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New studies in several countries and a large coronavirus outbreak in Massachusetts are bringing into question reassurances by US officials about the way the coronavirus is spreading.

Previously, officials have emphasized that the virus is spread mainly by people who are already showing symptoms, such as fever, cough or difficulty breathing.

If that's true, it's good news -- people who are obviously ill can be identified and isolated, making it easier to control an outbreak.

But it appears that a Massachusetts coronavirus cluster with at least 82 cases was started by people who were not yet showing symptoms, and more than half a dozen studies have shown that people without symptoms are causing substantial amounts of infection.

During a press briefing at the White House on Saturday, the administration's coronavirus response coordinator, Dr. Deborah Birx, seemed to strike a somewhat different note on asymptomatic transmission.

"Until you really understand how many people are asymptomatic and asymptomatically passing the virus on, we think it's better for the entire American public to know that the risk of serious illness may be low, but they could be potentially spreading the virus to others."

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