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US daily case rates of Covid-19 illnesses were lower among states that adopted mask mandates earlier, while hospitalisation growth rates declined during the first two weeks after implementing statewide mask mandates, researchers have found.

A study published in the Clinical Infectious Diseases journal on Sunday showed case-rate slopes of minus 1.08 per 100,000 per day among early-adopting states, and minus 0.37 per cent per 100,000 per day among late-adopters, compared with never-adopter states.

“Our findings support statewide mask requirements to mitigate Covid-19 transmission,” they said.

Peter Rebeiro, David Aronoff and Kevin Smith of Vanderbilt University School of Medicine found there were 15 early mask requirement adopters, 19 late adopters, and 17 never adopters among US states and the District of Columbia.

The median Covid-19 rates per 100,000 were 5.70, 5.59 and 5.99 cases a day, respectively. “These analyses advance the scientific evidence showing positive impacts of statewide mask requirements in the US,” they said.

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In a separate US Centers for Disease Control and Prevention study, hospital admissions growth rates declined by 5.5 percentage points among persons aged 18-64 years After mask mandates had been implemented for more than three weeks.

Even a two-week mandate resulted in a 2.9 percentage point decline among adults aged 40-64 years.

The study, led by Heesoo Joo and Gabrielle Miller, concluded that statewide mask mandates might be associated with reductions in transmission of Sars-CoV-2, the virus that causes Covid-19, and might contribute to reductions in Covid-19 hospitalization growth rates.

The researchers analysed admissions from March 22 to October 17 2020 at 10 hospitals and hospital networks.

“Mask-wearing is a component of a multipronged strategy to decrease exposure to and transmission of Sars-CoV-2 and reduce strain on the health care system, with likely direct effects on Covid-19 morbidity and associated mortality,” they wrote.