Update: Influenza Activity --- United States, December 7--13, 2003

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Influenza activity in the United States continued to increase during December 7--13, 2003*. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 7.4%, which is above the national baseline^§ of 2.5%. Thirty-six state health departments reported widespread influenza activity, 12 states and New York City reported regional influenza activity, one state and the District of Columbia reported local influenza activity, and one state and Puerto Rico reported sporadic influenza activity^¶ (Figure 1).

Laboratory Surveillance

During the reporting week of December 7--13, World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories reported testing 3,814 specimens for influenza viruses; 1,365 (35.8%) were positive. Of these, 262 were influenza A (H3N2) viruses, 1,080 were influenza A viruses that were not subtyped, and 23 were influenza B viruses (Figure 2).

Since September 28, WHO and NREVSS laboratories have tested 32,854 specimens for influenza viruses; 9,464 (28.8%) were positive. Of these, 9,395 (99.3%) were influenza A viruses, and 69 (0.7%) were influenza B viruses. Of the 9,395 influenza A viruses, 2,113 (22.5%) have been subtyped; 2,112 (>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 269 influenza viruses collected by U.S. laboratories since October 1 and characterized antigenically by CDC, 265 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 265 influenza A (H3N2) isolates that have been characterized, 62 (23%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 203 (77%) 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 13, P&I accounted for 7.2% of all deaths reported through the 122 Cities Mortality Reporting System. The epidemic threshold for that week was 7.7%.

ILI Surveillance

During the reporting week of December 7--13, the weekly percentage of patient visits^§§ to approximately 1,000 U.S. sentinel providers nationwide for ILI increased from 5.3% to 7.4%, which is above the national baseline of 2.5% (Figure 3). The percentage of patient visits for ILI increased in eight of the nine surveillance regions^¶¶ but has continued to decline in the West South Central region (8.0% for week 50 compared with 11.3% during week 47). On a regional level, the percentage of visits for ILI was highest in the Pacific region (10.6%), followed by the West South Central region (8.0%), West North Central (7.9%), Mountain (7.7%), East North Central (7.6%), South Atlantic (6.9%), East South Central (6.3%), and the New England and Mid-Atlantic regions (4.7%).

Activity Reported by State and Territorial Epidemiologists

During the week ending December 13, influenza activity was reported as widespread in 36 states (Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Massachusetts, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, New York, North Carolina, North Dakota, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, and Wyoming), regional in 12 states (Alabama, Alaska, Florida, Louisiana, Maine, Maryland, Michigan, Minnesota, New Jersey, Ohio, South Dakota, and Wisconsin) and New York City. Hawaii and the District of Columbia reported local activity, and New Hampshire and Puerto Rico reported sporadic 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.

Temperature of >100.0^º F (>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.

^¶ 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, 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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