LAUSR

Objectives Test the behavior of the case fatality rate 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 simulate three hypothetical case fatality rate scenarios that result from a different combination of vaccine effectiveness in preventing deaths and the efforts in detecting infections among both the vaccinated and unvaccinated individuals. Results In the presence of vaccines, the case fatality rate depends not only on the effectiveness of vaccines in preventing deaths, but also on the detection of breakthrough infections. As a result, a decline in the case fatality rate may not imply that vaccines are being effective in reducing deaths. Likewise, a constant case fatality rate can still mean that vaccines are effective in reducing deaths. Conclusions Unless vaccinated people are also tested, the case fatality rate loses its meaning in tracking the pandemic. This shows that unless efforts 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 case fatality rate.