Coronavirus Most Contagious Soon After Infection

TUESDAY, Nov. 24, 2020 (HealthDay News) -- People infected with the new coronavirus are most contagious in the first week after they develop symptoms, which shows the importance of identifying and isolating infected people early, researchers say.

They reviewed 79 studies and clinical trials, including 73 that included hospitalized COVID-19 patients only.

SARS-CoV-2 viral load appears to peak in the upper respiratory tract -- believed to be the main source of transmission -- over the first five days after symptom onset, according to the analysis published Nov. 19 in The Lancet Microbe journal.

Previous research suggested that people with the new coronavirus are infectious for nine days.

"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 five days after symptom onset, indicating the importance of self-isolation immediately after symptoms start," said review lead author Dr. Muge Cevik, from the University of St. Andrews in Scotland.

This study included only confirmed cases and not people who may have been exposed, so it can't provide guidance on how long people with possible exposure should be quarantined, Cevik and her team noted.

They also reviewed eight studies on SARS-CoV and 11 on MERS-CoV infection, and found that viral loads of those coronaviruses peak later, which may explain why SARS-CoV-2 spreads more rapidly in the community.

Among people with SARS-CoV-2 infection, viral loads appear to be similar between those who develop symptoms and those who don't, the review found. Most studies suggest that asymptomatic people may clear the virus faster from their body and be infectious for a shorter length of time.

SARS-CoV-2 genetic material may still be detected in respiratory or stool samples for several weeks among infected people, but no live virus (that can cause infection) was found in any type of sample collected more than nine days after the start of symptoms, the review found.

Many countries currently recommend that people with a SARS-CoV-2 infection should self-isolate for 10 days, which is supported by the review results, according to the authors.

"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 said in a journal news release.

Knowing when infected people are most contagious is crucial for developing public health measures to control the spread of the new coronavirus.

"These findings suggest that in clinical practice, repeat PCR [polymerase chain reaction] 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," Cevik said. "In patients with non-severe symptoms, their period of infectiousness could instead be counted as 10 days from symptom onset."

More information
For more on COVID-19, go to the U.S. Centers for Disease Control and Prevention.

SOURCE: The Lancet Microbe, news release, Nov. 19, 2020