Can infected people without symptoms transmit coronavirus?

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The new coronavirus can be transmitted by people who are infected but before their symptoms appear – pre-symptomatic carriers. There is evidence that infected people who never develop symptoms – asymptomatic carriers – can also transmit coronavirus. While pre-symptomatic carriers appear to be an important source of infection, the role of asymptomatic carriers is less clear.

What determines how many infections are transmitted?

The number of people infected by someone with COVID-19 depends on:

What is the role of pre-symptomatic carriers?

Studies of patients with COVID-19 indicate that viable virus can be detected in swabs up to six days before to the onset of symptoms.[1][2][3] Estimates of the percentage of all infections transmitted by an individual prior to the appearance of symptoms will vary according to how quickly they are isolated. Some studies suggest a substantial contribution of pre-symptomatic carriers to transmission: 73% in Wuhan and 62% in Tianjin, China; and 48% in Singapore.[4][5]

One modelling study calculated that, in the early stages of the epidemic in China, each pre-symptomatic carrier infected 0.9 other people on average, contributing 47% of all transmitted infections.[6] The value of 0.9 secondary infections is close to the threshold of 1, above which pre-symptomatic carriers could sustain an outbreak of COVID-19 on their own. By contrast, during the SARS coronavirus outbreak in 2003, pre-symptomatic carriers contributed almost no secondary infections, indicating that COVID-19 needs a different containment strategy from SARS.[6]

What is the role of asymptomatic carriers?

The role of asymptomatic carriers is difficult to assess because they are usually not tested at all during the course of infection, and therefore not counted. And they are not easily identified as a source of infection to others, even of close contacts who develop COVID-19.

It is, however, possible to assess the percentage of people who never develop symptoms from studies where everyone is tested during a COVID-19 outbreak, irrespective of who became ill and who they contacted. One such study found that 18% of infected passengers (who were mostly over the age of 60) on the Diamond Princess Cruise Ship did not develop symptoms.[7] Another investigation in a nursing facility in Washington State, United States, found that among 48 infected residents, 24 were pre-symptomatic at the time of testing, but only three remained asymptomatic throughout the course of infection.[3]

So far, few studies have attempted to investigate whether asymptomatic carriers are actually infectious to other people, and they have produced divergent results. One study in the town of Vo, Italy, found little difference in the viral load of symptomatic and asymptomatic carriers.[8] Another study in Nanchang, China, found that mild cases of COVID-19 had a lower viral load than severe cases and shed virus for a shorter period of time,[9] which raises the question of whether asymptomatic carriers also shed less virus. The modelling study which calculated that pre-symptomatic carriers in China generated 0.9 secondary infections also found that asymptomatic carriers generated only 0.1 secondary infections, contributing just 6% of all transmission events.[6]

Although uncertainties remain, it is clear that infected people without symptoms can transmit coronavirus. Pre-symptomatic carriers might contribute more than asymptomatic carriers, but this needs further investigation. Nevertheless, in controlling the spread of COVID-19, there are benefits to physical and social distancing irrespective of the presence of symptoms.

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  1. He X, Lau EHY, Wu P, et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nature Medicine. 2020 May;26(5):672-675. DOI: 10.1038/s41591-020-0869-5.

  2. Peak CM, Kahn R, Grad YH, et al. Comparative Impact of Individual Quarantine vs. Active Monitoring of Contacts for the Mitigation of COVID-19: a modelling study. medRxiv. 2020 Mar. DOI: 10.1101/2020.03.05.20031088.

  3. Arons MM, Hatfield KM, Reddy SC, et al. Presymptomatic SARS-CoV-2 Infections and Transmission in a Skilled Nursing Facility. The New England Journal of Medicine. 2020 Apr. DOI: 10.1056/nejmoa2008457.

  4. Xia W, Liao J, Li C, et al. Transmission of corona virus disease 2019 during the incubation period may lead to a quarantine loophole. medRxiv. 2020 Mar. DOI: 10.1101/2020.03.06.20031955.

  5. Ganyani T, Kremer C, Chen D, et al. Estimating the generation interval for COVID-19 based on symptom onset data. medRxiv. 2020 Mar. DOI: 10.1101/2020.03.05.20031815.

  6. Ferretti L, Wymant C, Kendall M, et al. Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing. Science. 2020 Mar. DOI: 10.1126/science.abb6936.

  7. Mizumoto K, Kagaya K, Zarebski A, Chowell G. Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Euro Surveillance. 2020 Mar;25(10). DOI: 10.2807/1560-7917.ES.2020.25.10.2000180.

  8. Lavezzo E, Franchin E, Ciavarella C, et al. Suppression of COVID-19 outbreak in the municipality of Vo, Italy. medRxiv. 2020 Apr. DOI: 10.1101/2020.04.17.20053157.

  9. Liu Y, Yan LM, Wan L, et al. Viral dynamics in mild and severe cases of COVID-19. The Lancet Infectious Diseases. 2020 Mar. DOI: 10.1016/S1473-3099(20)30232-2.

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