By: Dr. Geoffrey Modest
Here is a new analysis of Tdap vaccine effectiveness (VE) in the 2012 pertussis outbreak in Washington State (see DOI: 10.1542/peds.2014-3358). In a case-control study of adolescents born between 1993-2000, they correlated cases of pertussis in patients who had Tdap vaccination vs controls, matched by primary provider clinic and birth year. Those born between 1993-97 had a mix of whole-cell (DTP) and acellular (Tdap) vaccines, whereas those born from 1998-2000 had only the acellular Tdap vaccines. The 2012 pertussis outbreak in Washington State included nearly 5000 reported cases, with an unexpectedly high incidence in 13-14 year olds despite the high Tdap vaccination rate of 86%.
Results
–In those receiving only acellular vaccine (450 cases, 1246 controls), overall VE was 63.9% (50-74%), with
–VE in those receiving the last Tdap within 12 months was 73% (60-82%)
–VE in those receiving the last Tdap from 12-23 months was 54.9% (32.4-70.0%)
–VE in those receiving the last Tdap from 24-47 months was 34.2% (-0.03-58%)
–no difference if look at confirmed (B pertussis isolated) vs clinically-diagnosed cases
–In the older group receiving a mixture of DTP and Tdap vaccines (386 cases, 1076 controls), overall VE was 51.5% (26.1%-68.1%), with 77% having received their last Tdap at least 3 years before the outbreak (as an older cohort, they received their 11-12 yo booster longer before the epidemic). So, the authors stratified the patients by either having had the Dtap less than or more than 4 years before the outbreak and found essentially the same VE’s (51.5% and 52.2% respectively for <48 months and >48 months). Given the differences in times from the last vaccination, the authors were unable to compare directly the findings between those who received the mixed vaccines vs those with only Tdap.
–There was more protection with the Tdap brand Boostrix than Adacel, though the confidence intervals overlapped
See blog from a few months ago for a more detailed critique of the acellular pertussis component, though I did append it below. I also included below a couple of graphs from the CDC, the first showing the increase in pertussis since the first few years of the 2000 decade, and the second with the age distribution of cases
(CDC, National Notifiable Diseases Surveillance System and Supplemental Pertussis Surveillance System)
So, a few additional comments:
–the graphs below clearly show the dramatic increase in cases in those <1 yo, reinforcing the recommendation that pregnant women receive the Tdap vaccine during pregnancy, and the waning immunity noted above supports the recommendation that this be repeated for each pregnancy.
–the increase noted in the graph below in 7-10 year olds, a cohort that received only the acellular Tdap vaccine, also reinforces the attenuated protection, as found in this study
–the actual utility of Tdap vaccination in post-adolescence is pretty unclear. Seems like there is, at best, pretty transient protection (older people have been part of the outbreaks). all of this begs for a new pertussis vaccine with improved immunogenicity and longer duration of protection (the lower incidence of pertussis found on those adolescents receiving a mixture of DTP and Tdap vaccines suggest longer-lasting VE, even though the last DTP was many years before.)