*Defined by WHO as an “occurrence of disease cases in excess of normal expectancy. The number of cases varies according to the disease-causing agent, and the size and type of previous and existing exposure to the agent.”20
Abbreviations: COVID-19: coronavirus disease 2019; KQ: key question; MERS: Middle East respiratory syndrome; SARS: severe acute respiratory syndrome

2.3 Screening process

A team of experienced researchers screened all titles and abstracts based on predefined inclusion and exclusion criteria (Table 1). The first 30% of records were screened independently by two investigators. In cases of disagreement about eligibility, the two investigators reached consensus by discussion or by involving a senior reviewer. The remaining 70% of titles and abstracts were screened by a single investigator.
The review team retrieved the full texts of all included abstracts. Two reviewers screened all full-text publications independently. Disagreements were resolved by consensus or by involving a third, senior reviewer. The team conducted literature screening using the Covidence Systematic Review software.21
The list of Chinese studies provided by the WHO was checked dually and independently. Potentially relevant abstracts were retrieved in full text by WHO and were translated by the WHO collaborators from China.

2.4 Quality assessment

The review team assessed the risk of bias of the included cohort studies using the Risk Of Bias In Non-randomized Studies - of Interventions (ROBINS-I) tool.22 A single reviewer rated the risk of bias for each relevant outcome of each study; a second reviewer checked the ratings. Due to time constraints, we omitted an independent dual risk of bias assessment. Because no validated risk of bias checklist for transmission models is available, we rated the quality of modeling studies based on the best practice recommendations of the International Society for Pharmacoeconomics and Outcomes (ISPOR) for transmission models.23 We assessed whether the model was dynamic, whether the authors conducted uncertainty analyses on key assumptions, and whether the results provided estimates of the change in the burden of infection due to the intervention.
One experienced researcher assigned certainty of evidence ratings based on an approach developed by the Grading of Recommendations, Assessment, Development and Evaluation Working Group (GRADE).24,25 GRADE uses four categories to classify the certainty of evidence. High certainty of evidence means that investigators were very confident that the estimated effect lies close to the true effect; moderate certainty assumes it is likely to be close; a low certainty rating suggests that the estimated effect might substantially differ from the true effect; and very low means that investigators had little confidence in the effect estimate.

2.4 Data Extraction

One experienced researcher extracted data from the included studies into standardized tables; a second reviewer checked the data extraction for completeness and correctness. The data items for cohort studies included: author, year, country, study design, objective, characteristics of the study participants, description of the intervention and comparison, and results. For the modeling studies, the data items were: author, year, type of model, setting, time, data source and participants, interventions, and results.

2.5 Synthesis

We synthesized results narratively and in tabular form.

3 Results

Our searches identified 21 relevant studies.26-46 Of these, two focused on COVID-19,45,46 15 focused on SARS,26,28-39,42,43 two focused on SARS and other viruses,40 212,41 and two focused on MERS.27 29,44 204 Four of the included studies were cohort studies,26-29 while 17 were modeling studies using different types of transmission models.30-46 The cohort studies were from China, Korea, and Taiwan and included data on 178 461 ‬individuals. The modeling studies used data from outbreaks in Canada, China, Hong Kong, Japan, Korea, Singapore, and Taiwan. In the studies on SARS and MERS, we detected only three studies that employed general models that also took presymptomatic infectiousness into consideration.32,33,40
The PRISMA flow diagram in Figure 2 provides an overview of the study selection process; Table 2 presents the characteristics of the included observational and modeling studies, respectively. Annex B presents the results of each individual study.