CDC: Moderate Influenza Season So Far

Yael Waknine

February 16, 2007

February 16, 2007 — Influenza activity so far this season has been moderate, according to a report in the February 16 issue of the Morbidity and Mortality Weekly Report. The Centers for Disease Control and Prevention (CDC) reports that although patient visits for influenza-like illness exceeded the national baseline for 7 weeks of the season, mortality rates due to pneumonia/influenza have remained below epidemic threshold.

As of the week ending February 3, widespread influenza activity was reported by 9 US states; 19 and 7 states had regional and sporadic activity, respectively. In general, influenza activity had remained low since season onset in October 2006 before increasing in mid-December and then again in January.

Over the course of the season, 7.5% of specimens tested by the World Health Organization and the National and Enteric Virus Surveillance System US collecting laboratories were positive for influenza; the majority of these were type A viruses (82.7% vs type B, 17.3%).

Overall, most type A viruses were of the H1 subtype (88.9%), and 93 (94%) of 99 specimens sent to the CDC for antigenic characterization matched the H1 component of the 2006/2007 flu vaccine (A/New Caledonia/20/99).

The proportion of type A H3 viruses increased from 6.2% to 16.9% during the period from January 7 to February 3; a majority of these (61.9%) were reported from the Mountain and Pacific regions. However, 4 (57%) of 7 H3 viruses were characterized as A/Wisconsin/67/2005-like (the H3, the influenza A [H3] component of the 2006/07 vaccine), and 3 specimens (43%) had reduced titers with ferret antisera produced against A/Wisconsin/67/2005.

The percentage of patient visits for influenza-like illness during the current season has ranged from 1.0% to 3.2%; the rate exceeded the national baseline (2.1%) for 7 weeks. As of February 3, 8 of 9 surveillance regions reported influenza-like illness at or above region-specific baselines. However, pneumonia and influenza accounted for 7.4% of all deaths reported at that time, a rate that remained below the epidemic threshold of 8.2% (range this season, 5.6% - 7.5%).

According to the Emerging Infection Program, the estimated rates for pediatric hospitalization due to influenza are 0.34 and 0.05 per 10,000 children aged 0 to 4 years and 5 to 17 years, respectively. Data from the New Vaccine Surveillance Network suggest that the estimate is higher for children younger than 4 years (0.63 per 10,000). Thus far this season, 9 influenza-related deaths have occurred in children aged 3 months to 14 years (mean, 7.5 years); all were positive for type A virus.

The CDC notes that vaccination is the best method for preventing influenza and its complications. Although October and November are optimal months for vaccination, it is still beneficial at this time because US influenza activity usually peaks between December and March. As the season continues, the degree of antigenic match between the vaccine and circulating influenza strains will be determined.

Weekly influenza surveillance reports are available online from October to May at http://www.cdc.gov/flu/weekly/fluactivity.htm. Additional information regarding influenza viruses, surveillance, vaccination, and avian influenza is available at http://www.cdc.gov/flu.

MMWR. 2007;56(06):118-121.

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