The Hong Kong government said Thursday that it had detected two cases of a new variant identified in South Africa, which scientists have warned shows a "big jump in evolution" and could limit the effectiveness of vaccines.

The infections were detected in a man who had returned to Hong Kong from South Africa this month, and later in another man staying across the hall in the same quarantine hotel. (Hong Kong requires almost all overseas arrivals to quarantine in hotels for two to three weeks.) The virus’s genetic sequence was identical in both men, suggesting airborne transmission, according to the city’s Center for Health Protection. Both men were vaccinated.

Further sequencing by the University of Hong Kong confirmed that the viruses belonged to the new variant from South Africa, officials said, though they acknowledged that information about the variant’s public health impact was "lacking at the moment".

Some Hong Kong experts have questioned the length and efficacy of Hong Kong’s quarantines, noting that officials have recorded several cases of residents in quarantine hotels apparently infecting people who were staying in other rooms.

In the case of the latest variant infections, the government has blamed the first man for not wearing a surgical mask when opening his hotel room door, as well as "unsatisfactory air flow" in the hotel. As of Friday afternoon there had been no reports of infections in nearby rooms.

The presence of the new variant may complicate efforts to reopen the border between Hong Kong and mainland China. For months, Hong Kong officials have said that resuming quarantine-free travel between the Chinese territory and the mainland — virtually the only places in the world still pursuing a containment strategy that seeks full eradication of the virus — is their top priority, even though the strategy has damaged the city’s reputation as a global finance hub.

Mainland officials have said that Hong Kong is not doing enough to control the virus, even though the city has recorded just two locally transmitted cases in the past six months. The mainland has recently faced new domestic outbreaks; on Thursday, the National Health Commission there reported four new local cases.

On Thursday evening, Hong Kong’s No.2 official, John Lee, said mainland officials had told him earlier in the day that Hong Kong had "basically fulfilled" the conditions to reopen the border. He said details would still need to be worked out, including the introduction of a mainland-style "health code" app that has raised privacy concerns.

Asked by a reporter whether the new variant would delay reopening with the mainland, Lee said only that the Hong Kong authorities would "ensure that adequate research and tracking are done in this regard".

"Of course, we must manage and control any new risks," he said.