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Update: Influenza Activity -- United States

Influenza B and influenza A(H3N2) viruses continue to circulate throughout the United States. For the week ending February 1, 13 states* and the District of Columbia reported widespread outbreaks of influenza-like illness, and 18 states** reported regional outbreaks. The preceding week, five states reported widespread outbreaks, and 13 states reported regional outbreaks.

Although influenza B viruses are more prevalent, there is evidence of at least one mixed outbreak. Beginning January 14, an outbreak of influenza-like illness was observed among students seen at the student health center at North Carolina State University in Raleigh. Influenza viruses were isolated from nine students tested from January 19 to January 21; seven type B viruses and two type A(H3N2) were identified. This is the first outbreak this season where both virus types B and A(H3N2) were isolated and raises the possibility of co-circulation of these viruses in other outbreaks.

Influenza virus type A(H3N2) has been isolated from a nursing-home outbreak near Minneapolis-St. Paul, Minnesota, where approximately 33 (25%) of the 132 residents were ill between January 16 and January 28. Influenza type B viruses were isolated during the same week from two students in an outbreak of influenza-like illness at the high school in the same community as the nursing home.

Several more states have recently reported their first influenza isolates of the season. West Virginia has reported influenza type A(H3N2) virus; Maryland, Nebraska, and Rhode Island have reported influenza type B virus; and Delaware, Kentucky, North Carolina, and Ohio have reported both influenza type B and influenza type A(H3N2) viruses. Forty-two states and the District of Columbia have now reported influenza virus isolates this season. Thirty-eight states and the District of Columbia have reported type B isolates; 21 have reported type A(H3N2); and one, Hawaii, has reported type A(H1N1).

Tallies of patients with influenza-like illnesses seen by sentinel physicians nationwide continued to increase from an average of 7.6 for the reporting week ending January 15, to an average of 9.2 for the week ending January 22.

Pneumonia and influenza (P&I) deaths reported from the 121 U.S. cities for the week ending February 1 represent 5.8% of total deaths, compared with 6.1% reported for the preceding week. Reported by State and Territorial Epidemiologists; State Laboratory Directors; R Meriwether, MD, Div of Health Svcs, North Carolina Dept of Human Resources; E Balkovic, PhD, G Hsiung, PhD, Virology Laboratory, Veterans Administration Medical Center, West Haven, Connecticut; J Braun, MPH, Minnesota Dept of Health; Statistical Svcs Br, Div of Surveillance and Epidemiologic Studies, Div of Field Svcs, Epidemiology Program Office, WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. *Colorado, Georgia, Idaho, Michigan, Minnesota, Nebraska, New Hampshire, New Jersey, North Carolina, Oklahoma, Pennsylvania, Washington, and West Virginia. **California, Connecticut, Delaware, Illinois, Iowa, Kansas, Kentucky, Maryland, Mississippi, Missouri, Montana, Oregon, South Carolina, South Dakota, Texas, Vermont, Virginia, and Wisconsin. SCases reported by those members of the American Academy of Family Physicians Research Panel who serve as sentinel physicians for influenza.

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