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Weekly Report: Influenza Summary Update
Week ending April 8, 2006-Week 14
Error processing SSI fileSynopsis:
During week 14 (April 2 ?April 8, 2006)*, influenza activity continued to decrease in the United States. Three hundred specimens (15.7%) 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) was below the national baseline. The proportion of deaths attributed to pneumonia and influenza was below the baseline level. Ten states reported widespread influenza activity; 10 states reported regional influenza activity; 15 states, New York City, and the District of Columbia reported local influenza activity; 14 states and Puerto Rico reported sporadic influenza activity; and one state reported no activity.
Laboratory Surveillance*:
During week 14, WHO and NREVSS laboratories reported 1,908 specimens tested for influenza viruses and 300 (15.7%) were positive. Of these, 24 were influenza A (H3N2) viruses, 36 were influenza A (H1N1) viruses, 77 were influenza A viruses that were not subtyped, and 163 were influenza B viruses.
Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 119,202 specimens for influenza viruses and 15,113 (12.7%) were positive. Among the 15,113 influenza viruses, 12,910 (85.4%) were influenza A viruses and 2,203 (14.6%) were influenza B viruses. Five thousand two hundred thirty-four (40.5%) of the 12,910 influenza A viruses have been subtyped: 4,935 (94.3%) were influenza A (H3N2) viruses and 299 (5.7%) were influenza A (H1N1) viruses. During the past 3 weeks (weeks 12?4), the percentage of specimens testing positive for influenza has ranged from 26.4% in the South Atlantic region to 12.5% in the Mid-Atlantic region**. During this period, the proportion of isolates identified as influenza type B virus continued to increase. During weeks 12-14, regions reporting the highest proportion of isolates as influenza B included the Pacific (62.5%), Mountain (61.8%), and West North Central (61.2%) regions. Other regions reporting more than 30.0% of recent isolates as influenza B include the East North Central, West South Central, and New England regions.
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Composition of the 2006-07 Influenza Vaccine:
WHO has recommended that the 2006-07 trivalent influenza vaccine for the Northern Hemisphere contain A/New Caledonia/20/99-like (H1N1), A/Wisconsin/67/2005-like (H3N2), and B/Malaysia/2506/2004-like viruses. The influenza A (H3N2) and the influenza B components have been changed from the 2005-06 season vaccine components. A/Wisconsin/67/2005 is an antigenic variant of the current vaccine strain A/California/07/2004. Influenza B viruses currently circulating can be divided into two antigenically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. The updating of the influenza B component to B/Ohio/1/2005 (which is antigenically equivalent to B/Malaysia/2506/2004) represents a change to the B/Victoria lineage. This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and post-vaccination serologic studies in humans.
Antigenic Characterization:
CDC has antigenically characterized 575 influenza viruses [453 influenza A (H3N2), 42 influenza A (H1), and 80 influenza B viruses] collected by U.S. laboratories since October 1, 2005. Of the 453 influenza A (H3N2) viruses, 352 (77.7%) were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 101 (22.3%) viruses showed reduced titers with antisera produced against A/California/07/2004. Of the 101 low-reacting viruses, 75 were tested with antisera produced against A/Wisconsin/67/2005 (the H3N2 component selected for the 2006-07 vaccine), and 59 are A/Wisconsin-like. The hemagglutinin proteins of 39 (92.9%) influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99, and 3 (7.1%) showed reduced titers with antisera produced against A/New Caledonia/20/99. Twenty-nine (36.3%) of the influenza B viruses that have been characterized belong to the B/Yamagata lineage. Five were similar to B/Shanghai/361/2002, the recommended influenza B component for the 2005-06 influenza vaccine, and 24 were characterized as B/Florida/07/2004-like. B/Florida/07/2004 is a minor antigenic variant of B/Shanghai/361/2002. Fifty-one (63.8%) influenza B viruses were identified as belonging to the B/Victoria lineage and all were similar to B/Ohio/1/2005, the influenza B component selected for the 2006-07 vaccine.
Pneumonia and Influenza (P&I) Mortality Surveillance*:
During week 14, 7.4% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia or influenza. This percentage is below the epidemic threshold of 8.0% for week 14.
Influenza-Associated Pediatric Mortality*:
During October 2, 2005 ?April 8, 2006, CDC has received reports of 22 influenza-associated pediatric deaths, 20 of which occurred during the current influenza season.
Influenza-Associated Pediatric Hospitalizations*:
Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks? Emerging Infections Program (EIP) and New Vaccine Surveillance Network (NVSN). During October 1, 2005 ?April 1, 2006, the preliminary influenza-associated hospitalization rate reported by EIP for children aged 0-17 years was 0.98 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 2.31 per 10,000 and 0.31 per 10,000, respectively. During October 30, 2005 ?April 1, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years in NVSN was 4.0 per 10,000. EIP and NVSN hospitalization rate estimates are preliminary and may change as data continue to be collected.
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Influenza-like Illness Surveillance*:
During week 14, 1.9%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is below the national baseline of 2.2%. The percentage of visits for ILI ranged from 1.2% in the West North Central region to 4.6% in the West South Central region. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.
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Influenza Activity as Assessed by State and Territorial Epidemiologists*:
During week 14, 10 states (Connecticut, Delaware, Indiana, Kentucky, Maine, New York, Rhode Island, Vermont, Virginia, and Wisconsin) reported widespread influenza activity. Ten states (Hawaii, Massachusetts, Minnesota, Montana, North Dakota, Ohio, Pennsylvania, South Carolina, South Dakota, and West Virginia) reported regional influenza activity. Fifteen states (California, Colorado, Florida, Georgia, Illinois, Iowa, Maryland, Michigan, Nebraska, Nevada, New Hampshire, New Jersey, North Carolina, Tennessee, and Washington), New York City, and the District of Columbia reported local influenza activity. Fourteen states (Alaska, Arizona, Arkansas, Idaho, Kansas, Louisiana, Mississippi, Missouri, New Mexico, Oklahoma, Oregon, Texas, Utah, and Wyoming) and Puerto Rico reported sporadic influenza activity. Alabama reported no activity.
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Report prepared April 14, 2006
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