FDA study helps provide understanding of rising rates of whooping cough, response to vaccination

An FDA study is helping to provide a better understanding of vaccines for whooping cough, the common name for the disease pertussis. Based on an animal model, the study published November 25, 2013, in The Proceedings of the National Academy of Sciences, shows that acellular pertussis vaccines licensed by FDA are effective in preventing the disease among those vaccinated, but suggests that they may not prevent infection from the bacteria that causes whooping cough in those vaccinated or its spread to other people, including those who may not be vaccinated.

Whooping cough is a contagious respiratory disease caused by Bordetella pertussis bacteria. Whooping cough rates in the United States have been increasing since the 1980s and reached a 50-year high in 2012. There were 48,000 cases reported last year despite high rates of vaccination. Initial symptoms include runny nose, sneezing, and a mild cough, which may seem like a typical cold. Usually, the cough slowly becomes more severe, and eventually the patient may experience bouts of rapid, violent coughing followed by the “whooping” sound that gives the disease its common name, when trying to take a breath. Whooping cough can cause serious and sometimes life-threatening complications, permanent disability, and even death, especially in infants and young children.

FDA conducted the study in baboons, an animal model that closely mimics the way whooping cough affects people. The scientists vaccinated 2 groups of baboons-1 group with a whole-cell pertussis vaccine and the other group with an acellular pertussis vaccine currently used in the United States.

There are 2 types of pertussis vaccines, whole-cell and acellular. Whole-cell pertussis vaccines contain a whole-cell preparation, which means they contain killed, but complete, B pertussis bacteria. The acellular pertussis vaccine is more purified and uses only selected portions of the pertussis bacteria to stimulate an immune response in an individual. In response to concerns about the side effects of the whole cell pertussis vaccine, acellular vaccines were developed and replaced the use of whole-cell pertussis vaccines in the United States and other countries in the 1990s; however, whole-cell pertussis vaccines are still used in many other countries.

The animals were vaccinated at aged 2, 4, and 6 months, simulating the infant immunization schedule. The FDA study found that both types of vaccines generated robust antibody responses in the animals, and none of the vaccinated animals developed outward signs of pertussis disease after being exposed to B pertussis. However, there were differences in other aspects of the immune response. Animals that received an acellular pertussis vaccine had the bacteria in their airways for up to 6 weeks and were able to spread the infection to unvaccinated animals. In contrast, animals that received whole-cell vaccine cleared the bacteria within 3 weeks.

“These investigators recently developed the first animal model that reproduces some of the clinical symptoms of severe pertussis disease found in children,” Linda Lambert, PhD, chief of the Respiratory Diseases Branch in The National Institute of Allergy and Infectious Diseases' Division of Microbiology and Infectious Diseases, told Formulary.  “This study reports how the model is being used to address key research questions about pertussis vaccines and disease in humans. The ability of vaccinated animals to spread infection to non-vaccinated animals may help explain, in part, the recent resurgence in pertussis cases in the United States and elsewhere.”

“This study found that animals vaccinated with acellular vaccine are protected from severe disease, underscoring the importance of continuing to support and promote the use of current acellular vaccines in infants and other populations,” Lambert continued. “The findings of this study also indicate that pertussis-vaccinated individuals can still acquire infection and, if so, may transmit the infection to those who are naïve, including newborns.”

It did not directly evaluate currently recommended vaccination practices for pertussis.

Current recommendations are for adolescents and adults who anticipate having close contact with an infant under aged 12 months to receive a dose of Tdap if they have not previously received it-a strategy referred to as “cocooning”-and for pregnant women to be vaccinated in order to enable the transfer of maternal antibodies to the newborn.  This study did not directly evaluate the effectiveness of cocooning or of maternal immunization strategies.

“This animal model provides researchers with an important tool for improving our understanding of pertussis, which includes determining how the bacteria-that causes pertussis-is spread from person to person and opportunities to assess which aspects of the immune response to natural infection and vaccination may be most important to generate for protection from disease,” Lambert said.