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 |
Critical |
Uncontrolled confounding is likely to be very serious: the analysis adjusts only for age. |
| Selection of participants into the study |
Low |
No particular concerns in this domain. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No particular concerns in this domain. |
| Measurement of the outcome |
Moderate |
The indication for PCR testing is not described in the manuscript – this would be particularly important for the detection of asymptomatic cases with no indication for testing based on presence of COVID-19-like illness. The authors recognise that “screening and testing behaviours might differ among groups”. |
| 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 |
Critical |
|
| Overall comment | In addition to the published report, the supplementary material was used in data extraction and risk of bias assessment. The July 25, 2021 timepoint was extracted. 55.2% had received the Pfizer-BioNTech vaccine, 28.0% had received the Moderna vaccine, and 16.8% had received the Janssen (Johnson & Johnson) vaccine. Study start date of vaccinations was not reported; an estimate of December 2020 was used for California (https://covid19.ca.gov/vaccination-progress-data/). Substantial concern over the likelihood of uncontrolled confounding. |