The impact of COVID-19 vaccines on the Case Fatality Rate: The importance of monitoring breakthrough infections

Objectives: This study aimed to test the behavior of the case fatality rate (CFR) in a mixed population of vaccinated and unvaccinated individuals by illustrating the role of both the effectiveness of vaccines in preventing deaths and the detection of infections among both the vaccinated (breakthrough infections) and unvaccinated individuals. Methods: We simulated three hypothetical CFR scenarios that resulted from a different combination of vaccine effectiveness in preventing deaths and the effort s in detecting infections among both the vacci-nated and unvaccinated individuals. Results: In the presence of vaccines, the CFR depends not only on the effectiveness of vaccines in pre -venting deaths but also on the detection of breakthrough infections. As a result, a decline in the CFR may not imply that vaccines are effective in reducing deaths. Likewise, a constant CFR can still mean that vaccines are effective in reducing deaths. Conclusions: Unless vaccinated people are also tested for COVID-19 infection, the CFR loses its meaning in tracking the pandemic. This shows that unless effort s are directed at detecting breakthrough infections, it is hard to disentangle the effect of vaccines in reducing deaths from the probability of detecting infections on the CFR. (C) 2022 The Authors. Published by Elsevier Ltd on behalf of International Society for Infectious Diseases.

Automated summary

This study examines the behavior of the case fatality rate (CFR) in a population with vaccinated and unvaccinated individuals. The results show that the CFR is influenced by both the effectiveness of vaccines and the detection of breakthrough infections. Therefore, a decline in the CFR does not necessarily indicate the effectiveness of vaccines in reducing deaths, and a constant CFR can still mean that vaccines are effective.