A study published yesterday in The Lancet Microbe shows that COVID-19 is most contagious in the first 5 days after symptom onset, underscoring the importance of early case identification and quarantine.
Led by researchers at the University of St. Andrews in Scotland, the systematic review and meta-analysis included 98 studies on 7,997 patients infected with coronaviruses that cause COVID-19 (SARS-CoV-2), severe acute respiratory syndrome (SARS-CoV-1), or Middle East respiratory syndrome (MERS-CoV). Seventy-nine of the studies (81%) involved COVID-19 patients.
The duration of viral RNA shedding was, on average, 17 days in the upper respiratory tract, 14.6 days in the lower respiratory tract, 17.2 days in stool, and 16.6 days in serum. The longest times of shedding were 83 days in the upper respiratory tract, 59 in the lower respiratory tract, 35 days in stool, and 60 days in serum.
Eight studies that used respiratory samples from patients in their first week of illness successfully cultured live virus, but no live virus was found in any sample collected after 9 days after symptom onset, despite persistent high viral loads.
SARS-CoV-2 viral load peaked in the upper respiratory tract, believed to be the primary source of transmission, in the first 5 days after symptom onset. SARS-CoV-1 peaked at 10 to 14 days, and MERS-CoV peaked at 7 to 10 days, which the researchers said may be why COVID-19 spreads more quickly in the community and is more difficult to contain.
Infectious for about 9 days
The study found no difference between viral load peaks in COVID-19 patients with and without symptoms, but indications are that asymptomatic patients clear the virus faster and therefore could be contagious for a shorter time
While the authors said that they can't recommend an optimal duration for quarantine because their study involved only confirmed cases rather than possible exposures, the results appear to indicate that people with COVID-19 can infect others for about 9 days. Most countries currently recommend that COVID-19 patients quarantine for 10 days.
But the researchers caution that many patients in the studies they analyzed were hospitalized and received different therapies that may have altered the course of their infection and therefore their period of infectiousness.
"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," coauthor Antonia Ho, PhD, MBChB, MRes, of MRC-University of Glasgow Centre for Virus Research, said in a Lancet news release. "Further studies on viral shedding in this context are needed."
Immediate quarantine, contact tracing
Understanding the pattern of COVID-19 contagiousness is important for public health officials formulating measures to control its transmission, the researchers said.
"Our findings are in line with contact tracing studies which suggest the majority of viral transmission events occur very early, and especially within the first 5 days after symptom onset, indicating the importance of self-isolation immediately after symptoms start," lead author Muge Cevik, MD, MSc, of the University of St. Andrews, said. "We also need to raise public awareness about the range of symptoms linked with the disease, including mild symptoms that may occur earlier on in the course of the infection than those that are more prominent like cough or fever."
Cevik also said that repeat diagnostic testing for COVID-19 may not be necessary to decide whether a patient is no longer contagious, "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."