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Study: Covid19 Most Contagious Before Symptoms Appear

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Apr 21, 2020

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People infected with Covid19 may be most contagious during the two to three days before the onset of symptoms, a study published last week in the journal Nature, concluded. Most of the outbreak-control measures around the world seek to identify and isolate symptomatic people, a strategy whose effectiveness is unclear if the virus is spread by people with no symptoms.

Researchers at the Guangzhou Eighth People’s Hospital in China conducted this study by examining viral shedding, or the expulsion and release of infectious virus, in 94 Covid19-positive patients admitted there.

Having collected the patients’ nasal swabs, scientists found the virus production in a patient’s body decreases once the immune system kicks in to kill the coronavirus and produce symptoms. The researchers estimated that 44 percent of transmissions, in the conditions they studied, occurred during a patient’s presymptomatic stage.


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“Significant presymptomatic transmission would probably reduce the effectiveness of control measures that are initiated by symptom onset, such as isolation, contact tracing and enhanced hygiene or use of face masks for symptomatic persons,” the researchers concluded.

“More inclusive criteria for contact tracing to capture potential transmission events 2 to 3 days before symptom onset should be urgently considered,” the researchers advised. However, acknowledging the challenges that such an enormous scale of contact-tracing might pose, they concluded that enhanced personal hygiene and social distancing might be our best strategy for containing the spread of the virus within a community.

The study confirms previous findings of presymptomatic transmission that found infected individuals have a high viral load right around the time they begin experiencing symptoms. Tara Smith, professor of epidemiology at Kent State University, who was not part of the team of researchers, told USA Today. “That’s why its important to avoid people as much as possible, to wash your hands, to wear masks — to prevent you from spreading the virus,” she added.

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Written By Devrupa Rakshit

Devrupa Rakshit is an associate editor with The Swaddle. She is a lawyer by education, a poet by accident, and a painter by shaukh. She has her own podcast called #DateNightsWithD on Spotify. You can find her on Instagram @devruparakshit.

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