Covid-19 patients can shed fragments of the virus that causes the infection for up to 83 days in their respiratory or stool samples but they are unlikely to be infectious for as long. According to a study published in The Lancet Microbe, one of world’s top medical journals, no live virus has been isolated from culture of the respiratory or stool sample beyond day nine of symptoms despite persistently high viral RNA loads. This means that a person affected by Covid-19 is infectious for nine days after developing disease symptoms though tests may find presence of the virus for nearly three months. The study conducted by researchers from UK and Italy involved systemic review and meta-analysis of 79 studies that focused on SARS-CoV-2, the virus which causes Covid-19. “The majority of studies included in our review were performed in patients who were admitted to hospital. Therefore, our findings may not apply to people with milder infection although these results suggest those with milder cases may clear the virus faster from their body. Additionally, the increasing deployment of treatments, such as dexamethasone, remdesivir as well as other antivirals and immunomodulators in clinical trials are likely to influence viral shedding in hospitalised patients. Further studies on viral shedding in this context are needed,” Dr Antonia Ho of MRC-University of Glasgow Centre for Virus Research, UK, who is one of the authors of the study, said.
The Lancet Microbe study also suggests that people infected with SARS-CoV-2 are mostly likely to be highly infectious from symptom onset and the following five days. Therefore, the researchers said, it is important to self-isolate immediately after symptoms start. Understanding when patients are most likely to be infectious is of critical importance for informing effective public health measures to control the spread of SARS-CoV-2. The Lancet study looked at key factors involved in this: viral load (how the amount of the virus in the body changes throughout infection), viral RNA shedding (the length of time someone sheds viral genetic material (RNA), which does not necessarily indicate a person is infectious, as this is not necessarily able to replicate), and isolation of the live virus (a stronger indicator of a person’s infectiousness, as the live virus is isolated and tested to see if it can successfully replicate in the laboratory). The researchers found that the average length of time of viral RNA shedding into the upper respiratory tract, lower respiratory tract, stool and serum were 17 days, 14.6 days, 17.2 days and 16.6 days, respectively. The longest length of time that RNA shedding lasted was 83, 59, 35 and 60 days, respectively. “These findings suggest that in clinical practice, repeat PCR testing may not be needed to deem that a patient is no longer infectious, as this could remain positive for much longer and does not necessarily indicate they could pass on the virus to others. In patients with non-severe symptoms, their period of infectiousness could instead be counted as 10 days from symptom onset,” Dr Muge Cevik, the lead author of the study, said.