Clusters of influenza A infections identified : reports underscore importance of timely influenza vaccination
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Clusters of influenza A infections identified : reports underscore importance of timely influenza vaccination

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      Monday, October 20, 2003, 20:07 EDT (08:07 PM EDT)


      Reports Underscore Importance of Timely Influenza Vaccination This Health Alert Network notice describes recent reports of influenza A activity in Texas and other parts of the United States, presents results of preliminary laboratory analysis of influenza A(H3N2) isolates conducted at the Centers for Disease Control and Prevention, and outlines current recommendations for influenza vaccination.

      During the first, week of October, Texas health authorities reported cases and school outbreaks of laboratory-confirmed influenza A infections in the Houston metropolitan area. Testing in Texas identified influenza A(H3N2) virus and isolates were sent to the Centers for Disease Control and Prevention (CDC) for further characterization. At CDC, preliminary analysis has shown that 8 of 13 A(H3N2) isolates from Texas are antigenically similar to the A(H3N2) A/Panama/2007/99 vaccine strain, while five isolates are antigenic drift variants. Influenza subsequently has been reported from several counties in Texas.

      During August and September, CDC had received influenza A(H3N2) isolates from sporadic cases in Alaska, Connecticut, Wisconsin, Hawaii, New Hampshire, New York, Texas, and the District of Columbia and influenza A isolates from sporadic cases in Louisiana, Texas, and Washington. While influenza activity in the United States usually starts in November or December and reaches peak levels from late December through April, the timing of influenza activity is highly variable from year to year and influenza outbreaks have been reported in October in some previous years. Influenza cases and isolated outbreaks can occur at any time of the year.

      Similar to the Texas isolates, approximately 33% of influenza A(H3N2) viruses isolated worldwide between February and September have drifted antigenically from the current A(H3N2) A/Panama/2007/99 vaccine strain in laboratory tests. By contrast, influenza A(H1N1) and influenza B viruses generally have remained similar to their vaccine strain counterparts. Influenza vaccine is expected to provide good protection against influenza A(H1N1), B viruses, and A(H3N2) viruses that are similar to the vaccine strains. While vaccine protection against the A(H3N2) drift variants may be lower, the vaccine is expected to provide some degree of effectiveness although the level of protection cannot be predicted.

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