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Booster may be beneficial for children who experience Hib vaccine failure
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More than half of 167 children who participated in a study of Haemophilus influenzae type b antibody concentrations in the years following vaccination had antibody concentrations below 1.0 mcg/mL, the level that confers long-term protection.
“Survivors of Hib vaccine failure might benefit from an additional dose of the Hib conjugate vaccine several years after infection to provide long-term immunity against Hib disease,” researchers from the Netherlands wrote.
They used national surveillance data to identify children who experienced invasive Hib disease following vaccination with Hib conjugate, including those who received three doses in the first year of life; those who became ill more than one week following receipt of two doses administered in the first year of life; or children who had the infection more than two weeks following a single dose given after the first year of life.
Questionnaires and a blood sample request were sent to 323 families who had children that fit these criteria and who experienced disease from October 1992 to December 2005.
The researchers independently associated three factors with children whose antibody concentrations were lower than 0.15 mcg/mL (the level believed to provide short-term protection against the disease) — young age at disease onset, underlying medical conditions and a shorter time from Hib disease to follow-up.
“This last observation warrants further explanation, because normally, antibody concentrations tend to decrease after infection or immunization and would be expected to be lowest in children with a longer time from Hib disease to follow-up,” Lodewijk Spanjaard, MD, PhD, of the Academic Medical Center, Amsterdam, and Ger T. Rijkers, MD, of St. Antonius Hospital, Nieuwegein, both in the Netherlands, wrote in an accompanying editorial.
The researchers suggested that decreased natural boosting among both vaccinated and nonvaccinated populations due to reduced Hib carriage rates attributable to routine Hib vaccination may explain the increases in the rate of vaccine failure–associated invasive Hib disease observed since 1999.
Spanjaard and Rijkers asserted that the number of children with anti-PRP antibody levels lower than 0.15 mcg/mL — “the main laboratory parameter that would support this conclusion” — is too small to statistically support this hypothesis. Both groups called for further study.
Ladhani S. Clin Infect Dis. 2009;49:372-380.