Severity of whooping cough in England before and after the decline in pertussis immunisation.

Since the decline of pertussis immunisation, hospital admission and death rates from whooping cough have fallen unexpectedly. Although this might be taken to indicate that the disease is becoming less severe, a comparison of admissions and deaths in England and Wales before and after the decline in immunisation suggests that several factors other than disease severity--a shift in the social class distribution of the disease, an increase in the proportion of milder cases notified, and improved care--were responsible for this. The severity of attacks and the complication rates in children admitted to hospital were virtually unchanged. Very young infants, those from disadvantaged families, and children with chronic illness were at greatest risk.