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Wednesday, 29 April 2015

Weak vaccine causes surge in whooping cough

A Monash University expert has teamed up with the United States Centers for Disease Control and Prevention to pinpoint the reason behind a surge in whooping cough cases over the past 10 years.

Associate Professor Manoj Gambhir used mathematical models to reveal that the kind of vaccine currently used to prevent the disease is less effective than the drug it replaced due to policy changes between 1992 and 1997.

The earlier vaccine, known as DTP, was used for decades in the US prior to the changes. It consisted of inactivated cells of Bordetella pertussis, the bacteria that cause whooping cough. While it worked well to prevent the disease, its side effects were distressing for both infants and parents.

"It caused prolonged crying, fever, and in some cases convulsions," A/Prof Gambhir says.

In response to this, the US Advisory Committee on Immunization Practice decided to shift to an acellular formulation, made up only of chemical parts of the bacteria.

"The acellular vaccine was a result of years of intense research that attempted to isolate the specific biological components of the bacteria that raise an immune response ... These products were the pertussis toxin as well as various proteins that are usually found on or in the bacteria," A/Prof Gambhir adds.

The fact that whooping cough vaccinations are administered several times throughout childhood allowed A/Prof Gambhir's team to observe the change in overall protection over decades and identify the reasons behind the shifts by using mathematical models.

"The age-group that has the largest number of cases is infants under the age of 1. However, there is generally a second age group in which we see a notable 'bump' in disease, and this age group used to be adolescents," he explains.

In recent years, the second 'bump' in the pattern has moved down to 7-11 year olds.

"This shift downward in age is in accordance with what one might expect if the first group of kids who are less protected than their predecessors is moving into the 7-11 year old age group.

The lower level of protection of this group of kids is well-explained by the fact that they were among the first group to be entirely vaccinated by the acellular vaccine," Professor Gambhir says.

"We used a rigorous procedure for fitting several mathematical models to the data, and we found that a drop in protection was always necessary to reproduce the rising pattern of disease we've seen in the US," said Professor Gambhir.

"Crucially, our model also matches the age pattern of disease," he added.

The results of the study were published this week in PLOS Computational Biology, a peer-reviewed journal focussing on research that furthers insights into living systems of all kinds through computational approaches.

A/Prof Gambhir's team suggest in the findings that the problem could be solved by providing booster shots.

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