Most COVID-19 cases are spread by people without symptoms

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Published: May, 2021

You may be more likely to get COVID-19 from someone without symptoms than from someone who is feverish and coughing, according to a study published online Jan. 7, 2021, by JAMA Network Open. Study authors calculated that more than half of all COVID-19 cases were likely spread by someone without symptoms of the disease. Drawing on data from eight Chinese COVID-19 studies, the researchers developed a model that could estimate how people were infected, specifically looking at whether an infection would likely have come from someone with or without symptoms. They broke the asymptomatic people into two groups: those who were infected but hadn't yet developed symptoms (presymptomatic), and those who were infected but never got symptoms. Study authors assumed that it took five days from exposure for symptoms to develop or for someone to receive a positive test. The model also assumed that people were infectious for 10 days. Using these assumptions, they determined that 59% of people were infected by someone without symptoms. Thirty-five percent of the total would have been infected by someone who was presymptomatic, and 24% by someone who would never develop symptoms. These numbers rose when the researchers modified their model to assume that everyone was infectious a day earlier. Under this second model, 67% of cases were spread by someone without symptoms.

The researchers said their findings show the importance of public safety measures such as mask use, avoiding gatherings, distancing, and handwashing, all of which can reduce the spread of COVID-19 by people who don't know they have it.

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