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. Two new cases of the South Africa variant were confirmed Tuesday just outside the nation’s capital, in Montgomery County.

The mutation alters the structure of the virus’s spike protein — the target for vaccines and many naturally produced antibodies. The mutation may help the virus to elude detection and make neutralization by the human immune system less efficient. In effect, it makes the virus stealthier, a great concern to vaccine developers, who seek to train antibodies to zero in on recognizable invaders and destroy them.

The appearance of the E484K mutation in some infections caused by the U.K. variant is “a worrying development, though not entirely unexpected,” said Julian Tang, a clinical virologist at University of Leicester.

We have to come down hard on this variant,” said British Health Secretary Matt Hancock. “Our mission must be to stop its spread all together.” ...