Federal officials are warning that Canada could be on the brink of a fourth wave of COVID-19 driven by the highly contagious Delta variant if the country opens too fast before enough people have been vaccinated.

Canada's chief public health officer says long-term forecasts indicate that a hasty approach to reopening could portend a sharp resurgence of the virus by the end of the summer.

Dr. Theresa Tam says the new modelling underscores the need for caution in lifting public health measures as early signs of epidemic growth emerge in some areas.

Tam says officials expect that the Delta variant could fuel the spread of the virus among younger unvaccinated people, leading to a serious rise of case counts and hospitalization rates this fall and winter.

Tam says increasing vaccine acceptance among young adults aged 18 to 39 to a rate of 80 per cent would cushion the fallout of a potential fourth wave. As of last Saturday, 46 per cent of Canadians aged 18 to 29 were fully vaccinated, as were 54 per cent of those aged 30 to 39.

She says current COVID-19 case counts have plummeted by 93 per cent since the peak of the third wave, for an average of 640 new infections being reported daily over the past seven days.