Are People Infected with the Novel Coronavirus Contagious Before They Have Symptoms?
David MacIntosh | March 24, 2020
According to current information on the novel coronavirus (SARS-CoV-2) and the disease it causes, known as COVID-19, people infected with the virus are contagious before they have symptoms.
Research that Tells Us Asymptomatic Persons Are Contagious
A research letter published in the Journal of the American Medical Association on February 21, 2020 is perhaps the first description of this phenomenon.1 A 20-year old resident of Wuhan, China (called the “Index Patient”, meaning the first identified case in a group of cases) traveled to Anyang, China where she met with five relatives from February 10 – 12. All five relatives developed symptoms within 14 days after contact with the Index Patient and tested positive for SARS-Cov-2 by February 27, within 1 to 4 days of symptom onset. The Index Patient never developed symptoms but tested for SARS-Cov-2 on February 28. The authors reported that none of the patients traveled to Wuhan or had been in contact with anyone else who had traveled from Wuhan. An article in Emerging Infectious Diseases reported a similar set of circumstances for a two-family cluster of persons in Zhejiang Province, China.2
Why Measures to Prevent Community Spread of Coronavirus are Critical
Another early first description of asymptomatic transmission was published in the New England Journal of Medicine.3 The chart below reproduced from the article details the timeline of events. A German businessperson, Patient 1, became ill three days after meeting in Munich on January 20 – 21, 2020 with a Chinese partner from Shanghai, the Index Patient. The Index Patient developed symptoms the day after their meeting and was confirmed positive with SARS-CoV-2 four days later on January 26. Three associates of Patient 1, only two of whom attended the meeting with the Index Patient, developed symptoms within 5 days of contact with Patient 1; their contacts occurred prior to onset of symptoms for Patient 1.
Further evidence for transmission by asymptomatic individuals is provided in correspondence to New England Journal of Medicine.4 These authors tracked SARS-CoV-2 in nasal and throat swabs of 18 patients from two family clusters in Zhuhai, Guangdong, China. Seventeen of the patients were symptomatic and later were confirmed to be COVID-19 positive, while one of the patients never had symptoms but was in close contact with a patient who had a known case. The testing showed that loads of SARS-CoV-2 in noses and throats of the patients were highest soon after onset of symptoms. Notably, the viral load in the asymptomatic patient was similar to that of patients with symptoms, which suggests the potential for transmission by asymptomatic or low symptomatic individuals.
Research Supports Current Approaches to Prevent the Spread of COVID-19
The new coronavirus appears to be present and transmissible in individuals prior to the onset of symptoms. Although only limited evidence of this phenomenon is available to date, these findings illustrate the importance of social distancing, hand washing and surface disinfection, regardless of symptom status, as a means of controlling transmission. They also underscore the value of on-demand testing so that infected individuals can be identified early and managed appropriately.
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The coronavirus outbreak is evolving rapidly and over the course of time more information and data will become available. This post reflects information available as of the publication date.
- Bai et al. Presumed asymptomatic carrier transmission of COVID-19. Journal of the American Medical Association, February 21, 2020.
- Tong et al. Potential presymptomatic transmission of SARS-CoV-2, Zhejiang Province, China, 2020. Emerging Infectious Diseases, March 9, 2020.
- Rothe et al. Transmission of 2019-nCOV infection from an asymptomatic contact in Germany. New England Journal of Medicine, 382:10, March 5, 2020.
- Zou et al. SARS-CoV-2 viral load in upper respiratory specimens of infected patients. New England Journal of Medicine. 382:12, March 19, 2020.