Mask-wearing is even more important for preventing the virus' spread and the sometimes deadly COVID-19 illness it causes than social distancing and stay-at-home orders, researchers said, in the study published in PNAS: The Proceedings of the National Academy of Sciences of the USA.

Infection trends shifted dramatically when mask-wearing rules were implemented on April 6 in northern Italy and April 17 in New York City - at the time among the hardest hit areas of the world by the health crisis - the study found.

"This protective measure alone significantly reduced the number of infections, that is, by over 78,000 in Italy from April 6 to May 9 and over 66,000 in New York City from April 17 to May 9," researchers calculated.

When mask-wearing went into effect in New York, the daily new infection rate fell by about 3 per cent per day, researchers said. In the rest of the country, daily new infections continued to increase.

Direct contact precautions - social distancing, quarantine and isolation, and hand sanitising - were all in place before mask-wearing rules went into effect in Italy and New York City. But they only help minimise virus transmission by direct contact while face covering helps prevent airborne transmission, the researchers say.

"The unique function of face covering to block atomisation and inhalation of virus-bearing aerosols accounts for the significantly reduced infections," they said. That would indicate "that airborne transmission of COVID-19 represents the dominant route for infection".

The US Center for Disease Control and Prevention on Friday urged organisers of large gatherings that involve "shouting, chanting or singing to strongly encourage the use of cloth face coverings to lower the risk of spreading the coronavirus".

Australian Associated Press