Whooping cough pertussis is on the increase in the US despite a programme of compulsory vaccination.

The number of reported cases in 1993 rose by 82 per cent over the previous year to 5457, the highest since 1967, claiming seven lives.

Only 16 per cent of cases had not received the DPT (diptheria, pertussis, tetanus) vaccine, according to detailed information on a smaller group of sufferers.

The figures were worsened by large outbreaks of whooping cough in Chicago and Cincinnati, although there has been a steady increase in reported cases since 1977.

But the outbreak is worse than even the figures indicate. Health officials estimate that only 10 per cent of cases are ever reported, partly because doctors are not diagnosing accurately. Whooping cough in adolescents and adults is rarely diagnosed at all.

Pertussis is most dangerous among babies of under 12 months and they can catch it from previously vaccinated older children. American doctors are admitting that vaccinated adolescents can transmit the disease to younger children because immunity wears off, usually from four years after the last shot.

Doctors in America are responding to the outbreak by accelerating the vaccination programme.

Following the recommendations of the Advisory Committee on Immunization Practices, they want to vaccinate babies when they are six, 10 and 14 weeks of age. They also say the outbreak highlights the need for children to complete the course of three vaccinations.