Tdap for Teens, Adults Not Enough to Block Whooping Cough Outbreaks

July 31, 2013

By Anne Harding

NEW YORK (Reuters Health) Jul 31 - Giving adolescents and adults the reduced acellular pertussis (Tdap) vaccine offers some protection against infection, but not enough to prevent outbreaks, according to a new case-control 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 original whole-cell vaccines against pertussis, tetanus and diphtheria were highly effective in preventing whooping cough outbreaks, Dr. Baxter and his team note in their July 17 report in BMJ. But safety concerns led to the introduction of 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 despite high vaccine coverage in the U.S.

Consequently, the Tdap vaccine for adolescents and adults, who often serve as a reservoir of infection, was introduced in 2005. The U.S. Advisory Committee on Immunization Practices recommended use of the Tdap vaccine instead of the next scheduled tetanus diphtheria booster shot for everyone aged 11 to 64. In 2012, USACIP recommended the vaccine for everyone 65 and older.

To investigate whether giving adolescents and adults the Tdap vaccine was effective in fighting outbreaks, Dr. Baxter and his colleagues looked at data from 2006 to 2011, during which time California saw the highest incidence of pertussis in the last 50 years.

The researchers matched 668 PCR-confirmed cases of pertussis in people 11 and older from Kaiser Permanente Northern California (KPNC) records to 10,098 PCR-negative controls and an additional 21,599 matched controls drawn from KPNC records.

Twenty-four percent of the PCR-positive pertussis cases had been immunized with Tdap, versus 32% of the PCR-negative controls (p<0.001). The adjusted odds ratio for cases versus PCR-negative controls was 0.47, and 0.36 when compared to the Kaiser Permanente controls.

Based on the adjusted odds ratio, the vaccine was 53% effective when the PCR-negative control group was used, and 64% effective for the larger matched control group. By comparison, a single dose of the current measles-mumps-rubella vaccine confers immunity to all three viruses in 95% 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; 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.

"Strategies to decrease the incidence of pertussis should prioritize giving the Tdap booster to people who received only acellular pertussis vaccines as children, as California has done since the 2010 outbreak, requiring Tdap vaccination for middle school students," Dr. Baxter and his team write.

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

"That strategy would only work if we repeatedly vaccinated people who were around infants during pertussis season, and that's probably not going to happen," he said.

"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," Dr. Baxter added.

"I think we knew at the time we moved to the acellular vaccine that we traded safety for efficacy," Dr. Paul Offit, chief of the division of infectious diseases and 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."

Dr. Offit was not involved in the new study. He said 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 added. "But we would do better with a better vaccine, there's room for a better vaccine."


BMJ 2013.


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