The Woolcock Institute of Medical Research

Rates of whooping cough across Australia are on track to be higher than at any time since 2016. There have been 12,000 recorded cases in the past six months compared with just 2500 in the whole of 2023. The current epidemic is mirrored in developed countries around the world and, atypically, cases are concentrated in the 10-14 year age bracket.

Whooping cough (caused by Bordetella pertussis) is a severe acute respiratory disease. It's also known as the 100-day cough because the cough can last for up to 12 weeks. It starts off with very similar symptoms to a cold or flu – a runny nose, mild cough and low-grade fever that lasts for one to two weeks. During the second week, however, the cough becomes more frequent and severe. In babies and infants, bouts of coughing may end with vomiting, and the continuous coughing means they struggle to breathe and gasp for air, making the whooping noise which gives the disease its name. Complications can lead to brain damage and even death.

While much of the media commentary about the recent surge in cases has focussed on areas with lower vaccination rates such as Queensland’s Gold Coast, Sunshine Coast and northern NSW, the answer may be more complex and nuanced.

VACCINE NOT VACCINATION?

Dr Laurence Luu, a microbiologist at the University of Technology Sydney who spoke recently at the Woolcock’s Friday Seminar series, completed a PhD in 2018 on the re-emergence of whooping cough which led to a national call to improve the current vaccine.

He explained that in Australia, a whole cell vaccine for whooping cough was introduced in the 1950s and cases were reduced to very low levels. Due to side effects such as fever and increasing vaccine resistance following a report linking vaccination to autism, a three-component acellular vaccine was introduced in 1997. With the switch, the number of whooping cough cases steadily increased, resulting in two epidemics – in 2008-2012 and a smaller one in 2014-2017.

While whooping cough is cyclical in nature, with an increase in cases every three to four years usually peaking during spring (September to November), the number of cases seen during the 2008-2012 epidemic was massive at around 39,000 cases in Australia. The expected peak in 2020-2022 was averted due to COVID lockdowns and mask wearing, which resulted instead in record low levels but, right now, in winter in Australia, numbers are rising quickly.

CHANGING VACCINE TARGETS

So why are we seeing a re-emergence of whooping cough in a population which has vaccination rates as high as 95-96 percent? One reason is vaccine selection, says Dr Luu.

“When we changed from the whole cell to the acellular vaccine in Australia, the number of antigens targeted went from hundreds to just three which meant more selection pressure on those three to evolve. A study of cases over more than 40 years showed that in 2008 strains which no longer produced one of the antigens targeted by the vaccine were emerging and this had expanded to 90 percent of strains sequenced from 2013-2017. So, the vaccine has one less target.”

New strains have continued to emerge, change and adapt. In 2019, testing showed a further increase in resistant strains, a number of additional strains emerging and undetected cases of Bordetella holmseii, a closely related bacteria which can cause whooping cough-like symptoms and in severe cases can lead to meningitis and endocarditis.

“The whole cell vaccine is still being used in the developing world and we are not seeing the new strains of the bacteria or the surge in cases there. It would be really difficult to go back to that here, so we need to develop a new vaccine or improve the one we have. Protection will be required for other Bordetella species also.”

Dr Luu says there is real potential for better protection, with researchers identifying 22 proteins which are potential antigens and which cannot be inactivated like the one which has reduced the effect of the current vaccine.

SHIFTING AGE AT INFECTION

The answer as to who is affected may also lie with the vaccine.

”We usually see most cases in the 0-9 age range but this time the main age group affected is 10-14 year olds. We’re not really too sure why, but we know that it is reflected in Europe and America. One hypothesis is host-side – anyone born before 2000 would have gotten the whole cell vaccine first and the difference in that initial vaccine might have skewed their immunity.”

For younger children, since 2015 pregnant women have received vaccinations for both flu and whooping cough at between 20 and 32 weeks gestation in Australia which could offer protection, especially to newborns in their first 3-6 months of life.

Or it could be something simpler. “It could be that the age bracket has just shifted because we didn’t get the peak we expected in 2020-2022. It could be a different strain that’s circulating post-COVID, we're not sure because we don't have the strains to investigate that yet. It could be just that it got into schools and so that's where it's spreading.”

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