Table 1. Effectiveness of Two or Three Doses of mRNA Vaccine against SARS-CoV-2 Infection among Frontline Workers at Eight Study Sites (August 26, 2021, to January 22, 2022).
| Vaccination Status and Timing* | No. of Participants† | Days since Vaccine Dose | Person-Days | SARS-CoV-2 Detected | Vaccine Effectiveness | ||
|---|---|---|---|---|---|---|---|
| Adjusted‡ | Relative§ | ||||||
| median no. (IQR) | no. | median no. (IQR) | no. | % (95% CI) | |||
| Predominance of delta variant | |||||||
| Unvaccinated | 343 | NA | 27,594 | 85 (35–129) | 51 | Reference | NA |
| Second dose | 2773 | 267 (248–301) | 204,624 | 67 (38–124) | 134 | 65 (49–76) | Reference |
| Third dose | 1716 | 74 (52–93) | 110,852 | 67 (45–87) | 20 | 91 (84–95) | 86 (69–94) |
| Predominance of omicron variant | |||||||
| Unvaccinated | 259 | NA | 9,072 | 41 (21–48) | 77 | Reference | NA |
| Second dose | 1021 | 330 (272–357) | 38,171 | 48 (29–48) | 156 | 46 (25–61) | Reference |
| Third dose | 1719 | 93 (70–113) | 72,673 | 48 (41–48) | 189 | 60 (42–72) | 60 (40–73) |
A total of 3163 participants were included in the analysis during the period when the delta variant was predominant and 2831 participants were included during the period when the omicron variant was predominant. Although whole-genome sequencing was performed to confirm variant types, data are presented according to the period in which each variant became predominant in the geographic area of the study site. At the time of specimen collection, participants were considered to have received two doses of vaccine at least 14 days after the second dose, to have received three doses at least 7 days after the third dose, or to be unvaccinated if no vaccine doses were received. All other interim periods were considered to be indeterminate, so data that were collected during those periods were excluded from the analysis. NA denotes not applicable. IQR denotes interquartile range.
Contributing participants in vaccination categories do not equal the number of participants in the study because participants could contribute to more than one vaccination category since vaccination status was time-varying.
In all models, the adjusted vaccine effectiveness was inversely weighted for propensity to be vaccinated and a priori with local virus circulation, study site, and occupation as covariates to adjust for additional bias. After weighting, all covariates that met the balance criteria of a standardized mean difference of less than 0.2 were included in the final model if the estimated effectiveness was changed by at least 5%; all a priori variables were included in the final model regardless of their effect on vaccine effectiveness. For the model of the delta variant, the participant’s history regarding occupation and influenza vaccination did not balance, but only occupation changed the calculation of vaccine effectiveness by more than 5%. For the model of the omicron variant, all the covariates met the balance criteria after weighting except study site, occupation, daily medication use, influenza vaccination history, and local virus circulation; influenza vaccination history was found to change the estimate of vaccine effectiveness by at least 5% and was added to the final model.
The calculation of the adjusted relative effectiveness of a three-dose vaccine series as compared with a two-dose series was inversely weighted for propensity to be boosted with local virus circulation, study site, occupation, and number of days since the most recent dose as the covariates to adjust a priori for additional bias. For the models of both the delta and omicron variants, all covariates met the balance criteria after weighting.