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 |
Serious |
Analysis controlled for age, sex, geographic area, time and comorbidities; did not control for ethnicity, SES or symptoms at the time of testing (follow up time was split into unvaccinated and vaccinated and unvaccinated individuals would be unlikely to receive a vaccine if they had symptoms). |
| Selection of participants into the study |
Low |
No concerns in this domain. |
| Clasification of interventions |
Low |
No concerns in this domain. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
28% of positive PCR tests were not screened for the variants. This percentage varied by geography and time, so could potentially be associated with whether a test was delta-positive or alpha-positive. |
| Measurement of the outcome |
Moderate |
Vaccinated individuals may be more or less likely than unvaccinated individuals to seek a test, particularly if asymptomatic. |
| 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 |
Serious |
|
| Overall comment | The main concern is the potential for residual confounding. Analysis did not control for ethnicity, SES or symptoms at the time of testing (follow up time was split into unvaccinated and vaccinated and unvaccinated individuals would be unlikely to receive a vaccine if they had symptoms). |