The number of cases of the Hib infection among children and adults is continuing to rise despite an extensive vaccination programme.
Rates of the meningitis-causing bacteria in adults have reached levels higher than before 1992 when the routine vaccination of babies began, with 0.27 cases per 100,000 in 2003 compared to 0.17 in 1992.
Researchers told the British Medical Journal that rates in children have also started to increase, although infection rates remain below those seen before the vaccination was introduced.
Ironically, an expert at the Health Protection Agency suggested the introduction of the Hib vaccine could be blamed for the increase in infected adults.
"The drop in infection rates among children meant reduced exposure to the disease for adults, and therefore lower rates of infection," said Dr Mary Ramsay from the HPA's immunisation department.
"This means that the level of antibodies in adults, to enable them to fight Hib infection, was no longer being boosted. Therefore, as the disease started to rise once again amongst children, some adults were less able than before to fight the infection."
The Department of Health recently launched a Hib booster campaign aimed at all children aged from six months up to four years, which should also help reduce adult cases in the long-term.
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