A second case of the Covid-19 strain known as the South African variant has been detected in Santa Clara County, officials said Wednesday, stressing the need for continuing safety measures even as the state eases pandemic restrictions.

The B.1.351 variant was discovered in a specimen taken in early March and randomly selected for genomic sequencing, officials said, noting that an investigation is underway.

County public health experts said the case, the fourth confirmed in California, is believed to be due to community transmission and not travel-related. A travel-related case was detected in the county earlier this year.

“Considering the national trends, we have been operating under the assumption that these variants were circulating at some level in our communities,” county Public Health Officer Dr. Sara Cody said. “This latest case confirms that we do have community transmission and reminds us to not let down our guard in the middle of this pandemic.”

The county has also seen the number grow to 15 confirmed cases of B.1.1.7, another strain known as the UK variant. The actual number of variant cases in the community is unknown, “since current genomic sequencing capabilities only allow a fraction of positive cases to be sequenced,” officials said.

“There is still a lot we are learning, but we do know some very key things about Covid-19 and how to protect ourselves,” Santa Clara County Deputy Public Health Officer Dr. George Han said in a public announcement earlier this week. “Even with these variants circulating in our community, the best tools to protect ourselves and our loved ones remain familiar to us and are available now. Wear your masks, keep your distance from others, avoid gatherings, especially indoors, and get vaccinated when it’s your turn.”

Travel is strongly discouraged, officials said, and activities being allowed as areas move into less restrictive tiers in the state’s Blueprint for a Safer Economy should still be considered high risk and steps should be taken to minimize exposure.