Issue 47 / 3 December 2012

A PROGRESSIVE decline over time in the effectiveness of the acellular pertussis vaccine has been confirmed by US research, which has also found that unvaccinated children are eight times more likely to contract the disease than vaccinated children. (1)

Professor Peter McIntyre, director of the National Centre for Immunisation Research and Surveillance, told MJA InSight that although the US study confirmed earlier research showing immunity waned more quickly after the acellular vaccine than after the whole-cell vaccine, it also found that 7.8% of unvaccinated children contracted pertussis, while only 0.9% of vaccinated children contracted the disease.

The case–control evaluation of pertussis cases, published in JAMA, found that as time increased since the last DTPa (diphtheria–tetanus–pertussis vaccine) dose, the odds of being diagnosed with pertussis increased. It found an estimated vaccine effectiveness of 98.1% at 12 months or less since the fifth and final dose, which dropped to 71.2% 60 months or more after the final dose.

However, Professor McIntyre said the acellular vaccine appeared to be “doing its job” in terms of preventing severe cases of hospitalisation in young babies. “If you just look at the most severe end — the babies being hospitalised and the deaths — you see that despite much better and more widely used tests, we’re not finding more pertussis in those categories.”

While the US had a five-dose DTPa schedule by 6 years of age, Australia had only four doses since the 18-month booster was discontinued in 2003.  Professor McIntyre said it would be challenging to make the case to reintroduce the 18-month dose under Australian criteria for national program funding unless it could be shown that this would significantly reduce severe cases in young infants.

“As cases in older children have nuisance value, but are not severe enough to cause hospitalisation, the case for an additional dose would be difficult to make on economic grounds”, Professor McIntyre said. Australians tended not to get vaccines unless they were provided under the national program.

However, pertussis vaccination during pregnancy was on the table, he said. “Theoretically it should be a good thing, but there’s not actual data to confirm that, so it’s probably going to take a year or so before any data like that emerges.”

An accompanying editorial in JAMA said in addition to assessing the current schedules, consideration should be given to a return to whole-cell pertussis vaccine for the first dose, “which may prime for better immune response to subsequent doses of acellular vaccines”. (2)

But Professor McIntyre said this suggestion was “pie in the sky”. He said not only was the whole-cell vaccine no longer available in Australia, the side effects caused would be likely to once again deter parents from seeking booster vaccinations for their children.

Emeritus Professor Tony Basten, senior principal research fellow at the Garvan Institute, said the 89% vaccine effectiveness shown in the US research would seem to be fantastic, but for highly infectious diseases, like pertussis and measles, “it’s not good enough” with over 95% needed.

He said although the vaccine may need improvement, it shouldn’t stop people from becoming vaccinated.

Professor Basten and Professor McIntyre, with 10 other eminent scientists, recently prepared a document about vaccination for the Australian Academy of Science. The booklet, The science of immunisation, aims to provide parents with accurate information about immunisation to allay their concerns.

Professor Basten said despite overall steady vaccination rates, mini epidemics of childhood infections like whooping cough still occurred and the publication was a timely reminder of the severity of the diseases targeted by vaccination programs.

“The anti-vaccine lobby is becoming more vocal and that’s a worry for the community at large, so the aim of the academy was to bring together a group of experts in different aspects of vaccinology to provide objective information so individuals and families can make up their own minds”, he said.

There had been so much focus on the very rare severe adverse reactions to vaccination — particularly on social media — that the seriousness of the diseases these vaccines were preventing was often forgotten.

“The vast majority of reactions to vaccines nowadays are mild — local not systemic — but as people just haven’t seen what these infectious diseases can do, awareness of the desirability of vaccination is less”, Professor Basten said.

– Nicole MacKee

1. JAMA 2012; 308: 2126-2132
2. JAMA 2012: 308: 2149-2150

Posted 3 December 2012

Strongly held beliefs are often used to justify a parent’s refusal to have their children immunised. See Jane McCredie: The right reason

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