Analysis

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.

Allergic reactions reported in two health workers who received a dose of Pfizer’s vaccine in Alaska this week have reignited concerns that people with a history of extreme immune flare-ups might not be good candidates for the newly cleared shots.

The two incidents follow another pair of cases in Britain. Three of the four were severe enough to qualify as anaphylaxis, a severe and potentially life-threatening reaction. But all four people appear to have recovered.

WASHINGTON — Elizabeth Graves, an ardent supporter of President Trump, is not opposed to vaccines. She said she had taken flu shots and pneumonia shots and, having just turned 50, was interested in being vaccinated against shingles.

But Ms. Graves, a legal transcriptionist in Starkville, Miss., said she would not be taking a coronavirus vaccine — and the sight of Vice President Mike Pence rolling up his sleeve to get vaccinated on live television on Friday, she added, would not change her mind.