MILLIONS of British children have probably been infected with whooping cough even though they have been immunised against it.

A study has found that nearly two in five children who went to their GP with a persistent cough had suffered from whooping cough, though very few doctors diagnose it. The results suggest that the whooping cough vaccine is ineffective at preventing infection, but makes symptoms less severe — thereby concealing just how common it remains.

In BMJ online, a team from the University of Oxford, the University of Auckland in New Zealand and the Health Protection Agency report that in 85.9per cent of the cases they saw, the children had been vaccinated. But blood samples tested positive for antibodies to Bordetella pertussis, the cause of whooping cough, indicating recent infection.

The team studied 172 children aged 5-16 who visited their family doctor with a cough lasting 14 days or more. Immunisation records were checked, notes made on the symptoms and duration of cough, and blood samples taken for testing. They found that 37.2 per cent of the children had evidence of a recent pertussis infection. The results suggest that the condition is “endemic among younger school-age children”, they say, and that doctors should consider a diagnosis of whooping cough even if the child has been immunised.

They also found that children with whooping cough were more likely to have symptoms of vomiting and were more likely still to be coughing two months after the start of their illness. They were also more likely to cough more, and to disrupt their parents’ sleep.

The danger is that cases will go undiagnosed, which could pose a risk to younger siblings who have not yet been immunised. A correct diagnosis also saves time and money on treatment and further tests for conditions such as asthma.

“Younger children are more likely than adolescents to have a newborn sibling to whom they could transmit the infection, with potentially devastating consequences,” they say.

“For school-age children presenting to primary care with a cough lasting two weeks or more, a diagnosis of whooping cough should be considered even if the child has been immunised.” Whooping cough was once one of the diseases that almost all children suffered, with about 150,000 cases a year in Britain. But the introduction of the pertussis vaccine in the late 1950s changed that. The number of cases fell almost to zero by 1973.

Uptake was reduced after claims that the vaccine caused brain damage, and cases increased between 1980 and 1985. By the time that the scare had been dismissed as unfounded, there had been as many as 100 needless deaths.

Last year there were only 185 cases in babies under 12 months old, 104 cases in the 1-5 age group, and 83 in those aged 5-9. But this study suggests that millions of cases are being missed. Doctors usually diagnose the disease only in babies too young to have been vaccinated, or in those with the characteristic whoop.

Anthony Harnden, a lecturer at Oxford and a GP, was the lead author on the paper. He said the problem was that the vaccine did not last very long, not that the immunisation policy was not working.

Children are immunised at the age of 2, 3 and 4 months, and are also given a pre-school booster. The condition is not usually serious in older children but can kill babies in their first year.

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