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Whooping cough booster not enough to stop outbreaks

By Anne Harding

NEW YORK (Reuters Health) - Giving adolescents and adults a booster shot of the whooping cough vaccine offers some protection against the infection - but not enough to prevent outbreaks, according to a new study.

"The take-home message is, we need a new vaccine," lead author Dr. Roger Baxter, co-director of the Vaccine Study Center at Kaiser Permanente in Oakland, California, told Reuters Health.

The booster, known as Tdap, protects against tetanus and diphtheria as well as whooping cough, also called pertussis. A similar combination vaccine is given to children in five doses between two months and six years of age.

The original vaccines contained whole pertussis bacteria and were highly effective in preventing whooping cough outbreaks, Baxter and his team noted in their report in the British medical journal BMJ.

But safety concerns led to the introduction of so-called acellular vaccines in the 1990s.

Studies have suggested the acellular vaccines are weaker than the earlier versions, which may help explain why pertussis outbreaks are becoming more common in the U.S.

As a result, the Tdap booster for adolescents and adults was introduced in 2005. The U.S. Advisory Committee on Immunization Practices now recommends use of the vaccine instead of the next scheduled tetanus diphtheria booster shot for everyone age 11 and older.

To investigate whether that move was effective in fighting outbreaks, Baxter and his colleagues looked at data from 2006 to 2011, during which time California saw the highest incidence of whooping cough in the last 50 years.

The researchers matched 668 people age 11 and older with confirmed whooping cough from Kaiser Permanente Northern California records to 10,098 others who tested negative for whooping cough and 21,599 similar people who didn't get sick.

Twenty-four percent of people with pertussis had been immunized with Tdap, versus 32 percent of negative-testers.

Once people's age, race, gender and vaccine history were taken into account, the researchers calculated that the booster shot was 53 to 64 percent effective at preventing whooping cough.

By comparison, a single dose of the current measles-mumps-rubella vaccine confers immunity to all three viruses in 95 percent of people who receive it, according to the Centers for Disease Control and Prevention.

The rate of pertussis infection was highest among 11- to 14-year-olds, who had only received acellular vaccines as children. This younger age group also saw a greater absolute benefit from Tdap vaccination compared to older individuals who had received at least some whole-cell vaccines early in life.

Immunity conferred by the acellular pertussis vaccines is known to fade more quickly than that offered by the original whole-cell formulation, Baxter noted.

While the Tdap booster had been introduced to address this issue, he added, the new findings show that giving it to adolescents and adults isn't going to solve the problem.

"It's going to blunt those epidemics a little bit, but it's not going to be like the old whole-cell pertussis vaccine that really took those outbreaks away. We expect that we will see these periodic outbreaks actually increase over the next few years as we reach a time that no one has had whole-cell vaccines," Baxter added.

"I think we knew at the time we moved to the acellular vaccine that we traded safety for efficacy," Dr. Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia, told Reuters Health.

"I don't think it's been until the last few years that we realized how big of a trade that was," said Offit, who was not involved in the new study.

He added that the findings are consistent with a number of other recent reports showing that immunity from the newer vaccine fades more quickly, and that it's just not that effective.

"This is the best tool available and it clearly and definitely decreases your risk of getting whooping cough, it's a no-brainer," he said.

"But we would do better with a better vaccine, there's room for a better vaccine."

SOURCE: http://bit.ly/16mXyoG BMJ, online July 17, 2013.

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