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 |
The analysis adjusted for age, week infection was documented, number of previous PCR tests, demographic group and gender. Our prespecified important confounding domains of socioeconomic status, geographic location, comorbidities, symptoms at the time of vaccination hospitalisation and need for health care were not controlled for, leading to a likelihood of uncontrolled confounding. |
| Selection of participants into the study |
Serious |
Vaccine effectiveness calculated in over 60s vaccinated in late January 2021 for infection and serious disease occurring 11-31 July 2021. Those who were unvaccinated in January but were vaccinated between February and May are excluded from the comparison and therefore do not contribute to the estimate. Because those who were subsequently vaccinated may be different from those who remained unvaccinated, the estimate of vaccine effectiveness may be distorted. Furthermore, anyone (vaccinated or unvaccinated) who tested positive before 11 July 2021 is also excluded, creating further potential for selection bias. |
| Clasification of interventions |
Low |
No particular concerns for this domain. Follow up for outcomes doesn’t start until several months after treatment assignment – therefore not immortal time bias as everyone has the same follow up period. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
The database studied was a national one and it is likely that data were nearly complete for the variables analysed. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - outcome assessment is likely to be adequate. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol, though the results appear to be faithful to the analyses undertaken. |
| Overall risk of bias |
Serious |
|
| Overall comment | The preprint was used in data extraction and risk of bias assessment. Results were extracted for the group aged 60 and older who received the second vaccine dose in January. Concerns mainly over uncontrolled confounding, and a possibility of selection bias. |