COVID-19 is Contagious without Symptoms

Related: Masks are Effective at Preventing COVID-19, but Fit Matters

COVID-19 transmitted during presymptomatic phase, but 30% less often than during symptomatic phase (n=2861)

This prospective case-ascertained study in Taiwan included laboratory-confirmed cases of COVID-19 and their contacts. We enrolled 100 confirmed patients. Among their 2761 close contacts, there were 22 paired index-secondary cases. The overall secondary clinical attack rate was 0.7%. The attack rate was higher among the 1818 contacts whose exposure to index cases started within 5 days of symptom onset (1.0%) compared with those who were exposed later (0 cases from 852 contacts). The 299 contacts with exclusive presymptomatic exposures were also at risk (attack rate, 0.7%). The attack rate was higher among household (4.6%) and nonhousehold (5.3%) family contacts than that in health care or other settings. The attack rates were higher among those aged 40 to 59 years (1.1%) and those aged 60 years and older (0.9%). Conclusions and relevance: In this study, high transmissibility of COVID-19 before and immediately after symptom onset suggests that finding and isolating symptomatic patients alone may not suffice to contain the epidemic.

https://pubmed.ncbi.nlm.nih.gov/32356867/

Contact with asymptomatic COVID-19 is 65% less likely to infect

We included a total of 94 studies. The overall estimate of the proportion of people who become infected with SARS-CoV-2 and remain asymptomatic throughout infection was 20%. There was some evidence that biases in the selection of participants influence the estimate. In seven studies of defined populations screened for SARS-CoV-2 and then followed, 31% remained asymptomatic. The secondary attack rate was lower in contacts of people with asymptomatic infection than those with symptomatic infection (relative risk 0.35). Modelling studies fit to data found a higher proportion of all SARS-CoV-2 infections resulting from transmission from presymptomatic individuals than from asymptomatic individuals.

https://pubmed.ncbi.nlm.nih.gov/32960881/

Presymptomatic cases represent 41% of cluster transmission during early COVID-19 outbreak, most often one day before symptom onset (n=3184)

We analyzed 3,184 cases of coronavirus disease in Japan and identified 61 case-clusters in healthcare and other care facilities, restaurants and bars, workplaces, and music events. For 16 clusters, we determined the date of transmission from probable primary case-patients to other case-patients in a cluster and found 41% (9/22) of probable primary case-patients were presymptomatic or asymptomatic at the time of transmission; only 1 had a cough at the time of transmission. Of the 16 probable primary case-patients with the determined date of transmission, transmission occurred one day before illness onset for 5 (31%) case-patients and on the same day of illness onset for 4 (25%) case-patients. All age groups demonstrated presymptomatic or asymptomatic transmission.

https://wwwnc.cdc.gov/eid/article/26/9/20-2272_article

During early outbreak, 12.6% of COVID-19 cases indicate presymptomatic transmission (n=468)

We estimate the distribution of serial intervals for 468 confirmed cases of coronavirus disease reported in China as of February 8, 2020. The mean interval was 3.96 days (95% CI 3.53-4.39 days), SD 4.75 days (95% CI 4.46-5.07 days); 12.6% of case reports indicated presymptomatic transmission.

https://pubmed.ncbi.nlm.nih.gov/32191173/

During COVID-19 quarantine, presymptomatic transmission may be as high as 48% to 62% (n=189)

We used outbreak data from clusters in Singapore and Tianjin, China to estimate the generation interval from symptom onset data while acknowledging uncertainty about the incubation period distribution and the underlying transmission network. Results: The mean generation interval was 5.20 days (95% credible interval (CrI): 3.78-6.78) for Singapore and 3.95 days (95% CrI: 3.01-4.91) for Tianjin. The proportion of pre-symptomatic transmission was 48% (95% CrI: 32-67) for Singapore and 62% (95% CrI: 50-76) for Tianjin. Conclusion: Notably, quarantine and other containment measures were already in place at the time of data collection, which may inflate the proportion of infections from pre-symptomatic individuals.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7201952/

During initial outbreak, 6.4% of COVID-19 cases were acquired during presymptomatic phase, 1-3 days before symptom onset (n=157)

Clinical and epidemiologic findings of all COVID-19 cases in Singapore through March 16 were reviewed to determine whether presymptomatic transmission might have occurred. Presymptomatic transmission was defined as the transmission of SARS-CoV-2 from an infected person (source patient) to a secondary patient before the source patient developed symptoms, as ascertained by exposure and symptom onset dates, with no evidence that the secondary patient had been exposed to anyone else with COVID-19. Seven COVID-19 epidemiologic clusters in which presymptomatic transmission likely occurred were identified, and 10 such cases within these clusters accounted for 6.4% of the 157 locally acquired cases. In the four clusters for which the date of exposure could be determined, presymptomatic transmission occurred 1-3 days before symptom onset in the presymptomatic source patient.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7147908/

44% of COVID-19 cases predicted to be infected during presymptomatic stage; highest viral load at symptom onset (n=94)

We report temporal patterns of viral shedding in 94 patients with laboratory-confirmed COVID-19 and modeled COVID-19 infectiousness profiles from a separate sample of 77 infector–infectee transmission pairs. We observed the highest viral load in throat swabs at the time of symptom onset, and inferred that infectiousness peaked on or before symptom onset. We estimated that 44% (95% confidence interval, 25–69%) of secondary cases were infected during the index cases’ presymptomatic stage, in settings with substantial household clustering, active case finding and quarantine outside the home. Assuming an incubation period distribution of mean 5.2 days from a separate study of early COVID-19 cases, we inferred that infectiousness started from 2.3 days before symptom onset and peaked at 0.7 days before symptom onset. Infectiousness was estimated to decline quickly within 7 days. Disease control measures should be adjusted to account for probable substantial presymptomatic transmission.

https://www.nature.com/articles/s41591-020-0869-5