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Update: Influenza Activity --- United States, 2002--03 Season

This report summarizes influenza activity in the United States during September 29, 2002--March 8, 2003,* and updates the previous summary (1). Influenza activity was mild in the United States overall but varied by region. Preliminary data collected through the four components of the CDC influenza surveillance system suggest that national influenza activity peaked during the week ending February 8, 2003 (2).

As of the week ending March 8, the World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 59,731 specimens for influenza viruses, of which 6,433 (10.8%) were positive. The percentage of specimens testing positive for influenza exceeded 10.0% during the week ending January 18 and appears to have peaked at 25.0% during the week ending February 8. During the three most recent influenza seasons (1999--00, 2000--01, and 2001--02), the peak percentage of specimens testing positive for influenza ranged from 23.9% to 30.9% (3; CDC, unpublished data, 2003). Of the 6,433 influenza viruses reported during the 2002--03 season, 2,916 (45.3%) were influenza type A and 3,517 (54.7%) were influenza type B viruses. However, during the weeks ending February 22--March 8, influenza A viruses have been reported more frequently (57.0%) than influenza B viruses. Of the 1,329 influenza type A viruses that have been subtyped, 1,089 (81.9%) are influenza A (H1) viruses and 240 (18.1%) are influenza A (H3N2) viruses. For the season, influenza type A viruses have predominated in the New England, East North Central, Mountain, Pacific, and Mid-Atlantic regions, and influenza B viruses have predominated in the West South Central, South Atlantic, West North Central, and East South Central regions. However, during the weeks ending February 22--March 8, influenza A viruses were reported more frequently (71.1%) in the West South Central region than influenza B viruses.

CDC has characterized antigenically 266 influenza viruses submitted by U.S. laboratories since September 29, 2002: 65 influenza A (H1) viruses, 54 influenza A (H3N2) viruses, and 147 influenza B viruses. Of the 65 influenza A (H1) viruses, 45 (69.2%) had the N1 neuraminidase and 20 (30.8%) had the N2 neuraminidase. The hemagglutinin proteins of all 65 influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99 (H1N1). Of the 54 influenza A (H3N2) isolates that have been characterized, 47 (87.0%) were similar to A/Panama/2007/99, the H3N2 component of the 2002--03 influenza vaccine, and seven (13.0%) showed reduced titers to ferret antisera produced against A/Panama/2007/99. Of the 147 influenza B viruses that have been characterized, 146 (99.3%) belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/01, and one (0.7%) belonged to the B/Yamagata lineage and was similar to B/Sichuan/379/99.

During the weeks ending January 11--March 8, the weekly percentages of patient visits for influenza-like illness (ILI)§ to approximately 750 sentinel providers in 49 states ranged from 1.5% to 3.1% and exceeded the national baseline of 1.9% for 7 consecutive weeks (the weeks ending January 25--March 8). The peak percentage of patient visits for ILI (3.1%) occurred during the weeks ending February 8 and 15. For the week ending March 8, the percentage of patient visits for ILI was 2.0%. During the 1999--2000, 2000--01, and 2001--02 influenza seasons, the peak percentages of patient visits for ILI ranged from 3.2% to 5.6% (3; CDC, unpublished data, 2003).

Since October 20, influenza activity** has been reported by state and territorial epidemiologists as regional in at least one state each week. During the weeks ending December 7--21, widespread influenza activity was reported in Texas. Widespread activity was not reported again until the week ending January 18 and has been reported by two to 13 states each week since then. The greatest number of states reporting regional or widespread activity was 34 during the weeks ending February 8 and February 15. For the week ending March 8, nine states reported widespread influenza activity, and 23 reported regional influenza activity.

During the week ending March 8, of the deaths in the 122 Cities Mortality Reporting System, 8% were attributed to pneumonia and influenza (P&I). This percentage was below the epidemic threshold†† of 8.3% for that week. The percentage of P&I deaths has been below the epidemic threshold each week during September 29--March 8.

Reported by: L Brammer, MPH, A Postema, MPH, S Harper, MD, A Klimov, PhD, N Cox, PhD, WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; P Terebuh, MD, EIS Officer, CDC.

Editorial Note:

Although overall influenza activity has been mild this season, numerous outbreaks have been reported among school children, some leading to school closures, and severe illnesses and deaths associated with influenza have been reported in children. These severe illnesses and deaths were not associated with a single influenza virus type; both influenza A (H1) and influenza B viruses were identified.

CDC contributes to the international surveillance for influenza through the World Health Organization's (WHO) Global Influenza Programme as the WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza. In February, the Hong Kong Department of Health (DOH) confirmed influenza A (H5N1) infection in two patients from a single family of Hong Kong residents who had traveled recently to Fujian Province on the Chinese mainland. The first case occurred in a boy aged 9 years who was hospitalized in Hong Kong and recovered. The second case occurred in the boy's father, who died in a Hong Kong hospital on February 16. Additional family members had respiratory symptoms, and the boy's sister aged 8 years died while the family was in China. The Hong Kong DOH has intensified its influenza surveillance, and no additional human infections with A (H5N1) virus have been identified. No indication exists that the influenza A (H5N1) virus has spread outside Asia. On February 26, CDC issued recommendations to state health departments for enhanced influenza surveillance in the United States. CDC is in communication with WHO about these cases of influenza A (H5N1) and will continue to monitor the situation.

Influenza surveillance reports for the United States are published weekly during October--May and are available at http://www.cdc.gov/ncidod/diseases/flu/weekly.htm or through CDC's voice (telephone, 888-232-3228) and fax (telephone, 888-232-3299, document number 361100) information systems.

Acknowledgments

This report is based on data contributed by participating state and territorial epidemiologists and state public health laboratory directors, WHO collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, U.S. Influenza Sentinel Provider Surveillance System, and Div of Public Health Surveillance and Informatics, Epidemiology Program Office, CDC.

References

  1. CDC. Update: influenza activity---United States, 2002--03 season. MMWR 2002;52:26--8.
  2. CDC. Influenza activity---United States, 1999--2000 season. MMWR 1999;48:1039--42.
  3. CDC. Surveillance for influenza---United States 1997--98, 1998--99, and 1999--2000. In: CDC surveillance summaries (October 25). MMWR 2002;51(No. SS-7).

* As of March 14, 2003. Reporting is incomplete.

Includes both the A (H1N1) and A (H1N2) influenza virus subtypes. § Temperature of >100.0º F (>37.8º C) and either cough or sore throat in the absence of a known cause other than influenza.

The national baseline was calculated as the mean percentage of visits for ILI during noninfluenza weeks plus two standard deviations. Wide variability in regional data precludes calculating region-specific baselines and makes it inappropriate to apply the national baseline to regional data.

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

†† The expected seasonal baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected by using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I during the previous 5 years. The epidemic threshold is 1.654 standard deviations above the seasonal baseline (3).

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