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Weekly Report: Influenza Summary Update

Week ending October 9, 2004-Week 40

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The Centers for Disease Control and Prevention (CDC) collects surveillance data year-round and reports on U.S. influenza activity each week from October through May. The U.S. influenza surveillance system now consists of seven separate components: laboratory-based viral surveillance, sentinel physician surveillance for influenza-like illness (ILI), mortality surveillance as reported through the 122 Cities Mortality Reporting System, state and territorial epidemiologist reports of influenza activity, influenza-associated pediatric mortality as reported through the Nationally Notifiable Disease Surveillance System, and influenza-associated pediatric hospitalizations as reported through the Emerging Infections Programs in 9 sites and the New Vaccine Surveillance Network in 3 sites. Both influenza-associated pediatric mortality and influenza-associated pediatric hospitalizations are new surveillance components initiated to estimate the impact of influenza on children.

These surveillance components enable CDC to determine when and where influenza activity is occurring, determine what types of influenza viruses are circulating, detect changes in the influenza viruses collected and analyzed, track patterns of influenza-related illness, and measure the impact of influenza in the United States. All influenza activity reporting by states, laboratories, and health-care providers is voluntary.

Synopsis:

During week 40 (October 3 – October 9, 2004)*, influenza activity occurred at a low level in the United States. Five (1.0%) specimens 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) and the proportion of deaths attributed to pneumonia and influenza were below baseline levels. Seven states reported sporadic influenza activity, and 40 states, the District of Columbia, and New York City reported no influenza activity.

Laboratory Surveillance*:

About 70 WHO and 50 NREVSS collaborating laboratories located throughout the United States report the total number of respiratory specimens tested and the number positive for influenza types A and B each week. Some laboratories also report the subtype (H1N1 or H3N2) of the influenza A viruses they have isolated and the ages of the persons from whom the specimens were collected. Some of the influenza viruses collected by laboratories are sent to CDC for more testing.

From week 21 through week 39 (weeks ending May 29 – October 2) WHO and NREVSS laboratories tested 11,770 specimens for influenza and 50 (0.4%) were positive. Isolates were reported from all surveillance regions** during the summer except the East South Central region.

During week 40, WHO and NREVSS laboratories reported 518 specimens tested for influenza viruses, and 5 were positive. Of the 5 influenza viruses identified, 3 were influenza A (H3N2) viruses and 2 were influenza B viruses. Influenza A (H3N2) viruses were identified in the East North Central and Mountain regions, and influenza B viruses were identified in the South Atlantic region.

During May – September, 5 influenza A (H3N2) viruses and 31 influenza B viruses were collected and antigenically characterized by CDC. All five influenza A (H3N2) viruses were characterized as A/Fujian/411/02-like.

Influenza B viruses circulating currently can be divided into two antigenically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. B/Yamagata-like viruses have circulated widely since 1990. B/Victoria-like viruses had not been identified outside of Asia between 1991 and 2001. However, since March 2001, B/Victoria-like viruses have been identified in many countries, including the United States. Of the 31 influenza B viruses identified during May – September, 10 belonged to the B/Victoria lineage and were collected in the Pacific region. Two of these viruses were similar antigenically to B/Hong Kong/330/2001 and 8 were similar antigenically to B/Brisbane/32/2002. The other 21 influenza B viruses belonged to the B/Yamagata/16/88 lineage; all were B/Shanghai/361/2002 -like, which is the influenza B component recommended for the 2004-05 influenza vaccine.

Pneumonia and Influenza (P&I) Mortality Surveillance:

Each week, the vital statistics offices of 122 cities report the total number of death certificates filed and the number of those for which pneumonia or influenza was listed as the underlying or a contributing cause of death. The percentage of all deaths due to pneumonia and influenza are compared with baseline and epidemic threshold values calculated for each week.

During week 40, 5.8% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 6.7% for week 40.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality:

Effective October 1, 2004, CDC has added influenza-associated pediatric (i.e., persons younger than 18 years of age) mortality to the list of nationally notifiable diseases voluntarily reportable through the National Notifiable Diseases Surveillance System (NNDSS). This action is based on recommendations developed collaboratively by the Council of State and Territorial Epidemiologists (CSTE) and CDC, and approved at the 2004 CSTE annual meeting. The recommended methods for surveillance are described in the 2004 CSTE position statement for influenza-associated pediatric mortality (www.cste.org). As of week 40, there are no reported cases of influenza-associated pediatric mortality.

Influenza-Associated Pediatric Hospitalizations:

The New Vaccine Surveillance Network (NVSN) provides estimates of hospitalization rates for children less than 5 years old in 3 sites. NVSN estimated rates of hospitalization for influenza will be reported every 2 weeks, beginning October 29, 2004.

In addition, Emerging Infections Programs (EIP) in 9 sites are collecting data on laboratory- confirmed, influenza-associated hospitalizations among children less than 18 years of age. Hospitalization data from the EIP sites will be included as they become available.

Influenza-like Illness Surveillance*:

Each week, approximately 1,000 healthcare providers around the country report the total number of patients seen and the number of those patients with influenza-like illness (ILI) by age group. For this system, ILI is defined as fever (temperature of >100°F) plus either a cough or a sore throat, in the absence of a known cause other than influenza.

The percentage of patient visits to sentinel providers for ILI reported each week is weighted on the basis of state population. This percentage is compared each week with the national baseline of 2.5%. The baseline is the mean percentage of visits for ILI during non-influenza weeks plus 2 standard deviations.

During week 40, 0.9%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline of 2.5%. On a regional level**, the percentage of visits for ILI ranged from 0.3% to 2.1%. 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 sporadic in 7 states (California, Florida, New York, Pennsylvania, Michigan, Texas, and Utah). Forty states, the District of Columbia, and New York City reported no influenza activity, and 3 states did not report.

U. S. map for Weekly Influenza Activity
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* 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: October 15, 2004

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