As recently as last week, many public health experts were fiercely opposed to the Biden administration’s campaign to roll out booster shots of the coronavirus vaccines to all American adults. There was little scientific evidence to support extra doses for most people, the researchers said.

The Omicron variant has changed all that.

Scientists do not yet know with any certainty whether the virus is easier to spread or less vulnerable to the body’s immune response. But with dozens of new mutations, the variant seems likely to evade the protection from vaccines to some significant degree.