A coronavirus mutation that appears to limit the protection of vaccines against infection has appeared in the United Kingdom, which is already struggling with a highly transmissible and apparently more lethal virus variant. The worrisome mutation, at a site on the virus RNA called E484K, has drawn close scrutiny from infectious-disease experts, who have given it the nickname “Eeek.” In addition to its appearance in the U.K. variant, it has been seen in variants that spread rapidly in South Africa and Brazil. It has also been identified in recent days in a handful of cases in the United States.