Thread by @latimes

The first — and only — randomized controlled study of the use of face coverings to prevent the spread of COVID-19 is finished.

The conclusion: masks are most effective if everyone wears them. latimes.com/science/story/…

The study, which began in Denmark at the start of the pandemic, involved 6,000 participants.
Half of them were given 50 surgical face masks and were asked to wear them whenever they left home for the next month.
latimes.com/science/story/…

Overall, 95 of the 4,862 volunteers who made it to the end of the study became infected with SARS-CoV-2.

Researchers couldn't find data to support that mask-wearing participants were better protected than non-mask-wearing participants.
latimes.com/science/story/…

In a typical clinical trial, this is the point where researchers would say their intervention didn’t work. But in this case, the investigators went the other way. The problem, they said, wasn’t with the masks. The problem was that people didn’t use masks enough.

Their reasoning? The study didn’t actually measure the effects of mask wearing. It measured the effectiveness of mask mandates.
latimes.com/science/story/…

Only 46% of volunteers in the mask group told the researchers they followed all the rules about wearing masks in public, 47% said they “predominantly” wore their masks, and 7% said they didn’t follow the rules.
latimes.com/science/story/…

Another way the study fell short: Lock down orders and social distancing measures were in place in Denmark for much of the study, reducing the risk that participants were coming into contact with others, let alone people with COVID.
latimes.com/science/story/…

But why didn’t the science journal editors reject the study? After all, the results could be misinterpreted by those who oppose mask mandates.

Burying the findings of a well-conducted study that didn’t turn out as expected would be worse, editors wrote.