Infectiousness peaks in first five days of COVID-19 symptoms, research suggests
A health worker takes a nasal swab sample of a man to test for COVID-19 in Mumbai, India, Thursday, Nov. 19, 2020. (AP Photo/Rafiq Maqbool)
TORONTO -- New research suggests that infectiousness peaks early in COVID-19 patients, highlighting the need to quickly identify and isolate cases before the virus spreads.
The research, published by The Lancet Microbe journal on Thursday, suggests people with COVID-19 reach their highest viral load within the first five days with symptoms. And even though genetic material from the virus may be found in the body for weeks, no live virus capable of causing infection was found in human samples beyond the first nine days of symptoms.
The report is the most comprehensive of its kind and used meta-analysis, the large-scale review of prior studies, to assess 98 studies on COVID-19, SARS and MERS. Part of the reason researchers studied all three diseases was to determine why COVID-19 has spread more rapidly than the earlier diseases.
The reason, researchers say, is because the viral load in patients with COVID-19 appears to peak in the upper respiratory tract between day one of symptoms and day five of symptoms. The upper respiratory tract is believed to be the main source of transmission since COVID-19 is largely spread through particles that enter the air.
In SARS and MERS, viral loads peaked several days later than COVID-19, sometimes up to two weeks after the onset of symptoms.
Lead author author Dr. Muge Cevik of the University of St. Andrews, U.K. said in a statement that the new analysis “provides a clear explanation for why SARS-CoV-2 spreads more efficiently than SARS-CoV and MERS-CoV and is so much more difficult to contain.”
Researchers also found that viral loads appear to be similar among COVID-19 patients with and without symptoms. However, most studies indicated that patients without symptoms still have two key advantages: they may be able to clear the virus faster from their body, and they could be infectious for a briefer period of time.
“Several studies have found that individuals with asymptomatic infection may clear the virus faster, suggesting that those without symptoms may be as infectious as those with symptoms at the beginning of infection, but may be infectious for a shorter period. However, at this stage, there are limited data available on the shedding of infectious virus in asymptomatic individuals to inform any policy change on quarantine duration in the absence of testing,” Cevik said.
While the evidence so far on COVID-19 shows a pattern of nine days of infectiousness, researchers did not offer a suggestion as to how long quarantine periods should last, since their study only looked at confirmed cases and not individuals who may have been exposed.
In Canada, anyone with symptoms of COVID-19 must contact their public health authority and isolate at home for 14 days to avoid spreading the illness to others.
The new research reinforces previous studies that used contact tracing and determined spread happened fastest at the onset of infection.