Nigeria is the latest African country to detect the Omicron variant.

Nigeria’s public health institute said on Wednesday that it had identified the variant in three travelers who arrived from South Africa in the last week, Dr. Ifedayo Adetifa, its director general, said in a statement.

The institute, the Nigeria Center for Disease Control, had said earlier on Wednesday that it had also detected the Omicron variant in a sample from October, but then retracted that announcement. The earlier sample was of the Delta variant, according to a message shared on WhatsApp by the institute’s spokesman, Emeka Oguanuo.

Referring to the three Omicron cases, Dr. Adetifa said in the statement: “Follow up to ensure isolation, linkage to clinical care, contact tracing and other relevant response activities have commenced.”

Nigeria is the third country in mainland Africa — besides South Africa and Botswana — to report the variant, which is now known to have reached every continent except Antarctica. But scientists say it could be weeks before more is known about how transmissible the variant is, the severity of illness it causes and how much protection the current vaccines offer against it.

Still, governments have reacted with alarm and markets have tumbled in recent days, as the W.H.O. rated the global risk from the variant “very high.”

On Wednesday, Nigerian health officials urged state governments to make testing easily accessible, vaccinate many more people and ensure the public was adhering to public health measures including masking and social distancing. Nigeria has so far reported 214,113 positive cases and 2,976 deaths from the coronavirus.

With more than 200 million people, Nigeria has administered just over 9.8 million vaccine doses, according to the World Health Organization. Starting Wednesday, federal government workers were expected to either be vaccinated or show a negative Covid-19 certificate. Travelers into the country are expected to arrive with a negative Covid test and to take two more tests on the second and seventh day following arrival.