The move to an artificially created vaccine for whooping cough is behind an increase in cases of the deadly disease in the US, a new study suggests.
The findings highlight the need to do similar research in Australia where whooping cough cases have spiralled upward in the past decade, co-author Associate Professor Manoj Gambhir, from the University of Monash, says.
In 2012 the US saw the highest number of pertussis (whooping cough) cases since 1955.
At the same time there has been a shift in the age group reporting the largest number of cases from adolescents to 7 to 11 year olds.
In the paper, published today in PLOS Computational Biology, Gambhir and colleagues use mathematical modelling of 60 years of pertussis disease data to determine what best explains this increase.
Their research finds the level of protection of the currently used acellular vaccine is lower than that of the previously used whole-cell vaccine.
Gambhir says the original whole-cell vaccine developed in 1942 was very effective.
Following introduction of vaccination, the reported disease incidence in the US dropped from 150 cases per 100,000 each year before 1940, to the point of near elimination in the mid-1970s when there were just 0.5 reported cases per 100,000 population.
“Now in the past decade we have seen a rise from that low to about 10-20 cases per 100,000,” says Gambhir.
“It’s the kind of numbers we haven’t seen since the mid-1950s so we are right back to the period before vaccines were introduced.”
Gambhir, who led the study with Dr Thomas Clark at the Centers for Disease Control and Prevention, in Atlanta, Georgia, and Professor Neil Ferguson, of the Imperial College London in the UK, says while the number of cases remains low the trend is upward.
“It’s the sort of five-year by five-year rise that is the concern because the worry is it could go back to those levels at which we would start seeing infant deaths,” he says.
New vaccine to blame
Gambhir says in 1991 researchers developed a new vaccine to address public concerns that the whooping cough vaccine caused a reaction in some children.
This vaccine, known as acellular pertussis vaccine, used particular components of the bacteria that were believed to generate the immune response, but was essentially artificially created.
Gambhir says the impact of the change in vaccine has taken time to show in the data.
“You didn’t see an immediate increase [in whooping cough],” he says. “It has taken cohorts of children to have all of their doses to be the new vaccine for the increases in disease to manifest themselves.”
Gambhir says the efficacy of the acellular vaccine is still high – around 80 per cent protection for the first three doses – but there has been a “significant drop” when compared with the older whole-cell vaccine (90 per cent protection).
He says current global research into new pertussis vaccines needs to continue, however their study also suggests changes to the current schedule of pertussis vaccinations and further booster doses may be required.
Gambhir says while there is debate over vaccination in the media, they did not find a drop in the overall level of whooping cough vaccination in the US.
“It could be the case as an alternative explanation to some of this that there are small clusters of people who refuse vaccine and they tend to live together and socially network together in which case among those small clusters there may have been a rise in disease and that may have been caused by their refusal to vaccinate,” he says.
However he says this cannot account for the generalised rise in the disease.
Australian whooping cough rates
Gambhir is working with colleagues at the University of Melbourne who are tracking the disease in Australia, where the annual rate of whooping cough has hit more than 100 cases per 100,000 in the past decade.
While Gambhir says more sensitive testing in Australia would account for some of the increase, “it is quite urgent we come up with some explanations of what is going on here”.
Their work will also feed into a collaboration with the World Health Organisation on examining whooping cough figures in the US, UK and Australia to determine if the same trends are evident in each country.
Fact File: Whooping cough