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COVID-19 May Be Most Contagious One To Two Days Before Symptoms Appear

More than 4 in 10 coronavirus cases are spread by those not obviously sick, a study suggests

People with coronavirus infections may be most contagious one to two days before they start to feel ill, new research suggests.

What’s more, considering pairs of people in which one person definitely caught the coronavirus from the other, scientists estimate that about 44 percent of COVID-19 cases may spread from person to person before symptoms appear. The results indicate that efforts to track down contacts of people with COVID-19 should include people encountered several days before a person’s symptoms start, researchers suggest April 15 in Nature Medicine.

Scientists swabbed the noses of 94 coronavirus patients who were admitted to the Guangzhou Eighth People’s Hospital in China from January 21 through February 14. Amounts of the virus’s genetic material peaked quickly after the onset of symptoms, then declined over about 21 days, the team found. That result indicates that virus production may be strongest at the start of the infection before the immune system kicks in to kill viruses and produce symptoms.

Analyzing 77 infector-infectee pairs allowed researchers to calculate when people are most contagious. The team estimates that contagiousness starts 2.3 days before symptoms begin and peaks 0.7 days before symptoms start.

Those estimates are similar to findings based on a small number of people with COVID-19 in Germany, in which researchers isolated actual infectious virus, versus just genetic material, from the patients. That study found that infectious virus is produced before symptoms begin and in the first week of illness (SN: 3/13/20). One person in that study was asymptomatic, meaning he never even developed symptoms, but still produced infectious virus.

Tracing contacts and isolating cases alone may not be successful at stopping more outbreaks if more than 30 percent of cases result from presymptomatic or asymptomatic transmission, the authors of the new study say. If that’s the case, more than 90 percent of contacts would need to be identified to contain viral spread. Increased handwashing and continued social distancing for everyone are probably also going to be needed for controlling the pandemic in the absence of COVID-19 treatments or a vaccine, the researchers say.


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