Although our estimate of 81% vaccine effectiveness with partial vaccination is similar to results provided in other reports,
1,2,7,21,22 this estimate is based on a relatively brief follow-up period (with a median of 22 partially vaccinated days, as compared with 69 fully vaccinated days, per participant). Second, we could have overestimated vaccine effectiveness if we disproportionately failed to detect infections among vaccinated participants because of attenuation of viral RNA load after vaccination or because of reductions in the sensitivity of RT-PCR assays associated with specimen collection by participants and shipping of specimens.
23 Third, we have not completed genetic sequencing for all viruses. Fourth, because there was a relatively small number of breakthrough infections, we could not differentiate attenuation effects associated with partial vaccination from effects associated with full vaccination. Similarly, sparse data reduced the precision of estimates, although the consistency of trends across measures affirms the direction of the overall effect. Fifth, because of the sparse data and limited racial and ethnic diversity among participants, we were unable to fully examine or adjust for potential confounders of vaccine attenuation effects.