Findings from the risk of bias assessment using the ROBINS-I tool, stratified by study design
| Study | Confounding or co-intervention bias | Selection bias | Misclassification bias | Deviation bias | Missing data bias | Outcome measurement bias | Outcome reporting bias | Overall judgement | Likely direction |
| School closures—pooled multiple-area before-after comparison studies | |||||||||
| Auger et al14 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Favours experimental |
| Banholzer et al15 | Moderate | Low | Low | Low | Low | Moderate | Low | Moderate | Unpredictable |
| Brauner et al18 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Chernozhukov et al19 | Moderate | Low | Moderate | Low | Low | Low | Low | Moderate | Unpredictable |
| Courtemanche et al20 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Garchitorena et al24 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Hsiang et al26 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Jamison et al30 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Li et al35 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Liu et al36 | Moderate | Low | Low | Low | Low | Low | Moderate | Moderate | Unpredictable |
| Stokes et al46 | Moderate | Low | Low | Low | Low | Low | Moderate | Moderate | Unpredictable |
| Wu et al47 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Yang et al48 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Krishnamachari et al33 | Moderate | Low | Serious | Low | Low | Low | Low | Serious | Unpredictable |
| Dreher et al21 | Serious | Low | Moderate | Low | Low | Moderate | Low | Serious | Favours experimental |
| Li et al34 | Moderate | Low | Serious | Low | Low | Low | Low | Serious | Unpredictable |
| Papadopoulos et al39 | Moderate | Low | Moderate | Low | Low | Serious | Low | Serious | Unpredictable |
| Rauscher42 | Serious | Low | Low | Low | Low | Low | Low | Serious | Favours experimental |
| Yehya et al49 | Serious | Low | Low | Low | Low | Moderate | Low | Serious | Favours experimental |
| Zeilinger et al50 | Moderate | Low | Low | Low | Low | Serious | Low | Serious | Favours experimental |
| Kilmek-Tulwin and Tulwin32 | Critical | Moderate | Low | Low | Low | Moderate | Low | Critical | Favours experimental |
| Piovani et al40 | Critical | Low | Low | Low | Low | Serious | Low | Critical | Favours experimental |
| School closures—within-area before-after comparison studies | |||||||||
| Matzinger and Skinner37 | Moderate | Low | Low | Low | Low | Moderate | Low | Moderate | Unpredictable |
| Gandini et al23 | Serious | Moderate | Low | Moderate | Low | Moderate | Low | Serious | Unpredictable |
| Iwata et al29 | Serious | Low | Low | Low | Low | Moderate | Low | Serious | Unpredictable |
| Neidhofer and Neidhofer38 | Serious | Serious | Low | Low | Low | Low | Moderate | Serious | Favours experimental |
| Shah et al53 | Serious | Low | Moderate | Low | Low | Moderate | Low | Serious | Unpredictable |
| Sruthi et al43 | Serious | Low | Low | Low | Low | Moderate | Low | Serious | Unpredictable |
| Stage—closures | Critical | Low | Low | Low | Low | Moderate | Low | Critical | Favours experimental |
| School closures—pooled multiple-area comparisons of interventions in place at a fixed time point | |||||||||
| Juni et al31 | Serious | Low | Low | Low | Low | Low | Low | Serious | Favours experimental |
| Wong et al51 | Serious | Low | Low | Low | Low | Low | Low | Serious | Unpredictable |
| Walach and Hockertz52 | Critical | Low | Serious | Low | Low | Serious | Low | Critical | Unpredictable |
| School reopening studies | |||||||||
| Garchitorena et al24 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Harris et al25 | Moderate | Moderate | Low | Moderate | Low | Low | Moderate | Moderate | Unpredictable |
| Isphording et al28 | Moderate | Low | Low | Low | Low | Moderate | Low | Moderate | Unpredictable |
| Li et al35 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Gandini et al23 | Serious | Moderate | Low | Moderate | Low | Moderate | Low | Serious | Unpredictable |
| Ingelbeen et al27 | Serious | Low | Low | Low | Low | Moderate | Low | Serious | Unpredictable |
| Sruthi et al43 | Serious | Low | Low | Low | Low | Moderate | Low | Serious | Unpredictable |
| Stage—opening | Serious | Low | Low | Low | Low | Moderate | Low | Serious | Unpredictable |
| Beesley16 | Critical | Low | Moderate | Moderate | Low | Serious | Low | Critical | Favours experimental |
| Ehrhardt et al22 | Critical | Low | Low | Moderate | Low | Low | Low | Critical | Favours experimental |
| Stein-Zamir et al45 | Critical | Low | Low | Low | Low | Serious | Low | Critical | Unpredictable |
| School holiday studies | |||||||||
| Pluemper and Neumayer41 | Moderate | Low | Low | Low | Low | Low | Low | Moderate | Unpredictable |
| Bjork et al17 | Low | Low | Low | Serious | Low | Low | Low | Serious | Favours comparator |
| Beesley16 | Critical | Low | Moderate | Moderate | Low | Serious | Low | Critical | Favours experimental |
Scale applied: low, moderate, serious or critical.
‘Favours experimental’ indicates that the bias likely resulted in an exaggeration of the reduction in community transmission associated with school closures.
ROBINS-I, Cochrane Risk of Bias In Non-randomised Studies of Interventions.