A total of 21 cases of the XBB.1.5 COVID subvariant have been detected in Canada as of Wednesday, the country's public health agency confirmed.

XBB.1.5 is a sublineage of the Omicron XBB sub-variant and has been considered by the World Health Organization (WHO) as the "most transmissible" COVID strain so far.

The new subvariant has been detected in 29 countries to date, according to WHO.

In a statement to Global News, the Public Health Agency of Canada (PHAC) said they will continue to "carefully monitor the spread of XBB."

"Proportions and growth rates will not be reported until there is sufficient data."

"XBB.1.5 is currently considered to be detected only sporadically. As data accumulates, growth rates can be more accurately estimated," the agency said.

The subvariant is now responsible for about 70% of new COVID-19 cases in the northeastern U.S., representing a massive growth rate since early December, when it accounted for just four percent of new COVID-19 cases in the U.S., according to data from the U.S. Centers for Disease Control and Prevention (CDC).