Update: Influenza Activity --- United States, January 4--10, 2004

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The number of states reporting widespread influenza activity* continued to decrease during the reporting week of January 4--10, 2004. Health departments in 20 states and New York City reported widespread influenza activity. A total of 24 states reported regional activity, three states reported local activity, and sporadic activity was reported by two states, the District of Columbia, Guam, and Puerto Rico (Figure 1). The percentage of outpatient visits for influenza-like illness (ILI)^§ continued to decrease in all surveillance regions during the week ending January 10, with an overall national percentage of 2.8%. This percentage is above the national baseline^¶of 2.5%. The percentage of specimens testing positive for influenza also decreased; however, the percentage of deaths attributed to pneumonia and influenza (P&I) continued to increase.

Laboratory Surveillance

During the week ending January 10, World Health Organization (WHO) laboratories reported testing 2,670 specimens for influenza viruses, of which 319 (11.9%) were positive. Of these, 52 were influenza A (H3N2) viruses, 261 were influenza A viruses that were not subtyped, and six were influenza B viruses.

Since September 28, 2003, WHO and National Respiratory and Enteric Virus Surveillance System laboratories have tested 69,052 specimens for influenza viruses, of which 18,535 (26.8%) were positive. Of these, 18,422 (99.4%) were influenza A viruses, and 113 (0.6%) were influenza B viruses. Of the 18,422 influenza A viruses, 4,418 (24.0%) have been subtyped; 4,417 (99.9%) were influenza A (H3N2) viruses, and one (0.1%) was an influenza A (H1) virus.

Antigenic Characterization

Of the 518 influenza viruses collected by U.S. laboratories since October 1, 2003, and characterized antigenically by CDC, 511 were influenza A (H3N2) viruses, two were influenza A (H1) viruses, and five were influenza B viruses. The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. Of the 511 influenza A (H3N2) isolates that have been characterized, 98 (19.2%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 413 (80.8%) were similar to a drift variant, A/Fujian/411/2002 (H3N2)**. Four influenza B viruses characterized were similar antigenically to B/Sichuan/379/99, and one was similar antigenically to B/Hong Kong/330/2001.

P&I Mortality Surveillance

During the week ending January 10, P&I accounted for 10.2% of all deaths reported through the 122 Cities Mortality Reporting System. This percentage is again above the epidemic threshold of 8.1% for that reporting week (Figure 2).

ILI Surveillance

The percentage of patient visits^§§ to approximately 1,000 U.S. sentinel providers nationwide for ILI decreased from 5.5% for the week ending January 3 to 2.8% for the week ending January 10, but remained above the national baseline of 2.5% (Figure 3). The percentage of patient visits for ILI continued to decrease in all nine surveillance regions^¶¶ during the week ending January 10. Visits for ILI ranged from 3.4% in the Pacific region to 1.9% in the Mountain and West North Central regions.

Activity Reported by State and Territorial Epidemiologists

During the week ending January 10, influenza activity was reported as widespread in 20 states (California, Connecticut, Delaware, Georgia, Hawaii, Indiana, Iowa, Maryland, Minnesota, Mississippi, Montana, New York, North Carolina, Pennsylvania, South Carolina, Texas, Vermont, Virginia, West Virginia, and Wisconsin) and New York City. Regional activity was reported in 24 states (Alabama, Alaska, Arizona, Colorado, Florida, Idaho, Illinois, Kentucky, Louisiana, Massachusetts, Michigan, Missouri, Nebraska, New Hampshire, New Jersey, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Tennessee, Utah, Washington, and Wyoming). Local activity was reported in three states (Kansas, Nevada, and Oklahoma). Sporadic activity was reported in two states (Arkansas and New Mexico), the District of Columbia, Guam, and Puerto Rico. Maine did not report.

Weekly updates on influenza activity will be published in MMWR during the influenza season. Additional information about influenza activity is available from CDC at http://www.cdc.gov/flu.

^*Levels of activity are 1) no activity, 2) sporadic---small numbers of laboratory-confirmed influenza cases or a single influenza outbreak reported but no increase in cases of ILI, 3) local---outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of a state, 4) regional---outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least two but less than half the regions of a state, and 5) widespread---outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of a state.

^§Temperature of >100.0^ºF (>37.8^º C) and cough and/or sore throat in the absence of a known cause other than influenza.

^¶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.

** Although vaccine effectiveness against A/Fujian/411/2002-like viruses might be less than that against A/Panama/2007/99-like viruses, the current U.S. vaccine probably offers some cross-protective immunity against the A/Fujian/411/2002-like viruses and reduces the severity of disease.

The expected baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected by using a robust regression procedure that applies a periodic regression model to the observed percentage of deaths from P&I during the preceding 5 years; the epidemic threshold is 1.645 standard deviations above the seasonal baseline percentage.

^§§National and regional percentages of patient visits for ILI are weighted on the basis of state population.

^¶¶New England=Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont; Mid-Atlantic=New Jersey, New York City, Pennsylvania, and Upstate New York; East North Central=Illinois, Indiana, Michigan, Ohio, and Wisconsin; West North Central=Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota; South Atlantic=Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, and West Virginia; East South Central=Alabama, Kentucky, Mississippi, and Tennessee; West South Central=Arkansas, Louisiana, Oklahoma, and Texas; Mountain=Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming; and Pacific=Alaska, California, Hawaii, Oregon, and Washington.

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