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, gender, hypertension, diabetes, major cardiovascular diseases, chronic obstructive pulmonary diseases, kidney diseases, cancer. Geographic location was accounted for by study design while health seeking behaviour was accounted for by the testing policy of the region. Uncontrolled confounding is likely, for example arising from socioeconomic status, ethnicity and COVID-19 symptoms at the time of vaccination. The study also did not control for calendar time, specific populations, despite measuring these, or hospitalization and need for health care. |
| Selection of participants into the study |
Moderate |
Infection after 1 dose, severe disease and mortality: No particular concerns (Low risk of bias); For infection after 2 doses, we have a small concern about possible selection bias (Moderate risk of bias). |
| Clasification of interventions |
Low |
No particular concerns in this domain. |
| Deviations from intervention |
Low |
No concerns in this domain - the study is observational. |
| Missing outcome data |
Moderate |
This is a register-based study, so the likelihood of missing data may be low, but no information is provided by the authors. |
| Measurement of the outcome |
Moderate |
Infection outcomes: concern that vaccination status may affect propensity to get tested (Moderate risk of bias); Severe disease and mortality: no particular concerns (Low risk of bias) |
| Selection of the reported results |
Moderate |
There is no evidence of a protocol/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 published report was used in data extraction and risk of bias assessment. Of the vaccinated subjects, 52.8% received only one dose within the follow-up; 68.5% received BNT162b2 vaccine; 24.4% ChAdOx1 nCoV-19, and 7.0% mRNA-1273. Among those who received the second dose, 93.8% were vaccinated with BNT162b2, only 0.1% (n = 18) with ChAdOx1 nCoV-19, and the remaining 6.1% received mRNA-1273.
Concerns about uncontrolled confounding, a possiblity of selection bias for infection status after two doses, lack of information about missing data and possible bias in assessment of infection status. |