AsianScientist (May 8, 2015) – 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 ten years. The results of the study were published in PLOS Computational Biology.
Associate Professor Manoj Gambhir, Department of Epidemiology and Preventive Medicine, 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,” 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,” Gambhir adds.
The fact that whooping cough vaccinations are administered several times throughout childhood allowed 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 one. 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 seven to 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 seven to 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,” Gambhir says.
Gambhir explains how his model has helped identify gaps in the immunization program:
“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 Gambhir.
“Crucially, our model also matches the age pattern of disease,” he added.
Gambhir’s team suggest in the findings that the problem could be solved by providing booster shots.
The article can be found at: Gambhir et al. (2015) A Change in Vaccine Efficacy and Duration of Protection Explains Recent Rises in Pertussis Incidence in the United States.
———
Source: Monash University; Photo: NHS Employers/Flickr/CC.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.
