Summary Background Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is spread by direct, indirect, or close contact with infected people via infected respiratory droplets or saliva. Crowded indoor environments with sustained close contact and conversations are a particularly high-risk setting. Methods We performed a meta-analysis through July 29, 2020 of SARS-CoV-2 household secondary attack rate (SAR), disaggregating by several covariates (contact type, symptom status, adult/child contacts, contact sex, relationship to index case, index case sex, number of contacts in household, coronavirus). Findings We identified 40 relevant published studies that report household secondary transmission. The estimated overall household SAR was 18·8% (95% confidence interval [CI]: 15·4%–22·2%), which is higher than previously observed SARs for SARS-CoV and MERS-CoV. We observed that household SARs were significantly higher from symptomatic index cases than asymptomatic index cases, to adult contacts than children contacts, to spouses than other family contacts, and in households with one contact than households with three or more contacts. Interpretation To prevent the spread of SARS-CoV-2, people are being asked to stay at home worldwide. With suspected or confirmed infections referred to isolate at home, household transmission will continue to be a significant source of transmission.

Background Current SARS-CoV-2 containment measures rely on the capacity to control person-to-person viral transmission. Effective prioritization of these measures can be determined by understanding SARS-CoV-2 transmission dynamics. We conducted a systematic review and meta-analyses of three parameters: (i) secondary attack rate (SAR) in various settings, (ii) clinical onset serial interval (SI), and (iii) the proportion of asymptomatic infection. Methods and Findings We searched PubMed, medRxiv, and bioRxiv databases between January 1, 2020, and May 15, 2020, for articles describing SARS-CoV-2 attack rate, SI, and asymptomatic infection. Studies were included if they presented original data for estimating point estimates and 95% confidence intervals of the three parameters. Random effects models were constructed to pool SAR, mean SI, and asymptomatic proportion. Risk ratios were used to examine differences in transmission risk by setting, type of contact, and symptom status of the index case. Publication and related bias were assessed by funnel plots and Egger's meta-regression test for small-study effects. Our search strategy for SAR, SI, and asymptomatic infection identified 459, 572, and 1624 studies respectively. Of these, 20 studies met the inclusion criteria for SAR, 18 studies for SI, and 66 studies for asymptomatic infection. We estimated the pooled household SAR at 15.4% (95% CI: 12.2%, 18.7%) compared to 4.0% (95% CI: 2.8%, 5.2%) in non-household settings. We observed variation across settings; however, the small number of studies limited power to detect associations and sources of heterogeneity. SAR of symptomatic index cases is significantly higher than cases that were symptom-free at diagnosis (RR 2.55, 95% CI: 1.47, 4.45). Adults appear to be more susceptible to transmission than children (RR 1.40, 95% CI: 1.00, 1.96). The pooled mean SI is estimated at 4.87 days (95% CI: 3.98, 5.77). The pooled proportion of cases who had no symptoms at diagnosis is 25.9% (95% CI: 18.8%, 33.1%). Conclusions Based our pooled estimates, 10 infected symptomatic persons living with 100 contacts would result in 15 additional cases in <5 days. To be effective, quarantine of contacts should occur within 3 days of symptom onset. If testing and tracing relies on symptoms, one-quarter of cases would be missed. As such, while aggressive contact tracing strategies may be appropriate early in an outbreak, as it progresses, control measures should transition to account for SAR variability across settings. Targeted strategies focusing on high-density enclosed settings may be effective without overly restricting social movement.

Abstract Asymptomatic but infectious people have been reported in many infectious diseases. Asymptomatic and pre-symptomatic carriers would be a hidden reservoir of COVID-19. Aim This review identifies primary empirical evidence about the ability of asymptomatic carriers to infect others with COVID-19 pandemic and reflects on the implications for control measures. Methods A systematic review is followed by a narrative report and commentary inclusion criteria were: studies reporting primary data on asymptomatic or pre-symptomatic patients, who were considered to have passed on COVID-19 infection; and published in indexed journals or in peer review between January 1 and March 31, 2020. Results Nine articles reported on 83 asymptomatic or pre-symptomatic persons. Conclusions The evidence confirms COVID-19 transmission from people who were asymptomatic at the time. A series of implications for health service response are laid out.

After an outbreak in Wuhan, China, a growing number of countries are now suffering from an epidemic by SARS-CoV-2, which causes COVID-19. Undoubtedly, reports of the skyrocketing global spread of COVID-19 has shocked people globally, from Japan to the United States. Presently, the World Health Organization indicates that the fatality rate due to COVID-19 is about 2%, inferring that many positive subjects may potentially overcome the illness with mild influenza-like symptoms and no need for hospitalization at intensive-care units. Because COVID-19 is completely new to the human immune system, many throughout the world are likely vulnerable to becoming sick after their initial exposure to SARS-CoV-2. Besides hospitalized cases, many individuals are likely asymptomatic but potentially carry the virus. While our knowledge about carriers and their virus shedding is deficient, some studies modelling the viral transmission have considered the potential contribution of the asymptomatic carriers. Protocols for managing asymptomatic cases, for example for controlling them to restrict their contact with healthy people at public places or private residences, have not been established. In-house quarantine may as well be applicable to asymptomatic cases if they could be identified and diagnosed. Presumably now, the asymptomatic subjects potentially contribute to the transmission of COVID-19 without their knowledge, intention, or being diagnosed as carriers. Thus, managing the asymptomatic

uilt into them, these models suggest that the speed and extent of SARS-CoV-2 transmission cannot be accounted for solely by transmission from symptomatic persons. Each of the epidemiologic, virologic, and modeling studies described has limitations. However, in the aggregate, these diverse studies suggest that SARS-CoV-2 can be transmitted by persons with presymptomatic or asymptomatic infection, which may meaningfully contribute to the propagation of the COVID-19 pandemic. This literature summation was conducted to support changes in CDC recommendations to reduce the risk for asymptomatic transmission and was not a systematic review. These conclusions are drawn from the literature available at the time and may change, given the rapidly evolving nature of the evidence base for asymptomatic transmission. Public Health Implications of Transmission While Asymptomatic The existence of persons with asymptomatic SARS-CoV-2 infection who are capable of transmitting the virus to others has several implications. First, the case-fatality rate for COVID-19 may be lower than currently estimated ratios if asymptomatic SARS-CoV-2 infections are included ( 34 , 35 ). Second, transmission while asymptomatic reinforces the value of community interventions to slow the transmission of COVID-19. Knowing that asymptomatic transmission was a possibility, CDC recommended key interventions including physical distancing ( 36 ), use of cloth face coverings in public ( 37 ), and universal masking in healthcare facilities ( 38 ) to prevent SARS-CoV-2 transmission by asymptomatic and symptomatic persons with SARS-CoV-2 infection. Third, asymptomatic transmission enhances the need to scale up the capacity for widespread testing and thorough contact tracing to detect asymptomatic infections, interrupt undetected transmission chains, and further bend the curve downward. Science Questions to Inform Public Health Action The existence of SARS-CoV-2 transmission while infected persons are presymptomati

high risk. It has been reported that pregnant women with asymptomatic COVID-19 infection have delivered babies who are negative for the COVID-19 nucleic acid test, suggesting no vertical transmission among neonates born to COVID-19-infected mothers. 14 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 In Wuhan, a lower COVID-19 fatality rate and higher discharge rate were observed than in Beijing, China. It is essential to identify asymptomatic individuals and implement necessary control measures to prevent transmission. 21 In South Korea, 41 COVID-19 asymptomatic adults were identified (confirmed by RT-PCR) out of 213 individuals. 17 In another study among 100 asymptomatic cases, 60% developed delayed symptoms and none of the asymptomatic patients died, suggesting that asymptomatic transmission could take place during the incubation period. 18 Another study did not observe any difference in the symptomatic and asymptomatic COVID-19 transmission rates among patients. 22 In adults, CT imaging of asymptomatic COVID-19 individuals has advantages in highly suspicious cases with negative nucleic acid test results. 17 A serological investigation among 31 of 34 adult cases with asymptomatic COVID-19 infection did not require oxygen support during hospitalisation. 15 Theoretically, the quantified infection transmission rate shows the estimated risk ratio (RR) of infectivity of symptomatic against asymptomatic to be 3.9% (95% CI: 1.5, 11.8). In asymptomatic adults, the transmission was significantly smaller than in symptomatic cases. 16 No gender difference was observed for asymptomatic transmission. 20 Further longitudinal surveillance using nucleic acid testing is warranted to identify and assess viral load among asymptomatic COVID-19 adults. 24 In one study, four asymptomatic cases were quarantined for 14 days; thus, these individuals were unable to transmit the infection due to proper isolation management. 19 Asymptomatic COVID-19 transmission has been observed in children. 26 In

Asymptomatic infections are actually 'healthy' people