Undervaccination and severe COVID-19 outcomes – Authors' reply

Abstract

We thank our colleagues for their interest in our Article. In response to Ari R Joffe, we provided estimates of the number of severe outcomes associated with a fully vaccinated counterfactual. These estimates controlled for time since the last positive test. We believe that this provides sufficient context regarding the absolute risk reduction associated with full vaccination in an intuitively understandable form. There is no discernible unconditioned healthy vaccinee bias in our study population, which can be seen from table 1 in our Article, where rates of undervaccination decreased with our primary measure of health—number of QCOVID risk groups —indicating greater vaccine uptake in individuals with more health conditions. There is also no consistent evidence of a healthy vaccinee effect across the age group strata from table 2 of our Article. Our suggestion is that an uncontrolled selection effect might explain similar adjusted hazard ratios for severe COVID-19 outcomes in unvaccinated individuals versus vaccinated individuals, but with a deficit of at least one dose. The suggested mechanism is that the number of QCOVID risk groups was an imperfect measure of health and, even after conditioning on it, healthier individuals might have been less likely to be vaccinated. It is not possible to tell definitively whether this is the case or not from our study.

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https://doi.org/10.1016/S0140-6736(24)02866-6