The CDC changed its mask guidance for vaccinated Americans. Other countries are taking different approaches.

With some countries rapidly inoculating their populations against the coronavirus, and pandemic fatigue deepening, a common question has reverberated around the world: When can the masks come off?

In the United States, fully vaccinated individuals can now leave their masks behind when walking, jogging, biking or dining with friends outdoors, federal officials said Tuesday. Ditching a mask at small outdoor gatherings is also deemed safe, but the CDC still recommends masks for indoor activities.

More than 52 percent of eligible people in the United States have received at least one shot, and the Centers for Disease Control and Prevention guidance followed calls from public health experts to relax mask mandates outdoors, where transmission is less likely. But officials warned that crowded outdoor events — like sporting events and concerts — are still dangerous, so both vaccinated and unvaccinated attendees should keep their masks on.

Masks — and requirements to don them — have prompted debate around the world since the start of the pandemic. Widely accepted in some places, they have been protested or completely forgone in others. In the United States, federal guidelines have shifted over time, and individual states have imposed different requirements. Mask-wearing became highly politicized, complicating efforts to enforce usage. But in places like Hong Kong and Japan, face coverings were widely worn even before the pandemic, so everyday mask-wearing easily became routine.

Studies show that wearing masks is one of the most effective ways to reduce coronavirus transmission. Some public health experts argue that societal immunity isn’t yet high enough to take them off — particularly since vaccines are not 100 percent effective and questions remain about whether vaccinated people can transmit the virus. Some experts suggest that wearing a mask even if vaccinated can send a powerful signal to unvaccinated individuals — such as children — to do the same.