Australian researchers say doctors need be on the watch for new whooping cough cases after finding the old vaccine was more effective than the new one.
A study conducted by the Queensland Children's Medical Research Institute, the University of Queensland and Brisbane Royal Children's Hospital have found the old pertussis vaccine gave stronger immunity than the new version does.
A resurgence of the infection in younger children in recent years raised questions in the medical community about the immunity offered by the new vaccine.
Professor Stephen Lambert, who led the study, said children born in 1998 or after who had received the new vaccine were three times more likely to develop the infection.
"In 1998, a cellular vaccine made up of a few parts of pertussis bacteria was introduced with fewer side effects and what was thought to have roughly similar efficacy," he said.
But the new vaccine is not as effective as the older one, with immunity waning over the years.
Prof Lambert says clinicians should not exclude whooping cough as a diagnosis just because someone has had all of their vaccines.
Researchers hope their discovery will help lead to the development of a stronger vaccine and have warned doctors about their findings.
The findings will be published overnight in the Journal of the American Medical Association (JAMA).
The Madden family is not surprised by the findings, after having a brush with the disease two years ago.
Doctors at the time struggled to diagnose them because the children had been vaccinated.
Jean Madden knew something was wrong, especially after her husband Tim cracked one his ribs during a violent coughing fit.
Mrs Madden, along with her sons Harvey, now aged five, and two-year-old Gerard, who was a baby at the time, also contracted the disease from Mr Madden.
"It was actually quite scary because baby Geddy wasn't breathing properly," Mrs Madden said.
The family was eventually diagnosed by the third doctor they saw at the Wesley Hospital's emergency department and had to spend a week quarantined in their home.
Mrs Madden is now warning Australian parents to stay vigilant.
"Mothers, you know your babies and you need to trust your intuition," she said.
"You know when something's not right."
Researchers were keen to remind parents that vaccination was the best defence against the illness.