Younger children would be most affected in sustained transmission of influenza A virus

Although some older children and adults have been found to have cross-reactive antibodies to influenza A (H3N2) variant viruses (ie, A [H3N2]v), which were reported with increased frequency in 2011 compared with previous years, the Centers for Disease Control and Prevention (CDC) is warning that children aged 10 years and younger have few or no cross-reactive antibodies.

Twelve human infections with influenza A (H3N2)v were detected in the United States in 2011 compared with 8 cases in the preceding 2 years. Eleven of these cases were in children aged 10 years and younger.

Vaccination with the 2010-2011 seasonal influenza vaccine boosted cross-reactive antibodies for A (H3N2)v in adults and older children, reported the CDC, but the seasonal vaccine had no effect on cross-reactive antibody levels in children younger than 3 years.

The report also said a serologic study in Canada showed no evidence of cross-reactive antibodies in children younger than 10 years and that receiving influenza vaccine did not produce a cross-reactive antibody response in children aged 4 years and younger. Because the composition of the 2011-2012 seasonal trivalent influenza vaccine (TIV) is identical to the 2010-2011 version, the CDC said that it does not expect the vaccine to provide any cross-protection to A (H3N2)v in younger children, the population that would be the most susceptible in the event of sustained human-to-human transmission.

The CDC noted that the number of persons in each age group in the report’s findings was small and that current levels of cross-reactive antibodies in persons aged 4 to 17 years might be overestimated or underestimated. Also, the population covered by the findings might not be representative of all children aged 6 to 35 months, and the populations aged 4 to 9 years and 10 to 17 years had not been part of a vaccine study. The effect of immunization with TIV in these age groups was not determined.

An A (H3N2)v-specific vaccine has been developed and could be used to produce an H3N2v vaccine if needed, said the CDC. However, the agency continues to recommend seasonal vaccination to protect all age groups against circulating human influenza viruses.

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