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 controlled only for calendar time (although the result was similar to results adjusting for age, sex, race/ethnicity, geographical region, and county-level vaccination rate). There remains a risk of bias arising from uncontrolled confounding. |
| Selection of participants into the study |
Low |
No concerns in this domain - the study used state-wide surveillance data and there were no inappropriate exclusions. |
| Clasification of interventions |
Moderate |
Some people may have been vaccinated outside North Carolina; also individuals vaccinated through federal channels (Department of Defence, Veterans Health Administration, Indian Health Service, federal Bureau of Prisons) would be incorrectly classified as unvaccinated. It is unclear whether the risk of the outcome would be different in these subgroups, or what proportion of the population this might have applied to. |
| Deviations from intervention |
Low |
No concerns in this domain - the study was observational. |
| Missing outcome data |
Low |
No concerns in this domain. |
| Measurement of the outcome |
Moderate |
Vaccination status could be associated with the likelihood of getting a test. |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol or statistical analysis plan, and we have concerns that results could have been selected for reporting because of the findings. |
| Overall risk of bias |
Serious |
|
| Overall comment | The main concern is with uncontrolled confounding. The results extracted only controlled for calendar time (although were similar to results adjusting for age, sex, race/ethnicity, geographical region, and county-level vaccination rate).
As the exact date of the second dose of vaccine is not reported (Table 2 starts counting events after the first dose), the vaccine effectiveness (VE) for the time interval of 1-2 months was selected to reflect VE after full vaccination. |