COVID-19 vaccinations remain safe at 4-month intervals, expert panel says

COVID-19 vaccinations remain safe at 4-month intervals, expert panel says
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OTTAWA—Most Canadians are still waiting for their first coveted COVID-19 vaccine shot, but there’s no rush for the second one, says a federal vaccine advisory panel.

The National Advisory Committee on Immunization (NACI) said Wednesday it is sticking to a recommendation that allows provinces to delay the second COVID-19 vaccine shot by up to four months.

Because vaccine supplies are still limited in Canada, the panel said it’s better to get more first doses into more arms to have broader population-level protection than to only slightly boost individual protection with an earlier second dose.

NACI chair Dr. Caroline Quach-Thanh told reporters that as vaccine supplies increase in Canada, the gap can be shortened to two or two-and-a-half months or less, including for specific populations who might need an earlier second shot to increase their antibody responses.

“A second dose should be provided as soon as possible once all eligible population groups have access to a first dose. The priority goes to the highest-risk individuals of serious illnesses or risk of death due to COVID-19,” she said.

While some other countries have allowed a three-month interval between doses, Canada appears to be the only country to extend that time to up to four months.

But the expert panel concluded the benefits of quicker, broader population protection right now warrants giving provinces the flexibility to delay booster shots. It might even produce better immunity in the long run, they suggested.

NACI, a volunteer arms-length body that advises Health Canada, is also sticking to its previous advice to provinces to suspend the use of the AstraZeneca vaccine for Canadians under age 55 until it reviews new advice that emerged Wednesday from the European Medicines Agency.

Sabine Straus, chair of the committee that conducted the detailed review, tweeted that it analyzed 62 cases of clots that occurred in the brain, and 24 cases of clots that occurred in abdominal veins, 18 of which were fatal, that were listed in the European Union’s drug safety database as of March 22.

Although most cases occurred in women under 60, Straus said that due to different ways the vaccine in being use in different countries the committee “did not conclude that age and gender were clear risk factors for these very rare side effects.”

Dr. Howard Njoo, Canada’s deputy chief public health officer, said Health Canada will review the European decision to determine if there should be a change to Canada’s labelling of the AstraZeneca, and NACI’s vice-chair Dr. Shirley Deeks said the external expert panel would study the data as well before making any decision about whether to adjust its advice.

In Canada, all chief medical officers of health have decided to suspend use of the AstraZeneca vaccine in Canadians under 55. On Wednesday, they welcomed NACI’s advice to stay the course on extended dosing intervals.

Dr. Bonnie Henry, B.C.’s chief medical health officer, told reporters that provinces are unanimous in committing to a vaccine strategy that “that brings the greatest benefit to all of our populations.”

In January, when only the Pfizer-BioNTech and Moderna vaccines were authorized for use in Canada, NACI had recommended that the interval between first and second doses could be extended from three and four weeks respectively, to up to six weeks due to scarcity of supply.

In March, after the AstraZeneca vaccine was authorized by Health Canada, NACI extended that delay for all vaccines to up to four months, citing promising data about their effectiveness.

That picture has only gotten better.

Quach-Thanh said a staggered vaccine strategy is supported by ongoing data from clinical trial participants, mathematical modelling, real-world effectiveness studies and tried-and-true principles of vaccine science and immunology.

It may turn out that an even longer period between first and second shots — up to six months or a year — is more beneficial in boosting the body’s ability to produce not just antibodies, but also memory cells that can trigger a longer-lasting immune response. That’s often the case for other types of vaccines.

And the protection appears to be lasting. Results published Tuesday in the New England Journal of Medicine showed that antibodies produced in recipients of the Moderna vaccine persisted up to six months out. Pfizer also reported last week that its vaccine was 91 per cent effective against COVID-19 and produced antibody responses that were measured seven days through six months after the second dose.
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