Previous research has indicated that neither immunisation nor infection provides lifelong immunity to Bordella pertussis.
Between 2001 and 2005 researchers tested blood samples from 172 UK children aged five to 16 years who presented with a cough lasting 14 days or more.
They found evidence of B pertussis infection in nearly 40 per cent of the children. More than 85 per cent had been immunised.
Children with evidence of pertussis were more likely than others to still be coughing two months after the start of their illness.