Waning pertussis immunity in England/Wales, and bad reporting in Australia
A new study has come out from the UK, assessing the case for the National Health Service adding an adolescent booster of the pertussis vaccine to the schedule.
I’m sure many of us in countries such as Australia, New Zealand, US and some European countries may be surprised to hear that this routine (to us) shot is not on the NHS schedule, but they did pioneer an advanced infant schedule which saw increased protection from pertussis in this age group. As you can see from the figure below, introduction of 2, 3 and 4-month pertussis shots onto the UK schedule in 1990 was followed by an absolute decrease in infant pertussis cases in the < 3 mo, 3-5 mo and 6-11 mo age groups (top graph), and following this change in the schedule, the < 3 month age group went from representing under 15% of cases in infants to 40-50% of these cases.
However, as well as this clear evidence of effectiveness in the young, this earlier study also found pertussis to still be maintained in the adolescent and adult populations, where pertussis vaccine coverage is poor. As I’ve discussed before, duration of protection from the currently used acellular pertussis (aP) vaccine is estimated to last from four to twelve years, with recent result indicating the evolution of the bacterium may be contributing to early waning of immunity. While some countries have controlled pertussis in the adolescent cohort with later booster doses, the UK health authorities are currently considering adding such a booster to their schedule.
The authors of this new study set out to assess what proportion of persistent coughing illness in adolescents in the Thames Valley area was being caused by pertussis infection, and how prevalent previous encounter with pertussis may be in this cohort. The study authors considered adolescents with persistent cough that had evidence of previous pertussis immunisation, and the correlation between pertussis incidence and time since last booster was much like what we’ve seen previously:
The increased risk of pertussis infection in children who received pertussis immunisation over seven years ago is consistent with previous findings that immunity wanes over this time period, to the extent that non-recent vaccinees are at similar risk of catching the disease to unvaccinated controls.
Keep in mind that this study only included immunised children, so there is no quantification of absolute risk relative to the unimmunised, though, as discussed above, that absolute protection has been demonstrated. That said, these results do answer the researchers’ question of how risk of pertussis increases with time in this age cohort not covered by a booster shot.
The authors also assessed the number of times six patients coughed in a 24 hour period. Two coughed around 200 times, three around 500 times and one 1600 times, and on the basis of this the authors reinforce that, despite the established protection of recent vaccine-recipients from more severe pertussis disease, the once aP vaccinated can still suffer relatively severe symptoms on eventual infection. This is not the first time the authors have reinforced in a paper the importance of pediatricians considering pertussis as a potential diagnosis in cases of persistent cough, and hopefully it gets this point across to pediatricians who keep track of the literature.
However, the big, headline-grabbing result from this study is that, of cases of persistent cough in previously pertussis immunised children presenting to primary care physicians, 20% had evidence of recent pertussis infection. While this prevalence of pertussis is indeed a matter of concern, it is worth pointing out several nuances that do not seem to have made it into the media as well as this soundbite:
- The rate of pertussis in this age cohort found in this study is decreased compared to the rate seen prior to introduction of the preschool pertussis booster
- The cohort with the greatest infection rate (by 2-3 fold) in this study was one in which aP-induced immunity is known to have largely waned
While it’s quite clear that the anti-immunisation lobby is going to jump all over selective quotations of this paper as evidence the pertussis vaccine is useless, in actuality these results indicate less pertussis infections in this age group since the introduction of the preschool booster (about half of the previous rate, despite over a third of this study occurring during a pertussis epidemic), and that the preschool booster has pushed back the time of immunity waning by several more years.
Oh, and another outcome of these results that could lead to unwarranted excitation of the anti-immunisation lobby is misreporting. From Medical Observer on this study yesterday:
Furthermore, the risk of pertussis was more than three times higher in children who had received the pertussis booster less than seven years previously.
Which you might notice contradicts the results of the study, and the BMJ‘s plain language ‘Research News‘ article on the study which I suspect is were many news outlets got their interpretation of the study from:
The risk of pertussis was more than three times higher (21/53; 40% (26% to 54%)) in children who had received the preschool pertussis booster vaccination at least seven years before presenting with cough compared with those given the booster less than seven years previously (20/171; 12% (7% to 17%)).
With such a severe mis-reporting of the study’s results, it’s only a matter of time until the anti-immunisation lobby, which regularly repeat the lie that vaccines increase your susceptibility to the disease and who never bother to follow up on news reports to see whether their claims are backed by the primary sources, touts this article as evidence of its claims.
Oh wait, it’s already happening.
Screenshot from the Australian Vaccination Sceptics Network twitter page. As you can see the, this anti-vax group have quoted verbatim Medical Observer‘s headline: “Pertussis found in 20pct of vaccinated children“. Great choice of wording there, MO.
Update: As of “3.34pm 26 June 2014“, Medical Observer have ammended their article to correct the highlighted error, but have maintained the misleading click-bait headline “Pertussis found in one fifth of vaccinated children“.