2 May 2002 10:00 GMT

by Julie Clayton, BioMedNet News

Confusion and ignorance surround the reemergence of whooping cough among newborn babies, admit Europe's leading infectious disease specialists. They regard the rising number of reported cases, notably in France, Germany, and the Netherlands, as the tip of a "hidden epidemic" but remain uncertain about how best to provide protection as US authorities now look for guidance too.

Adolescents and adults, particularly those aged between 20 and 25, are the most likely carriers of the infectious agent, the bacterium Bordetella pertussis. But even if mass vaccination was feasible, issues concerning the safety and efficaciousness of the available vaccines are unresolved.

"A lot of countries agree to vaccinate adolescents and adults, but how can you achieve 90% coverage," asked Nicole Guiso, director of France's national reference laboratory for pertussis infections at the Pasteur Institute in Paris.

Furthermore, she added: "The composition of vaccine needs consensus." Guiso was speaking last weekend during a symposium, Pertussis: the hidden epidemic, on the final day of the 12th European Congress of Clinical Microbiology and Infectious Diseases in Milan.

Delegates looked for advice to the Netherlands where, after eradicating the disease in the mid-1980s, there were around 500 hospitalizations a year between 1996 and 2000 and a total of eight deaths of unvaccinated infants less than three months old.

"For eradication you need to vaccinate everyone every 10 years, which is not feasible," said Joop Schellekens, a medical microbiologist at the Dutch National Institute for Public Health and Hygiene (RIVM) in Bilthoven.

"But if the objective is to reduce hospital admissions and deaths, my favored approach would be to give the booster to fathers and pregnant women in their third trimester of pregnancy," he told delegates.

In France and Germany, the immunity of adolescents is being boosted, an approach that US authorities are now considering. Guiso herself advocates the targeting of parents and health care workers.

Existing vaccine programs tend to involve a primary immunization at the age of two to three months, followed by a booster during the next four years with "acellular" vaccines, which contain between three and five purified bacterial toxins and other proteins.

But adopting a strategy that involves boosting the immunity of adults raises the question of which vaccine to use, according to Andre Cassone, a medical microbiologist from the Institute of Health in Rome. "The acellular vaccines are all efficacious, but not to the same degree," he said.

Cassone also acknowledges the mystery over how antibody and T cell responses to the vaccines contribute to protective immunity. "There's no correlation between the presence of the antibody and protection," he noted.

The immune responses may vary between adolescents and adults, and no one knows whether the vaccines are safe for adults, he adds.

Guiso and others are investigating the suggestion that the recent rise in cases may be due to new mutants of B. pertussis. While both European and US scientists are isolating new genetic variants from patients, there is no evidence yet that these variants correspond to any change in antigenicity. Neither is it clear whether the variants arise from the natural evolution of the bacteria, or from exposure to vaccines.

"We need to continue surveillance: we don't know if the acellular vaccines protect against antigenic variants - because there's less than 10 years of data," added Guiso. "We don't yet know the impact of a late booster, or the evolution of the agent."

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