Information on commonly asked questions

Written by Saif Badran and Suhail A. Doi

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Respiratory droplets are mainly the route via which person-to-person transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) occurs. With this sort of transmission, the virus reaches a new host through contact with respiratory secretions that are incorporated into respiratory droplets, Such droplets are created when a person with infection coughs, sneezing, or even talks with another person and if such a droplet makes direct contact with the eyes, nose, or mouth of the opposite person. Another route is to touch an infected surface on which a droplet has landed and then touching ones eyes, nose, or mouth. Droplets typically do not travel more than six feet (about two meters) and do not linger in the air; however, in one study, SARS-CoV-2 remained viable in aerosols under experimental conditions for at least three hours. Given the current uncertainty regarding transmission mechanisms, airborne precautions are recommended routinely in some countries and in the setting of certain high-risk procedures in others. In the same study the authors report that the virus remains viable on plastic and stainless steel longer (>72h) than on cardboard and copper and decays exponentially (markedly lower titers) over time and is usually completely gone within 4 – 24 hours on copper or cardboard.

In a study of 1099 patients with confirmed symptomatic COVID-19, the median incubation period (time from infection to symptom onset) was four days (interquartile range two to seven days). Using data from 181 publicly reported, confirmed cases in China with identifiable exposure, one modeling study estimated that symptoms would develop in 2.5 percent of infected individuals within 2.2 days and in 97.5 percent of infected individuals within 11.5 days and the median incubation period in this study was 5.1 days. In general, the incubation period for COVID-19 is thought to be within 14 days following exposure, with most cases becoming symptomatic approximately four to five days after exposure.

The interval during which an individual with COVID-19 is infectious is uncertain. Most data informing this issue are from studies evaluating viral RNA detection from respiratory and other specimens. However, detection of viral RNA does not necessarily indicate the presence of infectious virus. However duration of shedding, is also variable and there appears to be a wide range, which may depend on severity of illness. In one study of 21 patients with mild illness, 90 percent had repeated negative viral RNA tests on nasopharyngeal swabs by 10 days after the onset of symptoms; tests were positive for much longer in patients with more severe illness. In another study of 137 patients who survived COVID-19, 25% of patients stopped shedding by 17 days, 50% stopped by 20 days and 75% stopped by 24 days. The maximum duration of shedding was 37 days. This concurs with other reports suggesting that the SARS-CoV-2 virus can initially be detected 1–2 days prior to symptom onset in upper respiratory tract samples and can persist for 7–12 days in moderate cases and up to 2 weeks in severe cases (WHO mission to China Report).

This time period coincides with available preliminary data, on recovery where the median time from onset to clinical recovery for mild cases was approximately 2 weeks and is at least three weeks for patients with severe or critical disease and if a person is destined to have severe disease this becomes evident in the first week.

Given this data the optimal duration of home isolation cannot be defined with certainty but certain guidelines can be proposed and one has been put out by the United States Centers for Disease Control and Prevention (CDC) for health care settings as well as another recommendation for discontinuation of home isolation which includes both test-based and non-test-based strategies. In summary, when a test-based strategy is used, a person may discontinue home isolation when there is resolution of fever and improvement in cough and other respiratory symptoms. The tests can then be done for COVID-19 on two consecutive nasopharyngeal swab specimens ≥24 hours apart and both should be negative. When tests are limited, it makes sense that testing occurs no less than 14 days from onset of symptoms. If a non-test-based strategy is required, patients may discontinue home isolation when at least seven days have passed since symptoms first appeared and at least 3 – 7 days have passed since resolution of fever and all respiratory symptoms.