mRNA vaccines preferred: new NACI recommendation


The National Advisory Committee on Immunization (NACI) is now recommending that Canadians wait for an mRNA vaccine if they can rather than take the first COVID vaccine they can get ( 

The controversial recommendation states that an mRNA COVID-19 vaccine should be preferentially offered to individuals. The AstraZeneca COVID-19 vaccine may be offered to people aged >30 years only if the individual does not wish to wait for an mRNA vaccine. Individuals are advised to do their own benefit-risk analysis, weighing the relative risks and consequences of COVID infection and vaccine-induce thrombotic thrombocytopenia (VITT, or VIPIT). Factors influencing COVID risk include the anticipated wait time until mRNA vaccination, and the individual’s ability to avoid COVID exposure.

The recommendation appears to contradict the previous advice to take any vaccine on offer, and tacitly acknowledges viral vector vaccines are associated with risks that can be avoided with mRNA vaccines. “If, for instance, my sister got the AstraZeneca vaccine and died of a thrombosis when I know it could have been prevented and that she is not in a high-risk area, I’m not sure I could live with it,” said Dr. Caroline Quach-Thanh, NACI chair, in an interview on the CTV program Power Play ( The comment followed a report of a VITT-related death in Canada.

Several published reports have described cases of VITT in recipients of the AstraZeneca vaccine (Greinacher et al. and Schultz et al. N Engl J Med 2021, epublished 9 April 2021, free full text at and

One VITT case occurred in the clinical trial of the Johnson & Johnson vaccine (FDA Briefing Document, February 26, 2021. Full text at The FDA paused distribution of that vaccine on April 12 after six cases were identified in the Vaccine Adverse Event Reporting System; an additional six cases were reported by April 21 (See et al. JAMA, epublished 30 April. Full text at There are no reports of VITT with the mRNA vaccines.

While the NACI recommendation has been widely criticized, the Joint Committee on Vaccination and Immunization in the U.K. also states that mRNA vaccines are preferred over the AstraZeneca vaccine, but only in adults aged < 30 years ( JCVI also notes that there are no reports of VITT following a second dose of the AstraZeneca vaccine.

The NACI recommendation may also reflect the current state of vaccine distribution in Canada. Eleven million doses of the Pfizer vaccine have been distributed to date and over 2 million doses per week are expected to be allocated over the next three months. In contrast, the 300,000 doses of J&J vaccine that have been purchased are not being distributed in Canada due to concerns about quality control. The doses were produced at the Emergent BioSolutions facility in Baltimore, which failed a recent FDA inspection ( The plant mixed up ingredients of the J&J and AstraZeneca vaccines, and could not properly sanitize the facility due to peeling paint on the walls and floor and inadequate disposal of medical waste. The mix-up resulted in 15 million vaccine doses being destroyed. The FDA ordered AstraZeneca to relocate its vaccine manufacturing and has suspended production of the J&J vaccine at this facility.

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