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 individuals by age, sex, ethnicity, neighborhood of residence, health-seeking behaviours (history of influenza vaccinations), population sector, number of coexisting conditions and trimester of pregnancy. Women who had interaction with the healthcare system in the previous 2 days were excluded, which may go some way towards addressing confounding due to symptoms at the time of planned vaccination. Uncontrolled confounding remains likely by socio-economic status. Matching by pre-existing medical condition may attenuate the risk of bias introduced by not matching for 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 concerns on this domain - vaccination status likely to be correctly classified. |
| Deviations from intervention |
Low |
No concerns on this domain - the study is observational. |
| 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 |
For infection and symptomatic disease outcomes: Propensity to get tested might depend on vaccination status, leading to a risk of bias in determining infection status (Moderate risk of bias). For severe disease: No particular concerns (Low risk of bias). |
| Selection of the reported results |
Moderate |
There is no evidence of a statistical analysis plan and fine details of the analysis are not reported in the paper (note: it is a rapid communication). |
| Overall risk of bias |
Moderate |
|
| Overall comment | Some concerns around uncontrolled confounding, possibility of informative censoring and (for infection and symptomatic disease outcomes) a possibility that propensity to get tested may depend on vaccination status. |