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 matched indiduals by age, sex, ethnicity, neighborhood of residence, health-seeeking behavious (history of influenza vaccinations, pregnancy and number of coexisting conditions.
Uncontrolled confounding remains likely, for example by BMI, smoking and socio-economic status. |
| Selection of participants into the study |
Low |
No particular concerns in this domain - the trial emulation approach should overcome key selection biases. |
| 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 obervational |
| Missing outcome data |
Moderate |
There is likely to be informative censoring when controls became vaccinated. Controls who became vaccinated are likely to be different from those who did not. Controls might not have got vaccinated if they developed COVID-19 symptoms, although the number of these may be relatively small. |
| Measurement of the outcome |
Moderate |
Infection status: propensity to get tested might depend on vaccination status, leading to a risk of bias in determining infection status. There is very little information about this in the study reports (Moderate risk of bias). Severe disease and death: No particular concerns (Low risk of bias) |
| Selection of the reported results |
Low |
Although there is not evidence of a protocol, the results appear to be faithful to the analyses undertaken. |
| Overall risk of bias |
Moderate |
|
| Overall comment | Concerns over uncontrolled confounding; informative censoring (control participants who went on to get vaccinated are likely to be different from those who did not); and (for data on infections) the likelihood of getting tested for infection may depend on vaccination status. |