Ontario's top doctor is advising those who have yet to receive a third dose of the COVID-19 vaccine to avoid celebrating with elderly loved ones in person this holiday break due to the spread of the 'aggressive' Omicron variant, putting a damper on the plans of many families hoping to get together.

Dr. Kieran Moore, Ontario's chief medical officer of health, said Friday to "avoid social contact with anyone older" even though one has received two doses of the COVID-19 vaccine.

"If you are going to interact, I'd do it outdoors with masking and distancing in place. I hope the weather stays reasonable across Ontario to enable that because I think all of us have to learn for the last 20 months to best protect those that are older in our communities," Moore said when asked if it's worth the risk to see a grandparent with three doses over the holidays.

"I'm sorry to recommend that."

Moore did say people should get their third dose or booster shot if they are planning to celebrate indoors with anyone vulnerable to infection.

"Once we get better understanding of the severity of this virus, we'll communicate that to all Ontarians," he said. "At present, it's affecting a younger population. We're very concerned about how it could infect others, so those that are older."

Many families have been looking forward to gathering this season after not being able to last year when vaccinations were just starting. In 2020, all Ontarians were told to only celebrate with members of their own household and connect with extended family members and friends virtually.

With most adults in Ontario double-vaccinated and immunization of children aged five to 11 beginning, it appeared that this year would be different and more festive.

However, as the holiday season approached, the number of cases in the province steadily climbed and was coupled with the emergence of Omicron, which has become the dominant variant in Ontario.

Even before Omicron, there have been concerns about waning immunity, prompting calls for a booster shot. Those calls grew as many experts became worried about the effectiveness of two doses of vaccine against the new variant.

Dr. Peter Juni, the scientific director of the Ontario COVID-19 Science Advisory Table, said Wednesday that the chance of someone who is six months out from their second dose getting infected could be as “high as somebody who was never vaccinated.” He added that the definition of fully vaccinated should now change to include the third dose.

On the same day, the province expanded booster eligibility to all Ontario adults and shortened the interval between the second and third doses to three months. It also reinstated capacity limits for large indoor facilities, including sports and entertainment venues.

The measures were welcomed, but many public experts said they were not enough to curb the variant spread. In addition to faster third dose rollout, the science table recommended reducing contacts to 50 per cent and reinstating capacity limits in more settings to help blunt the rapid growth of the variant.

On Friday, the Ford government reintroduced capacity and social gathering limits.

"Our goal now is to try to slow the virus down as we said to try to build our immunity through the immunization strategy," Moore said.

"I want to warn Ontarians -- this is a very aggressive virus. It spreads very quickly. Everyone has an individual responsibility to try to limit their total number of contacts."

Moore added: "This is a difficult time for our province. But I trust Ontarians to do their best to try to limit the spread of this virus and at the same time, promote immunization for those that are the most vulnerable to this virus."

Also this week, the province implemented a new measure at long-term care homes, only allowing vaccinated visitors inside.

The province said only two visitors will be allowed to see a resident at one time indoors and a maximum of four visitors will be permitted outdoors.