The more transmissible variants of the coronavirus first reported in the U.K. and South Africa have been identified in D.C. residents, according to DC Health Director LaQuandra Nesbitt.

Three cases of the U.K. strain and one of the South African strain were identified, Nesbitt said during a press conference on Thursday. No information on the individuals’ travel history could be provided.

Because the District is unable to test every positive sample for mutations, Nesbitt said it is difficult to know the exact number of variant cases in circulation as some could be going undetected.

The U.K. and South African variants, known as B.1.1.7 and B.1.351 respectively, have been reported in several local jurisdictions, including Prince George’s and Montgomery counties, and in Northern Virginia.

The variants haven’t been shown to be more lethal than other strains of the virus, but they are highly-transmissible — a cause for concern as the region attempts to keep case counts low and vaccination rates climbing.

The U.S. has reported at least 930 cases of the U.K. variant and at least nine cases of the South African variant, according to the CDC.

While D.C.’s overall COVID-19 cases, hospitalizations, and deaths are decreasing, Nesbitt noted that the presence of more transmissible variants could reverse those trends.

“We don’t want to get overzealous and compromise the progress that we are making,” Nesbitt said. “And I think that’s critically important for people to know.” 

Nesbitt stopped short of recommending residents to double-mask in light of the new variants; a recent CDC study that found double-masks and tight-fitting masks to be highly effective.

“If you’re wearing a cloth mask it should have two or more layers. But the important thing is that people should wear a mask that fits,” Nesbitt said.