Vaccine efficacy and control measures in pertussis Stephen

An outbreak of pertussis in primary schoolchildren in the St David's area of Pembrokeshire provided the opportunity to estimate pertussis vaccine efficacy. The estimate of efficacy was 88% when notified cases were used, but this fell to 68% when all children with bouts of coughing for two or more weeks were included. Notified cases were significandy less likely to have been vaccinated than other cases with similar symptoms. Therefore vaccine efficacy estimates based upon notified cases are likely to be biased. However, even the lower estimates suggest that pertussis immunisation is highly desirable and efforts to improve coverage should be increased. Public anxiety about the safety and efficacy of pertussis vaccine in the United Kingdom in the 1970s led to a sharp fall in vaccine uptake and a consequent increase in the severity of pertussis epidemics in 1977/9 and 1981/3.1 Fears about the safety of the whole cell vaccine have been addressed, but there is a continuing need to monitor vaccine efficacy, both for pertussis and other routinely administered vaccines.2 Measurement of vaccine efficacy requires the comparison of disease experience in vaccinated and unvaccinated children in a population in which both groups are equally exposed to the agent.3 Outbreaks present such opportunities to carry out field evaluation of vaccine efficacy. In an outbreak of pertussis centred on a primary school in the St David's area of Pembrokeshire in 1987 vaccine efficacy was measured and the use of the accelerated schedule for vaccine delivery was evaluated.4 follows. All preschool children registered in the practices were identified by the attached health visitor, families were contacted, and children who had a history of cough for two or more weeks were identified. A letter and questionnaire were sent to parents of all children in primary and secondary schools in the St Davids/ Solva area on 2 June. Parents reporting illnesses in children were interviewed by the health visitor. A notified case was a case formally reported by a doctor to the local authority. A probable case was a child with bouts of coughing lasting for two or more weeks with bouts followed by vomiting, whooping, or choking/blue colour but who was not a notified case. A possible case was a child with bouts of coughing for two or more weeks but without vomiting, whooping, choking or turning blue and who was not notified. Statistical comparisons were carried out by x2 test, t test, and the Mann-Whitney test. Vaccine efficacy was estimated by three methods for a cohort of children born between 1 January 1981 and 31 December 1985 registered in the two practices and identified from the child health register. (1) Screening method PCV= PPV-(PPVxVE) 1-(PPVxVE) Where PCV=proportion of cases vaccinated, PPV=proportion of population vaccinated, and VE=vaccine efficacy. (2) Direct population estimates VE= AR unvaccinated-AR vaccinated