COVID-19 cases drop, but variants point to dangers ahead 

The number of Americans testing positive for the coronavirus has dropped substantially from an early January zenith, easing the strain on hospitals across the nation that faced danger over the winter holidays.

But new and more transmissible strains of the coronavirus are circulating more widely across the world, and public health experts caution that, even with the beginnings of mass vaccination programs, Americans must be more vigilant than ever in protecting themselves and reducing the spread.

The pace of infections has followed a steady and disturbing pattern in the year since the virus was first identified in the United States: Cases rise alarmingly, first in core epicenters and then across the nation, followed by a plateau and a slight decline, before the cycle begins again.

What worries epidemiologists and health experts is that the declines have so far stalled at successively higher baselines, leading to larger spikes when infections begin accelerating anew.

New strains: The pattern is likely to accelerate once again as two more transmissible strains — one known in the scientific literature as B.1.1.7 and first identified in the United Kingdom, the second known as B.1.351 and first identified in South Africa — begins spreading faster. The Centers for Disease Control and Prevention (CDC) estimates those strains will become dominant in the United States in the coming weeks. On Thursday, the CDC said it had identified the first two strains of the South African variant in South Carolina.