New bivalent boosters could slow virus spread, but won't be a silver bullet
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Canada has just approved an updated COVID-19 vaccine to target the first highly contagious Omicron variant, with doses expected to start rolling out within days. But what exactly can we expect from these new shots when they land in the real world?
The updated Moderna vaccine is a combination of two strains, also known as a "bivalent" vaccine, that targets both the original virus and the Omicron variant BA.1 that emerged late last year and drove the largest wave of infection and hospitalization in the pandemic.
While the new shot doesn't directly target dominant Omicron subvariants BA.4 and BA.5, which the U.S. approved an updated shot for this week, Pfizer submitted an application for Health Canada approval for its BA.4-5 vaccine Friday and Moderna is expected to soon.
"The evidence we have to date shows that the bivalent vaccine with BA.1 offers good protection against BA.4 and BA.5," Deputy Chief Public Health Officer Dr. Howard Njoo said during a technical briefing Thursday.
"Certainly as the situation evolves we'll have to look at the evidence and see what happens with real-world effectiveness."
But what an updated booster can do for you depends on how vulnerable your immune system is, whether you're one of the millions of Canadians who've recently been infected with COVID and when you last had a vaccine.
Vaccine for Omicron variant approved by Health Canada
Health Canada has approved a new bivalent COVID-19 vaccine from Moderna, which targets the Omicron variant. It's the first vaccine of its type in Canada, but officials say another batch of booster shots, specifically targeting Omicron's BA.4 and BA.5 subvariants, could arrive later this fall.
Will updated vaccines stop COVID spread?
Infectious diseases experts, virologists, epidemiologists and immunologists are hopeful updated vaccines will be more effective at preventing transmission than the original shots — at least initially — but also caution Canadians not to expect them to be a silver bullet.
With limited data on the impact these vaccines will have, all eyes will be on the effect they have on slowing rates of infection and transmission and whether they better protect vulnerable groups in the population heading into the fall and winter months.
"We do not know what the impact is because that is not available," Chief Public Health Officer Dr. Theresa Tam said during a news conference Thursday when asked by CBC News how effective the updated vaccines will be at stopping the spread of the virus.
Tam said she is hopeful that the new bivalent BA.1-targeted vaccine from Moderna boosts protection against infection and transmission — at least through the fall.
Clinical trial data on the BA.1-targeted vaccines from Moderna and Pfizer-BioNTech suggests they provide slightly stronger immune protection against Omicron than the original vaccines, but exactly what that translates to in the real world remains to be seen.
"The immune responses to these variant-tuned boosters are modestly encouraging, but we are still waiting for definitive clinical evidence," said Dr. David Naylor, who co-chairs the federal government's COVID-19 Immunity Task Force.
"That said, even if their marginal advantages are small, they could have a meaningfully positive impact if their novelty rekindles public interest in getting boosted."
A new preprint modelling study, that has not been peer reviewed, suggested that updated vaccines may not be much more effective than existing boosters in a population with hybrid immunity from vaccination and infection — especially when it comes to protection against severe illness.
But that could still lead to significant protection at a population level, with the preprint also suggesting that for every 1,000 people vaccinated with an updated booster, an average of eight fewer people would be hospitalized compared with the original vaccines.
"We don't know yet the magnitude of this improvement, but it will likely be pretty limited," said Dr. Gaston De Serres, an epidemiologist at the Quebec National Institute of Public Health (INSPQ).
"Especially against severe outcomes like hospitalization for which, up until now, the original [vaccines] have been quite successful."
What can updated COVID vaccines do for you?
More than half of Canadians have been infected with COVID since the emergence of Omicron and its highly contagious subvariants, and the added protection from prior infection in a vaccinated population appears to be providing an edge.
A new research letter published in the New England Journal of Medicine looked at the risk of BA.5 infection among people in Portugal who had a prior infection with past variants, including BA.1 and BA.2, and found they had strong protection against the newer variant.
That's in part because Portugal has such high levels of vaccination, with more than 98 per cent of the population studied having at least two doses, meaning that much like in Canada our high levels of two-dose vaccination and infection provide strong immune protection.
Two Canadian preprint studies from May and June, which have not been peer reviewed, also found prior Omicron infections provided robust immunity against future reinfection and hospitalization — especially when combined with vaccination.
"In general, we can anticipate that those who have hybrid immunity are going to be better protected," said Dr. Danuta Skowronski, a vaccine effectiveness expert and epidemiology lead at the B.C. Centre for Disease Control and co-author of the preprints.
"And it is also true that vaccine strains that are a better match to the circulating variant will likely provide better protection."
The longer you wait after your last infection or vaccination is also important to consider, with emerging evidence suggesting getting a booster too closely after being infected or vaccinated can impact how effective the shot is.
A new small preprint study, which has not been peer reviewed, found that getting a booster within two months of an infection could negatively impact B cells that help generate immune protection against severe disease.
NACI recommends waiting three months after an infection before getting another shot, and three to six months between doses, but also said in its most recent guidance that anyone at high risk of severe COVID in Canada should be offered a fall booster.
"Timing is everything and in general the recommendations are to wait a few months following infection before receiving a booster dose," Skowronski said.
She added that people shouldn't immediately rush out to get a booster after infection because it could actually interfere with the immune response.
Updated vaccines 'will help,' but not a 'miracle'
Canada still has a "worrisome gap" in third dose coverage, Naylor said, with less than half of Canadians having received a booster even though it provides significant added protection against severe COVID-19. Only about 12 per cent have received a fourth dose.
"As we start to head into the fall and we start to see almost certainly cases rise again, the updated booster is definitely going to be better than not getting a booster at all," said Deepta Bhattacharya, an immunobiology professor at the University of Arizona.
"Given that BA.5 is still circulating, I don't see that there's really any downside at all to picking up these boosters and they'll probably work better than just another shot of the original."
Bill Hanage, an epidemiologist at Harvard's T.H. Chan School of Public Health in Boston, said it's important to consider that even though the original vaccines were far from an exact match to Omicron — they still provided strong protection against severe illness.
"But the closer matches now should do more," he said in an email. "Notably, there may well be a period following the shot when people are immune to infection."
Bhattacharya said the "big question" that is top of mind right now is how effective the updated vaccines will be at stopping transmission in the real world — and for how long.
"Do we start to restore some of the protection against any infection or symptomatic infection with these boosters? I expect we will," he said. "Exactly how long that effect will last, I think, is to be determined."
Dr. Allison McGeer, a medical microbiologist and infectious disease specialist at Toronto's Mount Sinai Hospital, said the updated shots may provide a slight boost in neutralizing antibodies that can prevent transmission — and that added protection is "not nothing."
"An extra dose of the original vaccine results in substantial increased protection against BA.4 and BA.5. It's not perfect, but I think we're finding out nothing is perfect," she said. "The bivalent vaccines, they may be a little better, but they're not going to be a miracle."
Bhattacharya said that while clinical trials showed there is only about a twofold increase in protective antibodies against the BA.1, BA.4 and BA.5 variants targeted by Moderna and Pfiizer in their bivalent vaccines — it's still significant protection.
"If you look at the data, it's a ton more antibodies that are being made," he said. "So I have every reason to expect that they will help — and probably help a lot."
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