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Whooping cough vaccine wearing off

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The incidence of whooping cough has surged amid worrying signs that the effectiveness of a widely-used vaccine is wearing off.

Communicable diseases experts have reported that a pertussis epidemic that has recently swept the country has been caused, in part, by a decline in the protection provided to young children by vaccination.

Writing in the Department of Health’s latest Communicable Diseases Intelligence report, National Centre for Immunisation and Research of Vaccine Preventable Diseases senior research fellow Dr Helen Quinn said that although part of the increased reported incidence of the disease was due to improved testing, there was also concerning evidence of weakened protection from the disease.

Dr Quinn said that in the recent outbreak there had been increased pertussis notifications among children from as young as six months to as old as nine years, “which had not been seen since the introduction of acellular pertussis vaccines”.

The communicable diseases expert said vaccination coverage was not to blame – it had remained steady at 92 per cent among 12-month-olds, 95 per cent at 24 months and 90 per cent at five years.

“Instead, waning of vaccine-induced immunity appears to be a factor,” Dr Quinn said. “The increase in cases aged six months to four years may have resulted from removal of the 18-month dose from the National Immunisation Program in 2003, thereby increasing the interval between the last dose of the primary series and the first booster dose.”

Studies in both Australia and the United States have shown that immunity wanes with age and time since the last dose.

Dr Quinn said that, even though the protection provided by vaccination may be waning, it still meant that vaccinated children who caught the high contagious disease had a milder form of the infection than would otherwise be the case.

The big concern was instead that vaccinated children with the disease would pass it on to vulnerable younger children, particularly infants, Dr Quinn said, citing a study from Perth which found that more than a third of cases reported in a four-year period were as a result of sibling infection, with the majority involving two to three-year-olds who had been fully vaccinated.

Dr Quinn said there was mixed evidence regarding some strategies to try and combat this. Vaccinating a child immediately upon birth may reduce the effectiveness of later vaccinations, while the “cocooning” strategy used in some states (vaccinating all close contacts of infants, such as parents) was effective, but only if the vaccination was comprehensive and timely.

Instead, she said, “it is clear that a third generation of pertussis vaccines, providing long-lasting protection, are required”, though adding that the most likely prospects were still “some years away” form development.

Adrian Rollins