TORONTO -- The National Advisory Committee on Immunization is being accused of creating confusion about COVID-19 vaccines after it restated that mRNA vaccines were “preferred” over the viral vector-based ones.

On Monday, NACI doubled down on its position that the Pfizer-BioNTech and Moderna vaccines were “preferred” over the Johnson & Johnson and AstraZeneca shots, and that Canadians should weigh the risks before they decide which one to receive.

The viral vector-based AstraZeneca and Johnson & Johnson doses have been linked to an extremely rare and potentially life-threatening blood-clotting syndrome called vaccine-induced thrombotic thrombocytopenia (VITT). The risk for developing this syndrome is estimated to be anywhere from one case in 100,000 doses to one case in 250,000. 

The Johnson & Johnson vaccine has been approved for use by Health Canada, but the agency said it was holding the first 300,000 doses in order to conduct further quality assurance tests after one of its ingredients was discovered to have been made in a problem-plagued vaccine manufacturing facility in the States.

While the vaccine-related blood-clotting syndrome is very rare, with only seven reported VITT cases in all of the approximately 1.7 million doses of AstraZeneca that have been administered in Canada so far, the committee’s vice-chair Dr. Shelly Deeks said it should be taken into consideration.

“The viral vector vaccines are very effective vaccines, but there is a safety signal, a safety risk… And the issue with the safety signal is that although it is very rare, it is very serious,” she said. “Individuals need to have an informed choice to be vaccinated with the first vaccine that’s available, or to wait for an mRNA vaccine.”

Viral vector-based vaccines use a weakened or attenuated common cold virus, which is not the same SARS-CoV-2 virus that causes COVID-19, and manipulate it so it can’t replicate or cause illness. The vector virus then delivers instructions to the body's cells to create a piece on the surface of the SARS-CoV-2 virus called the spike protein. The immune system can then learn to recognize the spike protein and fight off a potential infection if it’s confronted with the real coronavirus in the future.

The mRNA vaccines, on the other hand, use a technology that teaches the body’s cells to make a protein that can trigger an immune response to ward off an infection by the SARS-CoV-2 virus.

This latest announcement from NACI appears to contradict Health Canada’s oft-repeated guidance to Canadians that the best vaccine is the first one available to them.

When asked about this on CTV News Channel’s Power Play on Monday, NACI chair Dr. Caroline Quach-Thanh said it really boils down to where people live and their potential exposure to COVID-19.

“It basically depends on where you are. So if you are in an area where there’s no COVID, then waiting for mRNA is OK,” she said. “Do an individual risk assessment. If you’re in an area where there’s tons of COVID, the transmission is high, then I absolutely agree, the first vaccine that comes is the one that you take.”

Deeks said that while someone who is able to work from home in a region without many cases may want to wait for one of the mRNA vaccines, someone working in a manufacturing plant in a province with a lot of infections may want to get the first vaccine available to them.

While Quach-Thanh described NACI’s advice as a “transparent” message they needed to send to Canadians, several doctors took to Twitter to express their frustration with the conflicting guidance and its potential to contribute to vaccine hesitancy in Canada.

Dr. Iris Gorfinkel, a family physician and medical researcher in Toronto, said she was dreading attending her clinic the next day, when she anticipated having to answer patients’ questions about the latest NACI recommendations.

“The risk level remains acceptable. It’s NACI's mixed messaging that’s hard to fathom,” she wrote.  

Dr. Brian Goldman also shared his disappointment with the NACI advice in a post on Twitter.

“It pains me to say this, but it's past time to take NACI recommendations with a grain of salt. For the good of your health, DO NOT be choosy when it comes to #covidvaccines. Take the first one you're offered,” he said.

Dr. Naheed Dosani, a palliative care physician, also attempted to make sense of the new guidance by noting that it seemed as if NACI was recommending that front-line and essential workers take the less-safe viral vector vaccines.

“So let me get this straight: NACI is recommending that you should wait to get an mRNA vaccine but if you can’t wait (eg you’re an essential worker in a hotspot), you should just settle for a vaccine that has a higher risk of blood clots?” he wrote.

The news that Canadians should consider waiting for the “preferred” Pfizer-BioNTech and Moderna vaccines also sparked concern among those who had already received an AstraZeneca dose.

“I got AstraZeneca vaccine last week. Today’s NACI guidance has thrown me for a loop. Not only is NACI saying that I should have waited for Pfizer or Moderna, b/c I work from home, but that the extremely rare risk of blood clots I took is still acceptable for low-wage workers,” one woman said in a tweet Monday.

“I am so confused and in fact angry about this. My husband got the AZ vaccine last week. Should he have waited?? That is NOT what they’ve been saying all along. They’ve been saying people should get the first vaccine available,” a nurse wrote on Twitter

During the NACI announcement, Deeks attempted to assuage concerns from those who had already received the AstraZeneca shot by calling them safe and effective. She also said the committee is looking into the idea of mixing and matching vaccines for second doses.

“There is no reason for people to not receive a second dose of an AstraZeneca vaccine or an mRNA vaccine… NACI will be making a recommendation about mixed schedules as soon as we see some information from some studies,” she said.

In the House of Commons on Monday, Conservative MP and health critic Michelle Rempel questioned Health Minister Patty Hajdu about the confusion surrounding NACI’s preferred vaccines.

“This is a lot different than what we have been hearing. Does Health Canada advise taking the first vaccine offered or to wait, if one can, for an mRNA (Pfizer or Moderna) vaccine?” she asked.

In response, Hajdu said that Canadians should speak to their health-care provider if they want to know which vaccine is right for them.

Quach-Thanh, however, said she didn’t think the advice would contribute to confusion or vaccine hesitancy and that it is meant to help people make a decision on which vaccine to get based on their risk of contracting COVID-19.

“This needs to be an informed consent,” she said. “If, for instance, my sister was to get the AstraZeneca vaccine and die of a thrombosis when I know that it could have been prevented and she’s not in a high-risk area, I’m not sure I could live with it.”

So far, the majority of vaccine doses Canadians have received have come from Pfizer, followed by Moderna, and then AstraZeneca.

With files from’s Rachel Aiello and The Canadian Press