Acellular pertussis shot still the best option

THE acellular pertussis vaccine is less effective in preventing disease in the longer term than its whole-cell predecessor but, with greater tolerability, it remains the best current option, say two experts in paediatric infectious diseases.

They were commenting on research undertaken in Queensland, that found pertussis was three times more likely to be reported among children who had had a primary course of acellular pertussis vaccine (diphtheria–tetanus–acellular pertussis or DTPa) than in those who had had a primary course of whole-cell vaccine (DTPw). The study was published in a research letter to the Journal of the American Medical Association. (1)

Professor Peter McIntyre, director of the National Centre for Immunisation Research and Surveillance, said bringing back the whole-cell pertussis vaccine was not the answer because the high frequency of side effects resulted in low pertussis vaccine coverage. He said this was associated with more hospitalisations and deaths during pertussis epidemics in the 1990s.

He said in the whole-cell era, many children had only one or none of the recommended three doses of vaccine because parents, often encouraged by their doctors, refused more doses after their child experienced side effects.

Professor McIntyre said the recent Queensland analysis suggested that we may have traded the problem of low coverage with the whole-cell vaccine (which led to more severe pertussis cases in infants) for the lesser, but still important, problem of more rapid waning of immunity after the acellular vaccine.

“We now have almost 95% of children receiving three doses of the acellular vaccine by two years of age, which is working well in preventing severe disease in the short term, but we are seeing more, less severe, pertussis cases in older children due to more testing and more rapid waning of immunity than occurred among children who got their three doses of whole-cell vaccine.”

Study co-author Associate Professor Stephen Lambert, senior research fellow with the Queensland Children’s Medical Research Institute, said it was important to remember that the whole-cell vaccine was “pretty unpleasant”.

“Up to about 50% of children would have an unpleasant injection site reaction, which could include redness, swelling and pain”, he said.

Less common reactions included fevers up to 40 degrees, febrile convulsion, inconsolable crying and hypotonic–hyporesponsive episodes.

Professor Lambert agreed that the acellular vaccine was effective in preventing severe disease. “Children who are immunised tend to have milder illness and transmit less frequently”, he said. “Even from the first one or two doses, the risk of an infant being hospitalised or ending up in intensive care drops very dramatically.”

The current epidemic was not related to the effectiveness of the vaccine, Professor Lambert said. “This is just one piece of a larger, more complex puzzle. It certainly doesn’t explain why we’re experiencing a nationwide pertussis epidemic.”

He said one factor that may partly explain the higher rate of pertussis was improved detection of the disease with the expansion of PCR testing.

Professor Lambert said vaccine manufacturers were looking into modifying their acellular products by adding new adjuvants. A new live-attenuated vaccine was in the pipeline, but this was still many years away, he said.

Professor McIntyre said investigations were underway in Australia into the vaccination of newborns and the immunisation of pregnant women to evaluate whether this protected newborn babies earlier.

“On top of these options to use the existing vaccines better, there’s no doubt that we do need better vaccines against pertussis, but the message that the acellular vaccine is still working well to protect against severe disease in young infants who receive it is an important one in terms of maintaining confidence”, he said.

- Nicole Mackee

1. JAMA 2012; 308: 454-456

Posted 13 August 2012