of deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7,9 %. This percentage is above the epidemic threshold of 7,4 % for week 13 but represents a substantial decrease from the peak of 11,2 % seen in week 3. The percentage of pneumonia and influenza deaths has exceeded threshold values for this time of year for 27 of the past 28 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 13, influenza activity was reported as sporadic in 17 states. Twenty-eight states reported no influenza activity and 5 states did not report. No state reported regional or widespread activity.

 

Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 13, 1 % of patient visits to U.S. sentinel physicians were due to influenza-like illness (ILI). The percentage of ILI was within baseline levels of 0 % to 3 % in all 9 surveillance regions.

*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: April 7, 2000

National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

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