099 - Secondary Familial Attack Rates from Pertussis in Vaccinated and Unvaccinated Children

This study is a follow-up analysis of previous results by Kendrick and Eldering of a clinical trial in Grand Rapids, Michigan, of a whole-cell pertussis vaccine.

Rather than the whole study population, this study focused only on 165 families that were definitely exposed to pertussis from one of their own members. The other members' immunity status was known from history of vaccination or the disease itself.

What they saw was that, of all 78 vaccinated subjects, only 28 got whooping cough (36%). All these were under age 7. In contrast, of those without history of vaccination or disease, 79% got sick. Of those unvaccinated under age 7, 92% were attacked (the rate decreased with increasing age of subject). 36% vs. 92% for those most vulnerable? Not bad.

Most of the primary cases, that brought the disease to a family, were in the older category, confirming that older children often bring it to their younger siblings. Interestingly, with the subjects who had the disease before, 5 of the 6 that got sick were over 7 years old, so it seems there's a period after which the immunity is not so great, even after natural infection. The study was set up to analyze that in depth though.

Also noteworthy is that, of 172 primary cases that brought the disease to the families, 157 were unvaccinated (91%), 12 were vaccinated (7%), and 3 had the disease previously. There's some herd immunity in action, perhaps, but it's hard to tell.

This wasn't a very rigorous study (lack of blinding or placebo), but considering that an intimate exposure, such as in the same household, is probably the most difficult to have immunity against, there seems to be some effect. And apparently whooping cough is so contagious that 80-90% get sick when exposed to it this way, if not immune somehow.

References:

Kendrick, P. L. Secondary Familial Attack Rates from Pertussis in Vaccinated and Unvaccinated Children. Am. J. Epidemiol. 32-SectionA, 89–91 (1940).