Coronavirus patients may be most infectious in first week of symptoms, study finds

Mandy Zuo

The pandemic coronavirus could be most contagious in the first week the carrier develops symptoms, which could partly explain why the disease has spread so quickly, according to a Hong Kong study.

The researchers based their assessment on saliva samples from 23 patients confirmed with Covid-19, the disease caused by the coronavirus, at two hospitals in the city.

The results showed that the viral load in the patients – who were all aged between 35 and 75 – was highest during the first seven days after symptom onset and declined gradually after that, according to the paper, published in The Lancet medical journal on Monday.

“The high viral load within the first week of illness suggests that the virus can be transmitted from one person to another easily before the patient is hospitalised,” said Kelvin To Kai-wang, clinical associate professor at the University of Hong Kong’s microbiology department and a co-author of the paper.

The virus, which has infected more than 400,000 people around the world and killed 16,000, tended to be in greater numbers in older patients and could stay in the body for nearly a month, the researchers said.

In one case, the virus was detected 25 days after the patient showed symptoms.

“One-third of our patients had viral shedding for 20 days or more,” said To, adding that patients might have to stay in isolation wards for longer.

In mainland China, patients stay in isolation centres for 14 days after being released from hospital, and then another 14 days at home. In Hong Kong, discharged patients are not required to self-isolate after being discharged from hospitals but medical staff do follow up to monitor their progress of recovery.

To said the prolonged viral shedding did not necessarily mean the patient was infectious for a long time because their test only detected the presence of the viral genome – viral nucleic acid – and not the live virus.

“But from an infection control point of view, we need to assume that anyone with viral nucleic acid detected is infectious [and isolate patients long enough to reduce risks],” he said.

“[But] there may not be enough isolation wards if there are a large number of new patients.”

The researchers also suggested that it could be safer for the patients themselves to collect a saliva sample rather than to have medical workers to do the standard throat and nasal swabs.

“Collection of nasopharyngeal or throat swab specimens can induce coughing and sneezing, which generates aerosol and is a potential health hazard for healthcare workers,” the study said.

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