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Weekly Report: Influenza Summary Update
Week ending November 22, 2003-Week 47
The following information may be quoted:
Influenza activity in the United States continued to increase during week 47 (November 16 - 22, 2003). One thousand seventy-six (34.3%) of 3,138 specimens collected from throughout the United States and tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 3.8%, which is above the national baseline of 2.5%. The proportion of deaths attributed to pneumonia and influenza was 6.4%, which is below the epidemic threshold for the week. Ten state health departments reported widespread influenza activity, 9 states reported regional activity, 8 states reported local influenza activity, 16 states, New York City, Guam, and Puerto Rico reported sporadic influenza activity, and 4 states and the District of Columbia reported no influenza activity.
During week 47, WHO and NREVSS laboratories reported 3,138 specimens tested for influenza viruses, and 1,076 (34.3%) were positive. Of these, 136 were influenza A(H3N2) viruses, 934 were influenza A viruses that were not subtyped, and 6 were influenza B viruses. The percentage of specimens testing positive for influenza first exceeded 10% (an indicator of increased influenza activity) during the week ending October 25, earlier than any other year for which CDC has this information (1976-77 season to present).
Since September 28, WHO and NREVSS laboratories have tested a total of 14,933 specimens for influenza viruses and 3,337 (22.3%) were positive. Among the 3,337 influenza viruses, 3,326 (99.7%) were influenza A viruses and 11 (0.3%) were influenza B viruses. Seven hundred fifty-one (23%) of the 3,326 influenza A viruses have been subtyped; 741 (98.7%) were influenza A (H3N2) viruses and 10 (1.3%) were A (H1) viruses. Forty states and all 9 surveillance regions** have reported laboratory-confirmed influenza this season. One thousand eight hundred thirty-three (54.9%) of the 3,337 isolates were reported from the West South Central region, and 1,067 (32.0%) were from the Mountain region.
CDC has antigenically characterized 108 influenza A (H3N2) viruses collected by U.S. laboratories since October 1 and found that 19 (18%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 89 (82%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). The A/Fujian strain predominated in Australia and New Zealand during the recent Southern Hemisphere influenza season and is a drift variant related to the vaccine strain, A/Panama/2007/99. Antibodies produced against the vaccine virus cross-react with A/Fujian/411/2002-like viruses, but at a lower level than against A/Panama/2007/99 (H3N2). Vaccine effectiveness depends, in part, on the match between vaccine strains and circulating viruses and cannot be determined by laboratory testing. Although vaccine effectiveness against A/Fujian/411/2002-like viruses may be less than that against A/Panama/2007/99-like viruses, it is expected that the current U.S. vaccine will offer some cross-protective immunity against the A/Fujian/411/2002-like viruses and reduce the severity of disease.
Pneumonia and Influenza (P&I) Mortality Surveillance:
During week 47, 6.4% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia and influenza. This percentage is below the epidemic threshold of 7.4% for week 47.
Influenza-like Illness Surveillance*:
During week 47, 3.8%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 2.5%. On a regional level**, the percentage of visits for ILI was highest in the West South Central region (11.9%, which is lower than the 14.7% of visits during the previous week). The percentage of patient visits for ILI increased in all other regions; the Mountain (4.9%) and Pacific (4.4%) regions reported the highest percentages of patient visits due to ILI after the West South Central region. All other regions were below 4%. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.
Influenza Activity as Assessed by State and Territorial Epidemiologists**:
Influenza activity was reported as widespread in 10 states (Arkansas, Colorado, Idaho, Nevada, New Mexico, Pennsylvania, Tennessee, Texas, Utah, and Washington), regional in 9 states (Arizona, California, Florida, Mississippi, Montana, Nebraska, North Carolina, Oklahoma, and Wyoming), and local in 8 states (Alaska, Georgia, Indiana, Kansas, Maryland, North Dakota, Rhode Island, and Virginia). Sporadic influenza activity was reported in 16 states (Alabama, Connecticut, Hawaii, Illinois, Iowa, Kentucky, Maine, Michigan, Minnesota, New York, Ohio, Oregon, South Carolina, South Dakota, West Virginia, and Wisconsin), New York City, Guam, and Puerto Rico. Four states and the District of Columbia reported no influenza activity, and 3 states did not report.
* Reporting is incomplete for this week. Numbers may change as more reports are received.
** Surveillance Regions: New England (Connecticut, Maine, Massachusetts, New Hampshire, Vermont, Rhode Island); Mid-Atlantic (New Jersey, New York City, Pennsylvania, Upstate New York); East North Central (Illinois, Indiana, Michigan, Ohio, Wisconsin); West North Central (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota); South Atlantic (Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, Washington, D.C., West Virginia); East South Central (Alabama, Kentucky, Mississippi, Tennessee); West South Central (Arkansas, Louisiana, Oklahoma, Texas); Mountain (Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming); Pacific (Alaska, California, Hawaii, Oregon, Washington)
*** The national and regional percentage of patient visits for ILI is weighted on the basis of state population.
Report prepared November 28, 2003