Three Chinese males on a flight that arrived in Chongqing from India via Nepal were confirmed infected with a coronavirus variant that is spreading in India, according to the Chinese Center for Disease Control and Prevention.

The center said on Saturday in China CDC Weekly that the men, aged from 20 to 30, were infected with B.1.617, an emerging variant in India that was first identified in October.

The men, who were probably infected there, the China CDC said, were coworkers at a cellphone company based in Noida, India, since late 2019.

An epidemiological investigation returned negative COVID-19 tests on multiple occasions in India before their return trip, which began on April 19. Preventive measures were taken throughout the trip — wearing protective clothing, N95 masks, goggles and gloves. The men went from Noida to New Delhi by car, then flew to Kathmandu, Nepal, and stayed at a hotel there for two days. They flew to Chongqing on April 21.

But new coronavirus tests at Chongqing Customs were positive, and the men were transferred to the Chongqing Public Health Medical Center, the designated COVID-19 hospital.

Test specimens taken from the men were sent to Chongqing CDC and sequenced on April 28 and 30 and later found to be related to the Pangolin lineage B.1.617, the emerging strain. It was recently designated as a variant of interest by the World Health Organization.

Wu Zunyou, chief epidemiologist at the China CDC, said at a news conference on April 29 that the coronavirus has been constantly mutating since the start of the pandemic, which has had severe consequences worldwide.

According to the China CDC, China has been able to confirm cases of the coronavirus variant spreading in India in some cities and expects to see new cases imported from India. Even so, the possibility of further domestic infections is low because of strict prevention and control protocols.