The effect of an accelerated immunisation schedule on pertussis in England and Wales.

Notifications of pertussis in England and Wales have fallen dramatically from 65 810 during the epidemic year of 1982 to 3963 cases during the epidemic year of 1994, as vaccine coverage has risen. The incidence of pertussis has declined in all ages, including babies under 3 months of age who would have been at risk of disease from older siblings vaccinated under the accelerated schedule introduced in 1990 if immunity induced as result of this schedule had been short lived. To document the efficacy of the current whole cell vaccine under the accelerated schedule an enhanced surveillance scheme based on laboratory confirmed cases of pertussis was set up in 1994. Three deaths occurred in infants with confirmed pertussis, all of whom were under 8 weeks of age and unvaccinated. The overall vaccine efficacy for those over 6 months and under 5 years of age was 94%. This estimate may be inflated, as a number of biases could lead to the underascertainment of cases in vaccinated children, but it is similar to previous estimates obtained for children of the same age vaccinated under the 3, 5, and 10 month schedule. Vaccine efficacy was 89% for children aged over 5 and under 15 years. The enhanced surveillance scheme will enable us to monitor the duration of protection under the accelerated schedule and evaluate the continuing impact of pertussis infection.