TORONTO -- Several provinces have halted administration of the AstraZeneca COVID-19 vaccine to those under the age of 55 following new recommendations from Canada's National Advisory Committee on Immunization (NACI).

On Monday, the NACI recommended pausing administration of the AstraZeneca vaccine to those under the age of 55, pending further investigation on reported cases of vaccine-induced prothrombotic immune thrombocytopenia (VIPIT), a rare blood clotting disorder, in Europe.

“We are taking this precautionary measure while Health Canada as the regulator completes its updated risk benefit analysis based on emerging data,” Dr. Howard Njoo, deputy chief public health officer at the Public Health Agency of Canada, said Monday during a technical briefing on the matter.

“During this time of risk assessment, Canada has other vaccine options to address the ongoing risk of COVID-19 infection.”

The new recommendations will likely slow down the vaccine rollout, but not dramatically, according to Infectious disease expert Dr. Isaac Bogoch.

Following the recommendation, several provinces announced they would follow the guidance, including Alberta, Manitoba, Ontario, Quebec, British Columbia, Newfoundland and Labrador and New Brunswick.

Prince Edward Island indicated on Monday morning it will no longer give the AstraZeneca vaccine to anyone. It had been set aside for people between the ages of 18 and 29.

Health officials in Nova Scotia said the updated recommendation will not impact vaccine rollout as the AstraZeneca vaccine is currently only being offered to people between the ages of 60 and 64.

“We still know that these events are extremely rare,” Bogoch emphasized to CTV’s Your Morning on Tuesday, adding that NACI’s decision was made out of an abundance of caution, but did not mean the vaccine was completely off the table for those under 55.

“I wouldn’t say this chapter has been fully written yet … It still needs to be evaluated, it still needs to be monitored.”

VIPIT is a condition that refers to blood clots -- including blood clots in the brain -- stemming from receipt of the AstraZeneca vaccine. Symptoms include serious headache, seizure, blurred vision and shortness of breath and tend to develop between four and 16 days after receiving the vaccine.

“These are not ordinary blood clots, they can cause very serious disease in the brain and they can be fatal,” Infectious disease specialist Dr. Abdu Sharkawy told CTV’s Power Play.

“Even though they are extremely rare, they're very serious and it begs an explanation, it begs an investigation to make sure that if you are going to roll it out potentially over a scale of millions and millions of people, you certainly don't want to put them in harm's way.”

While there have been no cases of VIPIT in Canada, cases internationally have been identified primarily in women under the age of 55, though cases have been reported in men as well, according to the NACI.

Based on available information, the NACI said the fatality rate of VIPIT is about 40 per cent, but this may decrease as doctors and those receiving the vaccine are more aware of the condition.

“The exact mechanism by which the AstraZeneca vaccine triggers that is still under investigation and no other risk factors have been consistently identified in patients who develop VIPIT,” Dr. Shelley Deeks, vice-chair of the NACI, said in the briefing.

Previously, it was believed that cases of VIPIT occur in about one per million administered dosesof the vaccine. However, a recent report from the Paul Ehrlich Institute in Germany indicated it could be one in 100,000 doses.

“Where the true rate is, we just don't know at this point in time, but we are continuing to follow the data as it emerges,” Deeks said.

The NACI also recommended the continued use of the AstraZeneca vaccine among people over the age of 55 with informed consent, due to the lower risk of developing of VIPIT in older populations and the increased risk of severe COVID-19 infections among these age groups. 

“Overall, it's a vaccine that prevents complications and death in the elderly population,” Dr. Caroline Quach, a pediatric infectious disease consultant and chair of the NACI, said in French.

“In particular, 70 and over, in terms of effectiveness, that has been well studied. (It) works well and doesn't seem to have the risk of clots that we see in the younger populations.”

Bogoch agreed with the NACI’s guideline to continue recommending its use for older Canadians.

“I think the move by NACI today was very reasonable to say: ‘You know what, over 55, we’ll give it.’ That's very reasonable,” he told. “I hope people continue to take it in the over 55 crowd.”

Canada and many other countries had previously halted the use of the vaccine in seniors at the beginning of March, but rescinded that guidance two weeks later.

Sweden and Finland have already suspended distribution of the vaccine to those under the age of 65, while in Spain it is only administered to those between the ages of 18 and 65. In Denmark, health officials have extended their suspension of the vaccine until at least April 18. 

In a statement, AstraZeneca said it respects the NACI’s decision and that patient safety “remains the company’s highest priority.”

“Regulatory authorities in the U.K., European Union, the World Health Organization and Health Canada have concluded that the benefits of using our vaccine to protect people from this deadly virus significantly outweigh the risks across all adult age groups,” the company wrote in the statement.

“Tens of millions of people have now received our vaccine across the globe. The extensive body of data from two large clinical datasets and real-world evidence demonstrate its effectiveness, reaffirming the role the vaccine can play during this public health crisis.”

In a statement, Health Canada said there have been no reports of blood clots from the AstraZeneca vaccine in this country, but noted that cases have been reported in Europe and it is now requiring “additional terms and conditions on the authorizations of the AstraZeneca and Verity Pharmaceuticals/Serum Institute of India vaccines.”

“These will include a requirement that the manufacturers conduct a detailed assessment of the benefits and risks of the vaccine by age and sex in the Canadian context,” Health Canada said in the statement. “This information will support the ongoing evaluation of these rare blood clotting events, and allow Health Canada to determine if there are specific groups of people​ who may be at higher risk.”

Last week, Health Canada noted that the vaccine is not associated with an overall increased risk of blood clots and that the benefits of the vaccine outweigh its risks.

In Monday’s statement, Health Canada noted that this guidance still stands and it is working with international partners to evaluate the incoming data.

Canada is expecting to receive 1.5 million doses of the AstraZeneca vaccine on Tuesday from the United States, which has not yet authorized it for emergency use. The shipment will represent the first doses to come from the United States.

Another 500,000 doses that were delivered from the Serum Institute of India have already been distributed.

Last week, AstraZeneca reported its vaccine was 76 per cent effective in preventing symptoms and 100 per cent effective against serious infections that result in hospitalization.

“It really has the potential to save lives and it has done so and it will continue to do so,” Bogoch said. “I think if we use it appropriately, especially now in those over the age of 55, it can really do a lot of heavy lifting here in Canada, but it's not going to work if nobody takes it and this is where we really have to build trust or rebuild trust in this vaccine.”

With files from producer and writer Ryan Flanagan, and writer Solarina Ho