MADISON, Wis. (AP) — A second case of a new, more contagious form of the COVID-19 virus has been detected in Wisconsin, less than a month after it was first seen, health officials said Tuesday.

The variant was first detected on Jan. 12 and was identified again on Sunday by lab partners of the state Department of Health Services.

Wisconsin chief medical officer Dr. Ryan Westergaard calls the development “concerning” and there are likely “many more cases” of it in the state than have been detected, he said. Only a small number of tests collected can be checked for variants through whole genome sequencing.

“Wisconsinites must continue to be vigilant to stop the spread of COVID-19 by wearing masks, staying home, washing their hands, and getting vaccinated when they are able,” Westergaard said in a statement.

The first case was discovered in Eau Claire. The state health department did not disclose where the second positive came from.

The variant was first discovered in England in November and December. It’s since turned up across the United States.

Fears of variants taking hold come as vaccinations are increasing across the country. Wisconsin currently ranks 10th in the percentage of population that has received at least one dose of the vaccine, according to the U.S. Centers for Disease Control and Prevention.