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Weekly Report: Influenza Summary Update

Week ending February 5, 2005-Week 5

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The following information may be quoted:

Synopsis:

During week 5 (January 30-February 5, 2005)*, influenza activity continued to increase in the United States. Seven hundred eighty-two (22.7%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza viruses. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) is above the national baseline and the proportion of deaths attributed to pneumonia and influenza is below the epidemic threshold. There have been six influenza-associated pediatric deaths reported to CDC this season. Twenty-seven states reported widespread influenza activity, 16 states and New York City reported regional influenza activity, and 4 states and the District of Columbia reported local activity. Two states and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 5, WHO and NREVSS laboratories in the United States reported testing 3,446 specimens for influenza viruses, of which 782 (22.7%) were positive. Of these, 255 were influenza A (H3N2) viruses, 404 were influenza A viruses that were not subtyped, and 123 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 65,404 specimens for influenza viruses and 7,118 (10.9%) were positive. Among the 7,118 influenza viruses, 6,077 (85.4%) were influenza A viruses and 1,041 (14.6%) were influenza B viruses. Nineteen hundred eighty-six (32.7%) of the 6,077 influenza A viruses have been subtyped; 1,980 (99.7%) were influenza A (H3N2) and 6 (0.3%) were influenza A (H1N1) viruses. The largest numbers of specimens testing positive for influenza virus have been reported from the New England (1,189), Mid-Atlantic (1,240), and West North Central (1,127) regions**. The percentage of total isolates that are influenza type A virus is the highest in the New England (96%) and Mid-Atlantic (96%) regions. Influenza A viruses represent a smaller proportion of positive results in the other seven regions, ranging from 88% in the West North Central and East North Central regions to 60% in the Pacific region.

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Antigenic Characterization:

CDC has antigenically characterized 256 influenza viruses collected by U.S. laboratories since October 1, 2004: 2 influenza A (H1) viruses, 174 influenza A (H3N2) viruses, and 80 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. One hundred twenty-five (72%) of 174 influenza A(H3N2) isolates were characterized as antigenically similar to A/Wyoming/3/2003, which is the A/Fujian/411/2002-like (H3N2) component of the 2004-05 influenza vaccine. Forty-nine (28%) influenza A(H3N2) isolates had reduced titers to A/Wyoming/3/2003 and are most closely related to a recent reference strain, A/California/7/2004 (H3N2). Sixty of the influenza B viruses isolated this season belong to the B/Yamagata lineage and were characterized as B/Shanghai/361/2002-like, which is the influenza B component recommended for the 2004-05 influenza vaccine, and 4 showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Sixteen influenza B viruses belong to the B/Victoria lineage.

Influenza B viruses currently circulating can be divided into two antigenically and genetically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. B/Yamagata lineage viruses circulated widely between 1990 and 2001, during which time B/Victoria-like viruses were not identified outside of Asia. However, between March 2001 and October 2003, B/Victoria-like viruses predominated in many countries, including the United States, and the vaccine strains were changed accordingly. While both Victoria lineage and Yamagata lineage viruses have been reported worldwide during the past year, Yamagata lineage viruses have once again become predominant.

Pneumonia and Influenza (P&I) Mortality Surveillance*:

During week 5, 7.8% of all deaths reported by the vital statistics offices of 122 U.S. cities were attributed to pneumonia or influenza. This percentage is below the epidemic threshold of 8.2% for week 5.

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Influenza-Associated Pediatric Mortality*:

During week 5, 2 pediatric deaths were reported from two states (Mississippi and Vermont) bringing the total number of reports to 6. Pediatric deaths have been reported to CDC from 6 states (California, Maine, Massachusetts, Mississippi, Ohio, and Vermont).

Influenza-Associated Pediatric Hospitalizations:

Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks^{?/sup>: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). During October 3, 2004–January 22, 2005, the preliminary influenza-associated hospitalization rate for children 0? years old reported by the NVSN and EIP was 1.8 per 10,000 and 0.42 per 10,000, respectively. The EIP also monitors hospitalizations in children 5-17 years and the preliminary influenza-associated hospitalization rate for this age group was 0.07 per 10,000. The overall hospitalization rate reported by the EIP for children aged 0-17 years was 0.22 per 10,000. In years 2000?003, the end-of-season hospitalization rate for the NVSN ranged from 3.7 to 12 per 10,000 children. The 2003-04 end of season hospitalization rate for the EIP was 7.8 per 10,000 children 0-4 years and 0.8 per 10,000 for children 5-17 years. The difference in rates between the NVSN and EIP could be due to different case finding methods and the different populations monitored. For a summary of the methods used in each system please refer to the surveillance methods in the Flu Activity section of the CDC Influenza website.}

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Influenza-like Illness Surveillance*:

During week 5, 4.6%^†† of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 2.5%. The percentage of visits for ILI ranged from 2.4% in the New England region to 8.8% 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 5, 27 states (Colorado, Connecticut, Delaware, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, North Dakota, Pennsylvania, Rhode Island, Tennessee, Texas, Utah, Vermont, Virginia, Washington, and Wisconsin) reported widespread activity. Sixteen states (Arizona, Arkansas, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Maine, Mississippi, Missouri, Montana, New Hampshire, Oregon, South Dakota, and Wyoming) and New York City reported regional influenza activity. Four states (Ohio, Oklahoma, South Carolina, and West Virginia) and the District of Columbia reported local activity. Alabama, California, and Puerto Rico reported sporadic influenza activity. Alaska did not report.

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Foot notes

Report prepared: February 10, 2005

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