The covid-19 variants that have emerged in the United Kingdom, Brazil, South Africa and now Southern California are eliciting two notably distinct responses from U.S. public health officials. First, broad concern. A variant that wreaked havoc in the U.K., leading to a spike in cases and hospitalizations, is surfacing in a growing number of places in the U.S. This week, another worrisome variant seen in Brazil surfaced in Minnesota. And on Thursday, two cases of the South African variant were confirmed in South Carolina. If these or other strains significantly change the way the virus transmits and attacks the body, as scientists fear they might, they could cause yet another prolonged surge in illness and death in the U.S., even as cases have begun to plateau and vaccines are rolling out.