Risk of bias in included studies assessed by the ROBINS-I tool
| Risk-of-bias domains | ||||||||
| Study, year | Confounding | Selection of participants | Classification of interventions | Deviations from intended intervention | Missing data | Measurement of outcomes | Overall risk of bias | Selection of reported result |
| Vaccination before SARS-CoV-2 infection | ||||||||
| Al-Aly 202219 | Low | Low | Low | Low | Low | Moderate | Low | Moderate |
| Antonelli 202220 | Moderate | Low | Low | Low | Serious | Serious | Low | Serious |
| Ayoubkhani 202221 | Critical | Low | Low | Low | Moderate | Low | Low | Critical |
| Azzolini 202222 | Moderate | Low | Moderate | Low | NI | Critical | Low | Critical |
| Ioannou 202223 | Moderate | Low | Low | Low | NI | Low | Moderate | Moderate |
| Mohr 202224 | Serious | Serious | Low | Low | Serious | Moderate | Moderate | Serious |
| Pell 202225 | Serious | Low | Serious | Low | Serious | Moderate | Moderate | Serious |
| Tannous 202226 | Serious | Low | Low | Low | Low | Serious | NI | Serious |
| Taquet 202127 | Moderate | Low | Low | Low | NI | Low | Low | Moderate |
| van der Maaden 202228 | Serious | Low | Moderate | Low | Low | Low | Low | Serious |
| Zisis 202229 | Serious | Critical | Serious | Low | NI | Serious | NI | Critical |
| Vaccination after SARS-CoV-2 infection or after diagnosis of long covid | ||||||||
| Ayoubkhani 202230 | Low | Low | Low | Low | Moderate | Low | Low | Moderate |
| Simon 202131 | Serious | Low | Low | Low | Moderate | Low | Low | Serious |
| Tran 202132 | Moderate | Low | Low | Low | Low | Low | Low | Moderate |
| Wisnivesky 202233 | Serious | Serious | Serious | Low | Low | Low | Low | Serious |
| Wynberg 202234 | Serious | Moderate | Serious | Low | Low | Serious | Low | Serious |
NI=no information.