This is when people with COVID-19 are at their most contagious

Other data have suggested that 16% of coronavirus transmission is due to carriers not displaying symptoms or only showing very mild symptoms who, while they’re contagious, may not believe they have the disease.

One case study of the quarantined Italian town of Vò published in the peer-reviewed journal Nature in June revealed more than 40% of COVID-19 infections had no symptoms.

With a population of approximately 3,200 people, Vò reported Italy’s first COVID-related death on Feb. 20. As a result, the residents of the town were placed in quarantine for 14 days.

Some 2.6% of the town tested positive for SARS-CoV-2, the virus that causes COVID-19, at the beginning of the lockdown, but that figure fell to 1.2% after a couple of weeks. Throughout this time, 40% of those infections were people who displayed no symptoms. The researchers also concluded that it took 9.3 days for people who tested positive to be virus-free.

“Someone with an asymptomatic infection is entirely unconscious of carrying the virus and, according to their lifestyle and occupation, could meet a large number of people without modifying their behavior,” found the study, which was carried out by researchers at Imperial College London and the University of Padua.

“If we find a certain number of symptomatic people testing positive, we expect the same number of asymptomatic carriers that are much more difficult to identify and isolate,” according to Enrico Lavezzo, a professor in the University of Padua’s department of molecular medicine.

Related: Was COVID-19 made in a lab? Will a vaccine protect you forever? Does the sun help? Coronavirus myths are spreading in 25 languages — here are the most popular

That, health professionals say, raises questions about how contagious they are after contracting the virus, and for how long they remain so.

Knowing when an infected person can spread SARS-CoV-2 is just as important as how the virus spreads so rapidly. WHO recently published a scientific brief on how the virus spreads, particularly among those who don’t show symptoms.

The virus can be detected in people one to three days before their symptom onset, with the highest viral loads around the day of the onset of symptoms, followed by a gradual decline over time, according to the World Health Organization.

This level of contagiousness appears to be one to two weeks for asymptomatic persons, and up to three weeks or more for patients with mild to moderate disease. “Preliminary data suggests that people may be more contagious around the time of symptom onset as compared to later on in the disease,” it added.

“Transmission of SARS-CoV-2 can occur through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions or their respiratory droplets, which are expelled when an infected person coughs, sneezes, talks or sings,” WHO added. This makes asymptomatic transmission all the more prevalent, scientists say.

However, all studies on asymptomatic people have limitations, the WHO added: “For example, some studies did not clearly describe how they followed up with persons who were asymptomatic at the time of testing to ascertain if they ever developed symptoms. Others defined ‘asymptomatic’ very narrowly as persons who never developed fever or respiratory symptoms, rather than as those who did not develop any symptoms at all.”

The U.S. COVID-19 death toll could reach nearly 300,000 by Dec. 1, but consistent mask-wearing beginning today could save approximately 70,000 lives, according to the Institute for Health Metrics and Evaluation (IHME) at the University of…