Table 3.

Potential Reasons for Vaccine Effectiveness (VE) estimates that are different from vaccine efficacy results [3].

VE estimate valid	VE estimate not valid
•Population being studied has different VE for epidemiologic or biological reasons	•Error in implementation (e.g. enrollment of persons not meeting case definition, poor specimen collection/handling)
•Vaccine mishandling	•Biases
•Systematic error in vaccine administration	•Unmeasured or incompletely controlled confounders
•Problems with vaccine batch	•Chance finding; more likely with small sample size
•Waning immunity resulting in lower VE
•Different outcome or schedule is being evaluated from clinical trial
•Vaccine less effective due to mutations in SARS-CoV-2 virus
•Contribution of vaccine associated enhanced disease (VAED) (especially severe disease outcome)
•Prevalence of prior infection in population different from that of efficacy study