Health officials are monitoring a pair of new derivatives of the COVID-19 Delta variant in Western Canada — although more evidence is needed to determine whether they are more transmissible or cause more severe illness.

Dr. Saqib Shahab, chief medical officer of health for Saskatchewan, where the two sublineages AY.25 and AY.27 have been detected, said this week there is no data to determine whether the Delta derivatives are more transmissible.

"Do these sublineages transmit more rapidly than the original Delta strain? We are watching very closely. They may or may not," Shahab told reporters during a briefing on Tuesday.

Meanwhile, Alberta's chief medical officer of health, Dr. Deena Hinshaw, stressed during a separate press briefing that the sublineages are not new variants of concern.

"There's no evidence that it causes any more severe illness, that it evades vaccine protection, that it's significantly different from the Delta variant that has been circulating as the dominant strain in Alberta since late summer," Hinshaw said Tuesday.

The new mutations were first detected in the U.S. state of Idaho and spread to Canada in the summer.

AY.25 is now the dominant sublienage in Saskatchewan, says Angela Rasmussen, a virologist at Saskatoon's Vaccine and Infectious Disease Organization.

A chart shared recently at a town hall meeting for doctors in Saskatchewan showed the prevalence of AY.25 in the province, as well as Alberta and B.C.

"Whenever variants spread widely through a population, as it has done here in Saskatchewan and Alberta, it will acquire new mutations because that's what happens every time a virus replicates," Rasmussen explained.

"Essentially, it will become sort of a grandchild of the original variant that spawned it."

She says the takeaway should be for those who aren't vaccinated to do so, as the vaccines are effective against the Delta variant and its new "flavours."

With files from CTV Saskatoon