*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.