COVID-19 patients may be most contagious one to two days before symptoms appear, study finds – USA TODAY

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COVID-19 patients may be most infectious in the days before they began showing symptoms, a new study from researchers in China found.

Researchersexamined "viral shedding" in 94 patients withCOVID-19admitted to Guangzhou Eighth Peoples Hospital, according to a small studypublished Wednesday in the peer-reviewed journal "Nature."

Scientists took throat swabs from thepatientsand found that viral loads were highest when symptoms began andgradually decreased towards the detection limit at about day 21. This finding is consistent with other small studies done attwo hospitals in Hong Kong and withpatients inZhuhai in the Guangdong province ofChina.

The study also modeled COVID-19's infectiousness from a separate sample of 77 pairs of peoplein which one had infected the other with coronavirus.

Using pairs obtained from publicly available sources within and outside mainland China,researchers estimated that 44% of the transmissions occurred during the index patient's presymptomatic stage,"in settings with substantial household clustering, active case finding and quarantine outside the home."

They were then able to inferthat infectiousness started 2.3 days and peaked 0.7 daysbefore symptoms appeared. Infectiousness was estimated to decline quickly within a week.

"Our analysis suggests that viral shedding may begin 2 to 3 days before the appearance of the first symptoms," researchers wrote. "More inclusive criteria for contact tracing to capture potential transmission events 2 to 3 days before symptom onset should be urgently considered for effective control of the outbreak."

These findings align with previous guidance from the World Health Organization which found infected people can be contagious and test positive 1 to 3 days before they develop symptoms. Presymptomatic transmissionwas seen in several casesinSingapore.

Tara Smith, a professor of epidemiology at Kent State University who was not involved in the study, said the research confirms both previous findings ofpresymptomatic transmission and that infected people seem to have a high viral load right around the time they began showing symptoms.

"That seems to be when they are maximally infectious," Smith said.

Presymptomatic transmissionis also seen in viruses like influenza and measles, Smith said, but is not seen withother coronaviruses like SARS and MERS.

Smithsaid more testing needs to be done to determine the rolepresymptomatic and asymptomatic transmission is playing in the current pandemic. She said these findings emphasize how important it is to keep social distancing measures in place to prevent those kinds oftransmission.

"That's why its important to avoid people as much as possible, to wash your hands, to wear masks," she said."To prevent you from spreading the virus."

Follow N'dea Yancey-Bragg on Twitter: @NdeaYanceyBragg

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COVID-19 patients may be most contagious one to two days before symptoms appear, study finds - USA TODAY