Experts fear a new booster shot may be needed for 18-month-olds, and say it is more important than ever to immunise babies as early as possible to prevent exposure from people whose vaccines have worn off.

A study by NSW res-earchers found almost 80 per cent of whooping cough cases analysed were caused by a mutated bacterium that had stopped producing pertactin, a key protein the vaccine targets.

“It is a red light in terms of how well the vaccination works,” said Peter McIntyre, study author and director of the National Centre for Immunisation Research and Surveillance of Vaccine-Preventable Diseases.

“Immunisation is still the best and only way to protect against the most extreme cases of the disease.”

Cases in Australia have more than doubled since 1994.