During week 45 (November 8-14, 2009), influenza activity decreased slightly in the U.S.
- 3,106 (28.8%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division were positive for influenza.
- Over 99% of all subtyped influenza A viruses being reported to CDC were 2009 influenza A (H1N1) viruses.
- The proportion of deaths attributed to pneumonia and influenza (P&I) was above the epidemic threshold for the seventh consecutive week.
- Twenty-one influenza-associated pediatric deaths were reported. Fifteen of these deaths were associated with 2009 influenza A (H1N1) virus infection, and six were associated with an influenza A virus for which the subtype was undetermined.
- The proportion of outpatient visits for influenza-like illness (ILI) was 5.5% which is above the national baseline of 2.3%. All 10 regions reported ILI above region-specific baseline levels.
- Forty-three states reported geographically widespread influenza activity, Puerto Rico and seven states reported regional influenza activity, the District of Columbia reported local influenza activity, and Guam and the U.S. Virgin Islands reported sporadic influenza activity.
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Antiviral Resistance:
Since September 1, 2009, four influenza A (H3N2), one influenza B, and 353 2009 influenza A (H1N1) virus isolates have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir), and 856 2009 influenza A (H1N1) original clinical samples were tested for a single known mutation in the virus that confers oseltamivir resistance. In addition, one influenza A (H3N2) and 182 2009 influenza A (H1N1) virus isolates have been tested for resistance to the adamantanes (amantadine and rimantadine). Additional laboratories perform antiviral testing and report their results to CDC. The results of antiviral resistance testing performed on these viruses are summarized in the table below.
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*The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.
†Two screening tools were used to determine oseltamivir resistance: sequence analysis of viral genes or a neuraminidase inhibition assay.
‡Additional laboratories perform antiviral resistance testing and report their results to CDC. One additional oseltamivir resistant 2009 influenza A (H1N1) virus has been identified by these laboratories since September 1, 2009, bringing the total number to 11.
Over 99% of all of the subtyped influenza A viruses reported during week 45 were 2009 influenza A (H1N1) viruses, and the majority of 2009 H1N1 viruses tested since April 2009 have been resistant to the adamantanes (amantadine and rimantadine).
Antiviral treatment with oseltamivir or zanamivir is recommended for all patients with confirmed or suspected influenza virus infection who are hospitalized or who are at higher risk for influenza complications. Additional information on antiviral recommendations for treatment and chemoprophylaxis of influenza virus infection is available at http://www.cdc.gov/h1n1flu/recommendations.htm.
2009 influenza A (H1N1) viruses were tested for oseltamivir resistance by a neuraminidase inhibition assay and/or detection of genetic sequence mutation, depending on the type of specimen tested. Original clinical samples were examined for a single known mutation in the virus that confers oseltamivir resistance in currently circulating seasonal influenza A (H1N1) viruses, while influenza virus isolates were tested using a neuraminidase inhibition assay that determines the presence or absence of neuraminidase inhibitor resistance, followed by the neuraminidase gene sequence analysis of resistant viruses.
The majority of 2009 influenza A (H1N1) viruses are susceptible to the neuraminidase inhibitor antiviral medication oseltamivir; however, rare sporadic cases of oseltamivir resistant 2009 influenza A (H1N1) viruses have been detected worldwide. A total of 21 cases of oseltamivir resistant 2009 influenza A (H1N1) viruses have been identified in the United States since April 2009. In specimens collected since September 1, 2009, 11 cases have been identified in the United States, including seven newly identified cases since last week and one case reported during a previous week that was reclassified. All tested viruses retain their sensitivity to the neuraminidase inhibitor zanamivir. Of the 21 cases, 12 patients had documented exposure to oseltamivir through either treatment or chemoprophylaxis, eight patients are under investigation to determine exposure to oseltamivir, and one patient had no documented oseltamivir exposure. Occasional development of oseltamivir resistance during treatment or prophylaxis is not unexpected. Enhanced surveillance and increased availability of testing performed at CDC are expected to detect additional cases of oseltamivir resistant 2009 influenza A (H1N1) viruses, and such cases will be investigated to assess the spread of resistant strains in the community.
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Pneumonia and Influenza (P&I) Mortality Surveillance
During week 45, 7.5% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage was above the epidemic threshold of 6.9% for week 45. Including week 45, P&I mortality has been above threshold for seven consecutive weeks
Influenza-Associated Pediatric Mortality
Twenty-one influenza-associated pediatric deaths were reported to CDC during week 45 (Arizona [2], Colorado [3], Georgia [2], Louisiana [2], Minnesota, Mississippi, New York, New York City, Ohio, Oklahoma, Oregon [2], Texas, Virginia, Washington, and Wisconsin). Fifteen of these deaths were associated with 2009 influenza A (H1N1) virus infection, and six were associated with an influenza A virus for which the subtype is undetermined. The deaths reported during week 45 occurred between September 20 and November 14, 2009.
Since August 30, 2009, CDC has received 138 reports of influenza-associated pediatric deaths that occurred during the current influenza season (24 deaths in children less than 2 years old, 16 deaths in children 2-4 years old, 50 deaths in children 5-11 years old, and 48 deaths in children 12-17 years old). One hundred thirteen (82%) of the138 deaths were due to 2009 influenza A (H1N1) virus infections, and the remaining 25 were associated with influenza A virus for which the subtype is undetermined. A total of 171 deaths in children associated with 2009 influenza A (H1N1) virus infection have been reported to CDC.
Among the 138 deaths in children, 74 children had specimens collected for bacterial culture from normally sterile sites and 23 (31.1%) of the 74 were positive; Staphylococcus aureus was identified in eight (34.8%) of the 23 children. One S. aureus isolate was sensitive to methicillin, six were methicillin resistant, and one did not have sensitivity testing performed. Fifteen (65.2%) of the 23 children with bacterial coinfections were five years of age or older, and six (26.1%) of the 23 children were 12 years of age or older.
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