A Rapid Review of the Asymptomatic Proportion of PCR-Confirmed SARS-CoV-2 Infections in Community Settings

Background: Up to 80% of active SARS-CoV-2 infections are proposed to be asymptomatic based on cross-sectional studies. However, accurate estimates of the asymptomatic proportion require systematic detection and follow-up to differentiate between truly asymptomatic and pre-symptomatic cases. We conducted a rapid review and meta-analysis of current evidence regarding the asymptomatic proportion of PCR-confirmed SARS-CoV-2 infections based on methodologically-appropriate studies in community settings. Methods: We searched Medline and EMBASE for peer-reviewed articles, and BioRxiv and MedRxiv for pre-prints published prior to 05/05/2020. We included studies based in community settings that involved systematic PCR testing on participants and follow-up symptom monitoring regardless of symptom status. We extracted data on study characteristics, frequencies of PCR-confirmed infections by symptom status, and (if available) cycle threshold values and/or duration of viral shedding by symptom status. We computed estimates of the asymptomatic proportion and 95% confidence intervals for each study and overall using random effect meta-analysis. Findings: We screened 270 studies and included 6. The pooled estimate for the asymptomatic proportion of SARS-CoV-2 infections was 11% (95% CI 4%-18%). Estimates of baseline viral load appeared to be similar for asymptomatic and symptomatic cases based on available data in three studies, though detailed reporting of cycle threshold


Introduction
Reports of asymptomatic SARS-CoV-2 infection and potential transmission 1,2,3 have generated concern regarding the implications of undetected asymptomatic transmission on the effectiveness of public health interventions in the current COVID-19 pandemic 4 .
However, estimating the proportion of asymptomatic SARS-CoV-2 infections with viral shedding is challenging as the majority of testing is carried out on symptomatic individuals 5 .
Furthermore, longitudinal designs that include symptom follow-up are required to differentiate truly asymptomatic cases, i.e. those that never develop symptoms during illness, from pre-symptomatic cases, i.e. those that shed virus and therefore test positive prior to symptom onset (see Figure 1). While asymptomatic virus shedders have been suggested to comprise up to ~80% of SARS-CoV-2 infections 6,7 , data informing these figures are largely confined to cross-sectional reports that cannot distinguish truly asymptomatic cases from those who are pre-symptomatic at the point of testing (see Figure   1). Interchangeable use of these concepts, i.e. asymptomatic and pre-symptomatic, precludes accurate estimation of the asymptomatic proportion of potentially infectious SARS-CoV-2 infections. Detectible SARS-CoV-2 shedding based on reverse transcriptase polymerase chain reaction (PCR) testing cannot conclusively establish infectiousness in the absence of viral culture 8 9 . However, PCR cycle threshold values provide an informative estimate of viral load and, by extension, probable infectiousness 8 ; consequently, PCRconfirmed infection can provide a useful and accessible indicator of potentially infectious cases, including those without symptoms, for epidemiological modelling.
Given the widespread discussion and potential implications of asymptomatic transmission of SARS-CoV-2, we aimed to rapidly synthesize studies to enable us to estimate the asymptomatic proportion of PCR-confirmed cases in community settings (primary outcome) and viral load and duration of viral shedding in asymptomatic community cases compared to pre-symptomatic cases or those symptomatic from baseline (secondary outcome). We limited the review to include studies from community settings rather than hospitals and other medical facilities to prevent selection bias towards symptomatic cases. Only studies reporting PCR-confirmed cases rather than exclusive serological studies were included to estimate the proportion of asymptomatic SARS-CoV-2 infection with viral shedding. The review was not extended to estimate the overall asymptomatic proportion including nonshedding serological cases due to the limited number of serological studies, varying interpretation, and ongoing development of valid serological assays for SARS-CoV-2.
. CC-BY 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

Search Strategy
We used Ovid to search the Medline and EMBASE databases of peer-reviewed literature (2019-May 05 2020) using the following search terms for titles and abstracts: (Coronavirus* OR Covid-19 OR SARS-CoV-2 OR nCoV) AND (asymptomatic) AND (polymerase chain reaction OR PCR OR laboratory-confirmed OR confirmed). We also searched BioRxiv and MedRxiv for titles and abstracts of pre-print manuscripts using the terms "Covid-19" + "asymptomatic". We hand-searched the reference lists of all included studies to identify any additional relevant literature.

Selection Criteria
We included studies that met all of the following criteria: 1) human study; AND 2) presented original research or public health COVID-19 surveillance data; AND 3) available in English; AND 4) presented data on polymerase chain reaction (PCR) confirmed COVID-19 cases; AND 5) presented data on PCR testing of exposed or potentially exposed individuals regardless of symptom status (to avoid bias towards symptomatic cases); AND 6) had systematic follow-up at ≥ 1 time-point and reporting of symptom status among PCR confirmed cases (to differentiate pre-clinical shedding from truly asymptomatic cases); AND 7) presented data from a community setting (i.e. community and home contact tracing, population screening, traveller screening, community institutional settings such as care homes or schools). Studies were excluded if they met any of the following criteria: 1) studies or case series with <5 positive cases and/or <20 total cases (small sample size) due to likely low generalisability of asymptomatic proportions; OR 2) not possible to consistently ascertain the symptomatic status of participants across follow-up; OR 3) inadequate detail about testing strategy (i.e. not possible to discern if all cases were tested systematically); OR 4) recruitment/reporting from acute healthcare settings (e.g. hospitals, medical facilities) due to selection bias towards symptomatic cases.

Data Extraction and Analysis
One researcher performed the search, screened and selected studies, and extracted study details. Two researchers extracted primary outcome data independently and resolved any disagreement by consensus. We extracted the following variables of interest to assess the primary and secondary outcomes and the characteristics and quality of included studies: author names, year of publication, publication type (peer-reviewed article or pre-print), study design, study setting, study country of location, participant age (mean, median, or range as available), participant sex distribution, symptoms comprising symptomatic case definition, . CC-BY 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted May 23, 2020. ; https://doi.org/10.1101/2020.05.20.20108183 doi: medRxiv preprint duration of symptom history at PCR-confirmation, duration of follow-up symptom monitoring, testing criteria, sample size, number of participants who underwent PCR testing, number of PCR-confirmed cases, number of confirmed cases who remained asymptomatic throughout follow-up, and cycle threshold values, viral culture results, and duration of viral shedding for asymptomatic and pre-/symptomatic cases if reported.
We performed random-effects meta-analysis using the metaprop programme 10 in Stata Version 15 to compute the study-specific and pooled asymptomatic proportion -the primary outcome of this review -with its 95% confidence intervals (Wilson score method) and 95% prediction intervals 11 . The asymptomatic proportion is given as the number of consistently asymptomatic confirmed cases over the total number of PCR-confirmed cases who received follow-up ( Figure 2). It is important to note that the term asymptomatic proportion is sometimes used to alternatively refer to the asymptomatic proportion of all infections including those that do not shed virus and would not be PCR-confirmed (see Figure 2). We report available findings regarding the viral load and duration of viral shedding for asymptomatic and (pre)symptomatic cases, but did not conduct meta-analysis due to sparse reporting and inconsistencies in data presented.

Risk of Bias Assessment
We assessed risk of bias based using criteria relevant to the topic of this review adapted from the Joanna Briggs Institute critical appraisal tool for prevalence studies 12 (Table 1). Two researchers independently assessed the risk of bias for each included study and resolved any disagreement by consensus. Bias was assessed according to criteria described in Table 1, with studies graded as very low risk of bias if they were unlikely to have been affected by bias on any of the criteria, low if one criterion may have been affected, moderate if two may have been affected, and high if all three may have been affected.

Role of Funding Source
The funders were not involved in the design, delivery, analysis, or write-up of this study.
. CC-BY 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

(which was not certified by peer review)
The copyright holder for this preprint this version posted May 23, 2020. ; https://doi.org/10.1101/2020.05.20.20108183 doi: medRxiv preprint including two studies based in nursing homes 15,16 and four studies from general population samples potentially exposed to confirmed cases 14,17,18,19 and/or returning from travel to highrisk countries 18 . No further eligible studies were identified from hand-searching reference lists of included studies.

Asymptomatic Proportion of PCR-Confirmed COVID-19 Cases in Community Settings
Estimates of the the asymptomatic proportion of PCR-positive SARS-CoV-2 infections for included studies ranged from 4% (95% CI 2-10%; Park et al., 2020) to 43% (95% CI 27%-61%; Chau et al., 2020). Table 2 reports all asymptomatic proportions with 95% confidence intervals for as well as details of included studies. Based on random-effects meta-analysis indicated to be the most heterogeneous study -was the only study to systematically test participants using multiple specimen types (baseline saliva specimens and daily nasopharyngeal swabs) and appears to have the highest detection sensitivity for positive cases. This study was also, however, the most affected by potential non-participation bias, as 39% of PCR-confirmed cases chose not to participate in the symptom monitoring. This led to a moderate risk of bias score whereas all other studies were assessed as low overall risk of bias.

Discussion
Accurate estimates of the asymptomatic proportion of SARS-CoV-2 infections depend on appropriate study designs that systematically detect asymptomatic virus-shedding and follow these cases up to differentiate truly asymptomatic infection from pre-clinical shedding. We calculated that 11% of PCR-confirmed SARS-CoV-2 infections in community settings were asymptomatic, with a 95% confidence interval between 4% -18%. These findings do not support claims 6,7 of a very high asymptomatic proportion for PCR-confirmed infections (up to 80%) and highlights the importance of distinguishing between asymptomatic and presymptomatic cases. The careful screening of study design and methodology done as part of this review was reflected in the overall low risk of bias on assessed criteria for all but one included study. An additional strength of our review is the systematic search of both peerreviewed published literature and preprint studies which has enabled us to capture the most up to date estimates available.
Although this review identifies PCR-confirmed cases, PCR-confirmation and symptom-status alone cannot establish whether cases are infectious and, if so, the degree or duration of their infectiousness. Small case reports, however, have indicated potential transmission of SARS-CoV-2 from some asymptomatic index cases 1,2,8,18 . Limited evidence regarding the viral load and dynamics of SARS-CoV-2 in the present review indicates that asymptomatic cases had . CC-BY 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

(which was not certified by peer review)
The copyright holder for this preprint this version posted May 23, 2020. Only three of the six included studies 15,16,19 described the full range of symptoms included within their symptomatic case definitions, while a further two studies 17,17,18 reported details of symptoms endorsed by participants but did not specify whether additional symptoms were assessed as part of their case definitions. While a similar range of symptoms appear to have been monitored/endorsed across included studies, it is possible that symptomatic case identification may have been affected by reporting bias and consequently that the true proportion of symptomatic cases was underestimated. This is particularly relevant given that unusual symptoms such as dysosmia/anosmia -only explicitly investigated by one study 17,18 -and dysgeusia/ageusia -not referred to in any included study -may be the primary or sole symptom for some COVID-19 cases [22][23][24] . Demographic reporting across studies was also limited and it was not possible to stratify findings by age and sex. Estimates of the asymptomatic proportion may vary across population subgroups and this is a relevant area for future enquiry.
This review was also limited to estimating the asymptomatic proportion of virologicallyconfirmed infections. The asymptomatic proportion of infection varies depending on whether . CC-BY 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

(which was not certified by peer review)
The copyright holder for this preprint this version posted May 23, 2020. ; https://doi.org/10.1101/2020.05.20.20108183 doi: medRxiv preprint infections are identified using virological or serological methods 25 . PCR confirmation, which identifies infection with viral shedding, is informative for modelling transmission potential.
However, review of the asymptomatic proportion of total infections based on emerging serological evidencewhich identifies infections regardless of viral sheddingwill be informative to understand how far SARS-CoV-2 has spread within populations and investigate evidence of immunity following asymptomatic infection 26 . Overall, this review provides preliminary evidence that, when investigated using methodologically-appropriate studies, a relatively low proportion of active SARS-CoV-2 infections with viral shedding are truly asymptomatic.