It was just days ago that Ontario decided to halt first doses of the AstraZeneca vaccine across the province due to an increasing number of reports of vaccine-induced immune thrombotic thrombocytopenia (VITT), the feared though exceptionally rare blood clotting phenomenon that has been linked to the COVID-19 immunization type.

Ontario Chief Medical Officer of Health Dr. David Williams said the move was made on May 11 out of "an abundance of caution" as experts look more closely at data about the condition and its occurrence in patients who received the AZ/COVISHIELD jab.

He added that the government was still considering whether we would be able to use the brand — which is a viral-vector based vaccine like the Janssen/Johnson & Johnson shot, not an mRNA vaccine like the Pfizer and Moderna are — for second doses going forward.

The decision has now been made that yes, we will, at least when it comes to the remaining supply we have on hand, with 254,500 AZ doses coming in this month.

Public health officials are in agreeance that the risk of VITT is low enough to administer the inoculation to those who have already received an AZ shot.

Those who got their first dose between March 10 and 19 will be able to book their second this week — a noted reduction in the originally suggested 12 week interval between the two doses.

📢 #Ontario to offer #AstraZeneca #CovidVaccine as 2nd 💉 to those who received 1st 💉 b/w March 10-19 from May 24th.

Current data from the UK= rate of VITT (rare but serious blood clot) is now 1/600,000.

IMO 👍🏽 to give decision to ppl who understand risk/benefit of 2nd 💉 https://t.co/MVnr43yoRZ

— Dr. Krishana Sankar (she/her) (@KrishanaSankar) May 21, 2021

According to new data from the U.K., the province says, there is a much-reduced risk of VITT in second doses of the shot, around 1 in 600,000 versus the 0.9-1.7 in 100,000 the province found was the case with first doses.

The government also cites the fact that certain health bodies currently recommend against dosing with two different vaccine types — the viral-vector based and the mRNA — as not enough information about potential effectiveness of mixing the two has been garnered just yet.

Doug Ford and his team say in a press release that they will "provide further guidance with respect to administering doses of two different vaccines in the near future and in advance of the 12-week interval when most people are needing to receive their second dose of the AstraZeneca vaccine."

"When the time comes to receive a second dose, everyone who received a first dose of the AstraZeneca vaccine will receive either an AstraZeneca or mRNA vaccine for their second dose."

I think I heard everyone who got 1st dose of #AZ will have a 2nd shot of #AstraZeneca. There are 55K set to expire end of May—those will go to people who got 1st shot in March. Then we have 200K more vaccines—with about 800K people needing the shot. More AZ coming, too. #Ontario

— Loukia (@MrsLoulou) May 21, 2021

The province confirmed its first case of VITT linked to the vaccine in a man in his 60s on April 23, and has since seen 13 others develop VITT.

Meanwhile, nearly one million Ontarians aged 40 and over have had the AZ shot in the last two months.

Officials will, of course, continue to monitor the number of reports of the rare complication and change their guidance and rollout of the vaccine accordingly.