Short Papers Relative efficacy of two acellular pertussls vaccines during three years of passive surveillance Jann Storsaeter *~ and Patrick Olin* Prolonged unblinded passive surveillance of the trial cohort from the Swedish 1986-87 pertussis vaccine efficacy trial indicates that a two-component vaccine, containing pertussis toxoid and filamentous haemagglutinin, provided better long-term protection against pertussis than a monocomponent pertussis toxoid vaccine. The relative risk (RR) for culture-confirmed pertussis was 1.5 (95% confidence interval ( CI) 1.0-2.4), and RR for pertussis according to parents' diagnoses was also 1.5 (95% CI 1.1-2.1), for the monocomponent vaccine compared with the two-component vaccine. Keywords: Pertussis; vaccine efficacy; monocomponent ; two-component; randomized trial INTRODUCTION An international collaborative efficacy trial of two acellular pertussis vaccines was conducted in Sweden in 1986 to 1987. The pertussis incidence during 15 months blinded, active follow-up was estimated in one placebo and two vaccinated groups. The two acellular vaccines investigated were developed by the National Institute of Health in Japan (JNIH) and produced by the Kannonji Institute, Osaka University, Japan. The monocomponent vaccine JNIH-7 contained formalin-inactivated pertussis toxin (PT) as the only protective antigen, while the two-component vaccine JNIH-6 also contained fila- mentous haemagglutinin (FHA) 1. Efficacy against cul- ture-confirmed pertussis with cough during blinded follow-up in 1986-87 was estimated as 54% (95% confidence interval (CI) 26 72%) for JNIH-7 and as 69% (95% CI 47-82% ) for JNIH-6. Both vaccines were ~80% efficacious against more severe or typical culture-confirmed pertussis 2. The efficacy estimates were lower than expected, and the Division of Drugs, National Board of Health and Welfare in Sweden, judged that protection was lower than the historical experience with conventional whole cell vaccines 3'4. However, the active case ascertainment during the trial may have uncovered atypical cases of pertussis that would not have been detected in previous studies of whole cell vaccines 5. New efficacy trials are planned to resolve this question, directly comparing acellular and whole cell pertussis vaccines 4. Meanwhile, unblinded surveillance of the Swedish study cohort was continued for 36 months from 27 August 1987 until 9 September 1990 (covering 16 52 months after the second trial dose). *Department of Paediatrics, Sachs' Children's Hospital, Karolinska Institute, Stockholm, Sweden. +National Bacterio- logical Laboratory, Stockholm, Sweden. ~To whom corres- pondence should be addressed. (Received 13 June 1991; revised 23 August 1991; accepted 17 September 1991) 0264-410X/92/030142q33 (u; 1992 Butterworth-HeinemannLtd 142 Vaccine, Vol. 10, Issue 3, 1992 SUBJECTS AND METHODS Study children were born during March-September 1985 and were 6 11 months of age when receiving the first of two doses of placebo or vaccine, given with 2 months interval. Active, blinded surveillance of the study cohort was ended on 26 August 1987, and passive unblinded surveillance of 3619 still eligible children started on 27 August 1987. Children were censored from follow-up if they experienced culture-confirmed pertussis, if they received whole cell pertussis vaccine, or if the parents asked to withdraw from the study. No booster doses of pertussis vaccine were given. Laboratory-confirmed cases of pertussis with beginning of cough before 9 September 1990 were identified from the nationwide routine laboratory reports to the National Bacteriological Laboratory, Stockholm and were also reported spon- taneously to us by parents. Also, all eligible households were contacted by letter every 6 months, and parents were asked whether their child had had pertussis during the preceding 6-month period. Ninety-four per cent (3289/3501) of the eligible households responded to the last mailing in September 1990. Non-responders were included in the denominators of incidence rates of parentally reported illness. Two specially assigned study nurses interviewed parents of children with culture-confirmed and/or parentally reported pertussis about the clinical symptoms, using similar questions as in the blinded trial. Information on duration of cough was less complete for cases of parentally reported pertussis than for cases with culture- confirmed disease. Data on clinical symptoms are therefore reported for culture-confirmed cases only. Statistical methods Vaccine efficacy (VE) and relative risk (RR) were estimated by survival analysis. The actuarial method with intervals of 1 and 6 months was used for culture-confirmed and for parentally diagnosed cases,