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ATLANTA — Vaccination for pertussis (whooping cough) is highly effective in reducing transmission of the disease from person to person, even in "breakthrough" cases in which a vaccinated individual still develops pertussis, according to research conducted by scientists at Emory University and in Dakar, Senegal. The finding should help resolve a long-standing debate about the effectiveness of the vaccine in reducing contagiousness of the disease in addition to conferring immunity.
The research was conducted by Marie-Pierre Préziosi, MD, PhD, former visiting faculty member at Emory University’s Rollins School of Public Health and the Institut de Recherche pour le Développement in Dakar, Senegal, and M. Elizabeth Halloran, MD, DSc, professor of biostatistics at the Rollins School of Public Health. The article was in the online edition of Vaccine January 14 and will be published in the printed journal on May 16.
The incidence of pertussis is on the rise in rise in infants, adolescents and adults in the U.S. and other developed countries, and millions of cases occur annually in the developing world. To clarify and promote an effective vaccination policy, scientists have needed a better understanding of how pertussis is transmitted and how that is affected by vaccination.
Earlier studies had indicated that vaccination might not alter the number of circulating Bordetella pertussis bacteria and thus transmission of the disease, because the period between epidemics did not appear to vary with the level of vaccination. More recent studies, however, have shown that widespread vaccination is, in fact, linked to longer periods between epidemics. Confirming studies have shown that infants too young to be directly protected by vaccination have decreased levels of pertussis disease in populations with broader vaccination coverage and that incidence of pertussis in parents and younger siblings of vaccinated children is lower than in parents and siblings of unvaccinated children.
To measure the effectiveness of pertussis vaccination in reducing transmission from vaccinated compared with unvaccinated individuals, Drs. Préziosi and Halloran analyzed data from a followup study within a rural community of 30,000 residents in 30 villages in Niakhar, Senegal, where active surveillance of pertussis has been conducted since 1983. The investigators focused on the calendar year 1993 (an epidemic year). In particular, they considered a group of 3,021 children under age 15 that included 340 primary cases (index cases) in households and 2006 contacts with no history of pertussis. Of these contacts, 41% became suspected secondary cases.
The primary interest in the analysis was to estimate the reduction in transmission from vaccinated compared with unvaccinated primary cases. Vaccine efficacy for infectiousness (VEi) was defined as the relative reduction in the secondary attack rate (SAR) in contacts exposed to vaccinated cases compared to contacts exposed to unvaccinated cases. Using the main case definition, the VEi was a dramatic 85% with a 28-day cut-off and 67% with no cut-off period for the development of secondary cases.
Total vaccine efficacy (VEt) was defined as the relative reduction in the SAR when both the infectious case and the contact were vaccinated compared to if both were unvaccinated. The VEt was measured at 89% with a 28-day cut-off and 77% with no cut-off for the development of secondary cases.
"This work includes important and timely findings on the prevention of pertussis and provides a unique insight into an effect of vaccination not traditionally estimated," said Dr. Préziosi. "The focus on vaccine efficacy for infectiousness presents a dramatically new way to look at the effects of vaccination from a true public health perspective, and clearly shows that vaccination makes breakthrough cases less contagious."
"We hope this new evidence that pertussis vaccination not only provides immunity to those vaccinated, but also a substantial indirect benefit in reducing transmission will give individuals and public health programs additional arguments for vaccination," said Dr. Halloran.
This analysis was supported by the National Institute of Allergy and Infectious Disease and the French Ministry of Foreign Affairs, the Singer-Polignanc Foundation, and Aventis Pasteur. Data collection was financed by Institut de Recherche pour le Développement and Aventis Pasteur.