Note: The risk of bias by domain corresponds to the highest risk of bias among outcomes by domain.
The overall risk of bias corresponds to the overall highest risk of bias assessed among outcomes.
| Bias | Author's judgement | Support for judgement |
| Confounding |
Moderate |
The study controlled for almost all of our pre-specified confounding domains, either by matching or analysis or by demonstrating that the factors were largely balanced between groups. For the remaining factors, sensitivity analyses were reassuring. |
| Selection of participants into the study |
Low |
No concerns in this domain. The matched design closely follows target trial. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured. For infection after a second dose, misclassification of the second dose was possible in 1% of women, which is acceptable. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
When matched controls became vaccinated the entire matched pair was censored ( ~30% of matched pairs). This may have led to informative censoring as controls who became vaccinated likely differ from those who remained unvaccinated. Controls might not have got vaccinated if they developed COVID-19 symptoms, although the total number of COVID-19 infections in the cohort was low (vaccinated 118 vs unvaccinated 202). |
| Measurement of the outcome |
Low |
No concerns regarding outcome measurement (by PCR testing). |
| Selection of the reported results |
Moderate |
There is no evidence of an analysis plan, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Moderate concerns over possible residual confounding, informative censoring leading to potentially missing outcome data, and no access to a protocol or statistical analysis plan. |