Whooping cough outbreak despite vaccination

By Dr. Ananya Mandal, MDMar 21 2012

There is a spike in whooping cough cases that may have been caused by a new strain of the bacteria that is resistant to existing vaccines, new research suggests.

Last year the number of diagnosed whooping cough cases in Australia rose to 38,000 - the highest since records began in 1991. Microbiologists from the University of NSW studied samples taken from patients in four states and found the new strain of the bacteria now accounts for 84 per cent of whooping cough cases. Among the recent victims was a five-week-old baby boy who died in an Adelaide hospital 18 months ago.

Professor Lyn Gilbert, who is the director of the Centre for Infectious Diseases and Microbiology at Sydney's Westmead Hospital, says the strain is breaking through the protection of vaccines. “What we suspect, although it's circumstantial evidence at this stage, is that one of the reasons that there has been a significant increase in Australia and many other countries in the last few years is that this strain is not affected as much by the vaccine as the older strains were,” she said. “So that it's been able to sort of break through the protection offered by the vaccines.”

Although the strains were present in Australia as early as 2000, they accounted for only 31 per cent of all samples collected between 2000 and 2007 – suggesting they have flourished alongside the current vaccine against the potentially fatal respiratory infection. The strains have “swept across Australia during the epidemic period” according to Ruiting Lan, from the school of biotechnology and biomolecular sciences. More than 13,000 whooping cough cases were diagnosed in 2011 – an all-time high.

The Children's Hospital at Westmead treated 76 children for whooping cough in 2011, up from 47 the previous year. The Sydney Children's Hospital treated 34 children in 2011, up from 16 the previous year.

Doctor Jeremy McAnulty from New South Wales Health says that does not mean people should not be vaccinated. “Before vaccinations against whooping cough it was a major killer, there were hundreds of deaths,” he said. He says even where the vaccine does not prevent the bacteria, it still reduces its severity.

“The vaccine is still the best way to reduce transmission of the disease and reduce cases, but it appears to be less effective against the new strain and immunity wanes more rapidly. We need to look at changes to the vaccine itself or increase the number of boosters,” said Dr Lan, whose analysis of cultured bacteria from 194 whooping cough patients was published last week in the Journal of Infectious Diseases.

Peter McIntyre, the Director of the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, said he had begun a project with Associate Professor Lan to identify any links between disease severity and different strains in Australia. “It's the missing piece of the jigsaw,” he said. “We need to collect clinical data to confirm or refute the idea that there's something different about the so-called mutant strains.”

Whooping cough deaths were higher during the 1990s, when immunisation rates were lower, than during the current epidemic, he said. “The vaccine may not be as good as we'd like but it does seem to be preventing the most extreme cases,” said Professor McIntyre, who suggested the reintroduction of a pertussis booster jab at 18 months old might help check the disease's spread.

However this has strengthened the opponents of vaccination. Meryl Doorey from the Australian Vaccination Network says the research is only the latest in many published articles over the last 20 years showing the bacteria has mutated. “The vaccine no longer contains the same strain of the bacteria as that which is circulating in the community,” she said. “In addition, the newer strain of the bacteria seems to be more virulent which is why we're seeing infants dying of whooping cough, which we haven't seen for a very long time.”

A spokeswoman for the Department of Health and Ageing said officially recorded cases of whooping cough had increased sevenfold since 2007, saying “suboptimal vaccine coverage” was among possible reasons, along with better detection. Dr Lan's work and other research linking particular strains with the continuing epidemic was “constantly under consideration and review”, she said. “Optimizing the schedule of boosters of existing vaccine ... is likely to be important at this time,” said the spokeswoman. She said the department's immunization advisers were reviewing the vaccine schedule to determine whether new jabs should be recommended for babies, who are most at risk from the infection.