A WHOOPING cough epidemic has led NSW to record its highest number of new cases of the disease.
More than 13,000 people were infected in 2011, and one baby died, surveillance figures published in the CSIRO's NSW Public Health Bulletin show.
Experts do not know how to stop the spread of the disease, as even people who are vaccinated are at risk because the vaccine begins to wear off after only a couple of years.
Alexander Rosewell, who led the surveillance report team, said 2011 was an epidemic year.
"It is largely due to a build-up of susceptible people in the community," he said. "When there are low levels of protective antibodies in the community you are more likely to see an outbreak."
Mr Rosewell said most people vaccinated their children, but the high rates of whooping cough helped create a chain-reaction.
"When there's more disease around then … the people who are susceptible have a greater chance of being exposed," said Mr Rosewell, who is a vaccine-preventable diseases epidemiologist with Health Protection NSW.
The director of health protection for NSW Health, Jeremy McAnulty, said adults could be unaware they had it.
"When you are an adult you might just have a cough and it doesn't go away, but you can spread it to little babies who are the most vulnerable to the disease,'' he said.
Overall, vaccine-preventable diseases increased in 2011, with rates of measles more than tripling from 26 cases in 2010 to 90 in 2011.
Vaccination rates are currently greater than 90 per cent for one and two-year-old children, according to separate research published in the same journal.
Study leader Brynley Hull, an epidemiologist from the National Centre for Immunisation Research and Surveillance, said a high-profile increase in so-called "conscientious objectors" across Australia had not affected vaccination rates.
But there were still areas where rates were lower, particularly among teenagers who needed booster shots.
"It means there are pockets of susceptibility," he said.
Helen Quinn, a research fellow with the centre, said people who were vaccinated but caught whooping cough got a less severe form, but could still pass it on.
"Age is a big factor with pertussis (whooping cough); being young is more dangerous," she said.
NSW, in line with the rest of the world, had switched vaccines more than a decade ago to one that had fewer side-effects but appeared to not last as long. But adding more booster vaccinations probably would not work.
"You can't fund it and people aren't going to want to have vaccines that often," she said. "There is still a need for long-lasting better vaccines".
A senior research fellow at the school of public health at the University of Sydney, Julie Leask, said there had been too much attention paid to vaccine-objectors, when many people were simply stopped by ''logistical issues'' such as poverty, and difficulty getting to their doctor on time.