Understanding Pertussis

Whooping cough is rife if you’ve seen the news. Certainly 2012 is possibly the worst UK outbreak for 20 years, the south west is particularly bad. It is extremely difficult to get a clear picture of the reasons, and part of that is poor use of statistics and terms. Sloppy thinking will kill us all in the end if something else doesn’t get us first đŸ˜‰

The twit-o-sphere is rife with skeptics apologizing to vaccine denialists because they assumed it was vaccine denial. Their reason for apologizing is that “vaccination rates” (for DTAP vaccine) are at an all time high. Okay the apology is probably not sincere since vaccination is still the most effective method we have for battling Pertussis, not vaccinating is a bad idea and kills kids.

“Vaccination rates” are not what matters, what matters is the proportion of people with immunity to Pertussis. This largely depends on historical vaccination rates, how effective the vaccine is, and how many people have had pertussis (which presumably affects the cyclical nature of the disease), but vaccination for the last 10 years has been at good levels in the UK, so vaccine coverage is probably at an all time high.

The DTaP vaccine is probably slightly less effective than its predecessor, but was preferred due to having fewer side effects. Pertussis may be a particular risk for returning if herd immunity falls. There is some suggestion that other strains of pertussis may be causing a problem.

So we have:

A disease where vaccines are much better at preventing serious break-through disease, than at preventing infection.

A period of years since we started using a marginally less effective vaccine.

A cyclic upswing in cases.

Increased surveillance programmes.

The arrival of mutant strains against which the vaccine may be less effective (data about which that is available appears locked behind academia’s pay walls).

So the explanation is probably due to a combination of things, the change is probably not as large as it appears.

The absolutes level of pertussis in the UK is still something like a 2% of the level it was in the last year before vaccination. Your risk in the southwest this year of a clinically demonstrated pertussis  infection was of the order of 1 in 5000, less if vaccinated, considerably higher (10 to 20 fold higher) if not vaccinated.

The big risk group is babies before they can be vaccinated, the government response to vaccinate pregnant mothers appears rational.

It seems likely that more booster vaccinations in adults who interact with babies would also help.

20% of the worlds children receive no DTaP vaccine, generally the poorest with the least access to medical care, and thus those who would benefit most. Vaccinating them would not only save 100,000’s of lives a year it might also reduce the scope for mutant pertussis variants to arise.

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