Update: Influenza Activity --- United States, December 14--20, 2003

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Influenza activity in the United States continued to increase during December 14--20, 2003*. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 7.7%, which is above the national baseline^§of 2.5%. Influenza activity^¶was reported as widespread by health departments in 45 states, New York City, and the District of Columbia; four states reported regional influenza activity; and one state reported local influenza activity (Figure 1) .

Laboratory Surveillance

During the reporting week of December 14--20, World Health Organization (WHO) laboratories reported testing 3,693 specimens for influenza viruses; of which 1,297 (35.1%) were positive. Of these, 323 were influenza A (H3N2) viruses, 964 were influenza A viruses that were not subtyped, and 10 were influenza B viruses (Figure 2). Because data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories for the week ending December 20 were not available at the time of this report, numbers might change substantially next week.

Since September 28, WHO and NREVSS laboratories have tested 40,075 specimens for influenza viruses; of which 11,982 (29.9%) were positive. Of these, 11,902 (99.3%) were influenza A viruses, and 80 (0.7%) were influenza B viruses. Of the 11,902 influenza A viruses, 2,934 (24.7%) have been subtyped; 2,933 (99.9%) were influenza A (H3N2) viruses, and one (0.1%) was an influenza A (H1) virus. All 50 states have reported laboratory-confirmed influenza this season.

Antigenic Characterization

Of the 330 influenza viruses collected by U.S. laboratories since October 1 and characterized antigenically by CDC, 326 were influenza A (H3N2) viruses, two were influenza A (H1) viruses, and two 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 326 influenza A (H3N2) isolates that have been characterized, 80 (25.0%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 246 (75.0%) were similar to a drift variant, A/Fujian/411/2002 (H3N2)**. Both influenza B viruses characterized were similar antigenically to B/Sichuan/379/99.

Pneumonia and Influenza (P&I) Mortality Surveillance

As of the week ending December 20, P&I accounted for 7.8% of all deaths reported through the 122 Cities Mortality Reporting System. The epidemic threshold for that week was 7.8%.

ILI Surveillance

During the reporting week of December 14--20, the weekly percentage of patient visits^§§ to approximately 1,000 U.S. sentinel providers nationwide for ILI increased to 7.7%, which is above the national baseline of 2.5% (Figure 3). The percentage of patient visits for ILI increased in five of the nine surveillance regions but has continued to decline in the West South Central region (6.8% for week 51 compared with 11.3% for week 47). On a regional level^¶¶, the percentage of visits for ILI was highest in the East North Central region (9.6%), followed by Pacific region (9.0%), West North Central region (8.8%), South Atlantic region (7.9%), East South Central region (7.8%), West South Central region (6.8%), New England region (6.3%), Mid-Atlantic region (5.8%), and the Mountain region (4.4%).

Activity Reported by State and Territorial Epidemiologists

During the week ending December 20, influenza activity was reported as widespread in 45 states (Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, and Wyoming), New York City, and the District of Columbia. Regional activity was reported in four states (Florida, Louisiana, New Hampshire, and Oklahoma), and Hawaii reported local activity.

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.

^§ 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---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.

** 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 will offer some cross-protective immunity against the A/Fujian/411/2002-like viruses and reduce 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 previous 5 years; the epidemic threshold is 1.645 standard deviations above the seasonal baseline percentage.

^§§ National and regional percentage 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, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, Washington, D.C., 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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