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, sex, comorbidities, occupation, hospitalization with co-morbidities in past 5 years, and contact with a COVID-19 case or suspected case. The study was conducted between April and June 2021 and within Bihar State, but there was no adjustment for calendar time or geography within these. Additionally, they adjusted for COVID-appropriate behaviour (mask wearing and others) in the past 14 days - this is potentially on the causal pathway between vaccination and infection. |
| Selection of participants into the study |
Serious |
Consent rates varied among cases and controls. In addition, cases were selected from among those presenting at a clinic whereas controls were selected from among those tested in the laboratory (at the same institute); a relatively large proportion of those tested were positive but these were not selected as cases. |
| Clasification of interventions |
Serious |
Study used self-reported vaccination status obtained via interviews with study participants or their proxy, with limited attempt to verify this. The authors report: “We also requested them to confirm the vaccination details by cross-checking with the CoWIN app or the message received following vaccination”. |
| Deviations from intervention |
Low |
No concerns in this domain - the study is observational. |
| Missing outcome data |
Low |
No particular concerns in this domain. |
| Measurement of the outcome |
Moderate |
It is unclear whether testing behaviour is associated with vaccination status. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Serious |
|
| Overall comment | In addition to the published report, the supplementary material were used in data extraction and risk of bias assessment. Follow-up time was estimated using the date of start of vaccination for the elderly and individuals aged above 45 years with comorbidities (March 1, 2021); however, for healthcare workers (6% of the study sample) and front-line workers it was earlier (January 16, 2021). The main concerns were uncontrolled confounding, adjustment for a variable potentially on the causal pathway, selection bias and potential differential misclassification of vaccination status. |