Figure 1.

Bias classification framework for observed negative VE against infection. Sources (or mechanisms) of bias capable of producing false-negative VE against infection either affect (1) true levels of infection by vaccination status or (2) observed levels of infection by vaccination status. An example of a mechanism of bias for (1) is differences in exposure (arrows), where higher exposure for vaccinated individuals compared to unvaccinated individuals produces higher infection levels among vaccinated. This bias persists when all infected and uninfected vaccinated and unvaccinated are observed (dark blue and red). In contrast, an example of a mechanism of bias for (2) is differences in testing by vaccination status (due to testing behavior or testing access) where testing differences only result in the perception of higher infection levels. This bias occurs because the unobserved uninfected and infected (light blue and light orange) are excluded from VE measurements. VE, vaccine effectiveness.