By Andrew Czyzewski, medwireNews Reporter
The risk for pertussis infection in children who receive the combined diphtheria, tetanus, and acellular pertussis (DTaP) vaccine increases by an average of 42% per year after the last dose, a US study shows.
The Californian cohort included a period in 2010 marked by a pertussis outbreak in the state and Nicola Klein (Kaiser Permanente, Oakland, California) and colleagues suggest that "waning efficacy among school-age children played a key role in both allowing and sustaining the outbreak."
In the early 1990s, the USA started to make the transition from whole-cell pertussis vaccines to DTaP. Receipt of five doses of DTaP is mandatory for school entry in many states, including California, with the fifth dose usually administered in children between 4 and 6 years of age.
However, pertussis outbreaks still occur every 3 to 5 years, with an increase in the peak incidence with each successive outbreak.
To investigate, Klein et al performed a case-control study in children aged 4 to 12 years who were vaccinated with DTaP, of whom 277 were polymerase chain reaction (PCR)-positive for pertussis and 3318 were PCR-negative controls.
They used logistic regression to examine the risk for pertussis in relation to the duration of time since the fifth DTaP dose.
Klein et al found that the time since the fifth dose of DTaP was significantly longer in PCR-positive children at an average of 1699 days than for PCR-negative controls at an average of 1028 days.
Comparing PCR-positive children with PCR-negative controls, with adjustment for calendar time, age, gender, race or ethnic group, and medical service area, the odds ratio for pertussis was 1.42 per year indicating that each year after the fifth dose of DTaP was associated with a 42% increased odds of acquiring pertussis.
The researchers point out that if DTaP effectiveness is initially 95%, so that the risk for pertussis in vaccinated children is only 5% that of unvaccinated children, then the risk would increase after 5 years by a factor of 1.42 to 29% that of unvaccinated children.
Likewise, if the initial effectiveness of DTaP was 90%, it would decrease to 42% after 5 years.
"Our findings highlight the need to develop new pertussis-containing vaccines that will provide long-lasting immunity," the researchers comment in The New England Journal of Medicine.
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