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 analysis adjusted for age, sex, socioeconomic status (index of multiple deprivation (percentile), ethnicity, geographic location, health-seeking behaviour (visit frequency), comorbidities (presence of long-term health conditions), calendar time and need for health care (direct or indirect contact with a hospital or care-home) and additionally for household size and multigenerational household. This addresses almost all of our prespecified confounding domains, although time-varying confounders were not explicitly adjusted for. There is potential for uncontrolled confounding, given the observational nature of the data. |
| Selection of participants into the study |
Low |
We have no particular concerns about selection bias in this domain. The study sample was a random sample and analysis was based on monthly visits to participants by study staff. |
| Clasification of interventions |
Low |
No particular concerns in this domain - vaccination status is likely to be adequately measured (using a national database supplemented with self report where this was unavailable). |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Moderate |
Study outcomes were assessed at regular visits to the participants and it is likely that there were missing data. However, there is insufficient information to assess the amount of missing data or the implications for risk of bias. |
| Measurement of the outcome |
Low |
No particular concerns in this domain - determination of infection and disease unlikely to be biased. |
| Selection of the reported results |
Moderate |
A protocol is available for the study, although not specifically for this analysis. Given the sophistication of the analysis, there is a risk that results may have been selected for presentation due to the nature of the findings. |
| Overall risk of bias |
Moderate |
|
| Overall comment | This is a strong, sophisticated analysis, although uncontrolled confounding cannot be ruled out given the observational nature of the data. |