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ATLANTA — Children under the age of 10 years old may be especially susceptible to a new strain of influenza, according to an article published Friday in the Center for Disease Control and Prevention's "Morbidity and Mortality Weekly Report."
As many as 12 infections with influenza A (H3N2)v, a variant virus with genes from avian, swine and human strains, that had been crossed with an influenza A (H1N1) strain have been tracked since August. Eleven of those cases occurred in children under the age of 10 years. And in six cases, no history of recent exposure to swine was noted, suggesting that human-to-human transmission had occurred.
The CDC conducted a preliminary analysis to evaluate the age-specific presence of serum cross-reactive antibody in U.S. populations vaccinated or not vaccinated with the 2010-2011 seasonal trivalent influenza vaccine. The results indicated that little or no cross-reactive antibody to A (H3N2)v exists among children under 10 years; that the 2010-2011 influenza vaccine had no impact on cross-reactive antibody levels in those under the age of 3 years; cross-reactive antibody was detected in 20% to 30% of those over 10 years old; and, among adults, vaccination with the 2010-2011 influenza vaccine provided a modest boost to the level of cross-reactive A (H3N2)v antibodies.
Receipt of seasonal influenza vaccine continues to be recommended to protect against circulating human influenza viruses for all age groups and might provide limited protection against A (H3N2)v infection in the adult population. A vaccine virus specific for A (H3N2)v has been developed and could be used to produce an H3N2v vaccine, if needed, the CDC reported.
Human infections with influenza A (H3N2)v were reported with increased frequency in 2011 compared with previous years, however enhanced surveillance might be a contributing factor, noted MMWR editors.
The composition of the 2011-2012 seasonal triumvirate vaccine is identical to the 2010-2011 vaccine evaluated in this report and is expected to provide limited cross-protection from A (H3N2)v in adults and no cross-protection in young children. In the event of sustained human-to-human transmission of (H3N2)v, an A (H3N2)v-specific vaccine would provide optimal protection for all ages. An A (H3N2)v reassortant vaccine strain based on the A/Minnesota/11/2010 virus has been developed and could be used to produce an H3N2v vaccine, if needed.
Updated information and guidance documents related to A (H3N2)v viruses are available online from CDC here.