COVID-19

LONDON — As nation after nation rushed this week to close their borders with Britain, the moves brought back memories of the way the world reacted after the coronavirus first emerged broadly in the spring. Most of those initial travel prohibitions came too late, put in place after the virus had already seeded itself in communities far and wide.

This time, with countries trying to stop the spread of a new, possibly more contagious coronavirus variant identified by Britain, it may also be too late. It is not known how widely the variant is already circulating, experts say, and the bans threaten to cause more economic and emotional hardship as the toll wrought by the virus continues to grow.

Pulse oximeters are one of the most commonly used tools in medicine. The small devices, which resemble a clothespin, measure blood oxygen when clipped onto a fingertip, and they can quickly indicate whether a patient needs urgent medical care.

Health providers use them when they take vital signs and when they evaluate patients for treatment. Ever since the pandemic started, doctors have encouraged patients with Covid to use them at home.

But in Black patients, the devices can provide misleading results in more than one in 10 people, according to a new study.