Immunocompromised children require double H1N1 vaccination
MedWire News: Pediatric liver transplantation patients require two doses of the pandemic influenza A vaccine, say Australian researchers who found a single dose did not give these children adequate immunity to swine flu.
"After the announcement of a worldwide pandemic in June 2009, a single dose of a monovalent pandemic H1N1 2009 influenza A (pH1N1/09) vaccine was advocated for all Australians who were 10 years and older because of excellent immunogenicity trial results for healthy children and adults," explain Wolfram Haller (Royal Children's Hospital, Melbourne, Victoria) and co-workers.
Noting that immunocompromised patients have lower seroconversion rates after routine vaccinations than other individuals, the team investigated the immune response to the pH1n1/09 vaccine in 21 children aged at least 10 years who had undergone liver transplantation.
Seroconversion was defined as a four-fold increase in the antibody titer from the baseline value with a minimum titer of 40 after immunization, while seropositivity, the marker for seroprotection, was defined as a hemagglutinin inhibition (HAI) titer of 40 and more.
At baseline, 33.4% of patients were already seropositive for the virus.
Six weeks after a single intramuscular injection of pH1N1/09 vaccine (15 µg), 62% of patients showed seroconversion. Seroconversion occurred in 86% of patients who were seropositive at baseline compared with 50% of patients who were not.
Six of the seven patients who did not seroconvert returned for a second vaccination a median of 208 days later and 67% seroconverted, raising the total seroconversion rate to 89.5% of patients.
"A second dose of the vaccine is necessary to achieve an immune response comparable to that of healthy controls, at least in a pandemic setting," say Haller et al, noting that seroconversion rates to the pH1N1/09 vaccine in nonpandemic circumstances has yet to be determined.
They conclude: "Further research into new vaccine strategies in order to improve and monitor vaccine responses in immunosuppressed patient groups is urgently needed."
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By Lynda Williams