According to analysis by data expert Justin Hart, who has been following COVID-19 data for months, demonstrated in a Sunday Twitter thread that states with mask mandates had a greater number of COVID cases per 100,000 people than states without mandates.

See thread below:

And while there were some objections to Hart's analysis - such as whether there might be bias towards getting tested for mask-wearers, or regional differences in population density, many of the replies to Hart's thread support his findings:

And a hypothesis:

Maybe the CDC, WHO, Dr. Fauci and the Surgeon General were right in February when they said masks don't work? On the other hand, they're so useful for other things...