Japan’s National Institute of Infectious Diseases (NIID) on Sunday said it had detected a new variant of the coronavirus in four travelers arriving from Brazil.

The newly-discovered mutant strain of Covid-19 was found to share some of the mutations in common with those of concern for increased infectivity, the institute said, referring to highly infectious strains recently discovered in the U.K. and South Africa.

However, at this time any information about the new variant is limited to its genetic make-up, the NIID said. The institute added it is difficult to immediately determine how infectious the new strain is and the effectiveness of vaccines against it.

The World Health Organization has been contacted by CNBC to confirm whether it has been informed about the new mutant strain of the virus. The United Nations health agency was not immediately available to respond on Monday morning.

Japan’s identification of a new variant of the virus comes as countries scramble to contain two other contagious variants that have emerged in the U.K. and South Africa. Public health experts have expressed concern the new strains could pose a threat to inoculation efforts.

To be sure, while the variants discovered in Britain and South Africa spread more easily, there is no clear evidence of the mutated viruses being associated with more severe disease outcomes. But being more transmissible means more people can get infected, and this could mean more serious infections and more fatalities.

In recent weeks, optimism about the mass rollout of Covid vaccines appears to have been tempered by the resurgent rate of virus spread worldwide.