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Mismatched Influenza Vaccine Gave Some Protection

Kate Traynor

A report in today's Morbidity and Mortality Weekly Report (MMWR) concludes that last season's influenza virus vaccine formulation provided modest protection despite a mismatch between the vaccine's influenza Type A (H3N2) strain and the H3N2 strain that circulated in the community.

According to the report, children ages 6–23 months who were fully vaccinated against influenza virus were 25 percent less likely than unvaccinated or partially vaccinated counterparts to suffer influenza-like illness. Fully vaccinated children were also 49 percent less likely to be diagnosed with pneumonia or influenza.

For children under nine years of age who are administered influenza virus vaccine for the first time, full vaccination requires the administration of two doses of vaccine. The MMWR report stated that young children who were partially vaccinated received no protection against influenza-like illness or pneumonia and influenza during the 2003–04 influenza season.

Among healthy adults ages 50–64 years, last season's vaccine was deemed 52 percent effective at preventing laboratory-confirmed influenza. The vaccine was 38 percent effective among adults in this age group who had medical conditions that increase the risk for influenza-related complications.

Both studies described in the report involved patients who lived in Colorado during the 2003–influenza season. The pediatric study examined electronic medical records obtained from Kaiser Permanente Colorado to retrospectively evaluate vaccine effectiveness. Vaccine protection in adults was examined using a case-control study, with data obtained through telephone interviews.

The vaccine-effectiveness studies examined data from 5,139 children and 1,185 adults.

Last season's influenza vaccine mismatch occurred because the vaccine did not contain an antigenically comparable strain to the virulent type A-Fujian-like viruses that characterized the influenza season.

The 2004–vaccine strain selection consists of A/Fujian/411/2002 (H3N2)-like, A/New Caledonia/20/99 (H1N1)-like, and B/Shanghai/361/2002-like antigens, according to the Centers for Disease Control and Prevention (CDC).

According to CDC, 143 children, including 58 children under age two years, are reported to have died from influenza-related disease last season. CDC now recommends that all children ages 6-23 months and their caretakers and household contacts receive an influenza vaccine.