Why the timing of your next COVID shot is so important
Waning immunity and rise of new Omicron subvariants threatens future surge
The timing of your next COVID-19 vaccine may be more important than ever, as highly contagious Omicron subvariants are on the rise in Canada and waning immunity from previous vaccination and infection threatens to fuel another surge.
Canada is once again a hotbed for variants, with BA.2.12.1 now making up more than 40 per cent of COVID cases, while BA.4 and BA.5 are quickly gaining ground at more than 10 per cent combined in late May — a major jump from less than one per cent weeks earlier.
But the latest available federal data is weeks out of date and modelling experts CBC News spoke to estimate the true proportion of BA.4 and BA.5 cases is more than 20 per cent — and could be as high as 50 — with one of them likely to become dominant in the coming weeks.
"COVID-19 has shown us over the past few years that there may be more surprises ahead," Canada's Chief Public Health Officer Dr. Theresa Tam said during a press conference Friday.
"The virus is still circulating in Canada and internationally and factors such as viral evolution and waning immunity are anticipated to impact COVID-19 activity moving forward."
Bracing for 'potential resurgence'
Tam said Canada's path forward with COVID will not be straightforward, and officials are bracing for a "potential resurgence" that could lead to "severe impacts" in the future as the Omicron subvariants battle for dominance, and new variants could still emerge.
"Omicron has evolved and it's so much different than our vaccines and infections prior to Omicron — the type of immunity that you got is just a different beast," said Sarah Otto, an expert in modelling and evolutionary biology at the University of British Columbia.
"And so what we're seeing with vaccine protection is that it's not so much the number of doses as it is how recent your last dose has been, and I think that's because the neutralizing antibodies in our bloodstream, they're not recognizing the virus as well."
That's why virologists and immunologists say timing our next shots ahead of another potential wave or when new variants start to rise in Canada is so important, so we don't get caught scrambling to roll out doses in the midst of a rapidly worsening wave — like when Omicron first hit in December.
When should you get a 4th dose?
Canada's National Advisory Committee on Immunization (NACI) strongly recommended second boosters for seniors aged 80 and up and other vulnerable groups back in April, but stopped short of recommending a fourth shot for all Canadians.
The reason for that decision may have come down to timing, with updated vaccines on the horizon that might fare better against Omicron subvariants, and a quieter stretch of COVID activity in the summer buying us time before another vaccine rollout may be needed.
But the NACI guidance also recommends that those eligible wait six months after their last dose for a second booster, adding that timing "may need to be balanced with local and current epidemiology" and that "shorter intervals" could be needed if another wave hits.
"When we start to see BA.4 and BA.5 leading to rising cases, then I think we should be nimble and expect to need to get the boosters — but it's this waiting game," said Otto.
"The longer you wait to get the vaccine, the more recent it is and the more powerful it is when the next wave happens. So you don't want to get it months before the next wave — Canada should be really pushing for vaccines right at the beginning of a wave."
Otto said Canada will likely see another wave driven by BA.4 and BA.5, but how bad it gets and whether it's already started isn't yet clear, although some provinces like Ontario and British Columbia are seeing a recent uptick in COVID wastewater surveillance.
Updated vaccines may fall short
Virologists and immunologists are also concerned that repeated boosters with COVID-19 vaccines tailored to the original strain of the virus may no longer be sufficient, and that updated vaccines may be needed to blunt another potential wave in the coming months.
Moderna's bivalent vaccine is one strategy many are pinning their hopes on, but experts worry targeting the original Omicron strain may not be sufficient due to research showing a lack of cross protection immunity against the vastly different BA.2.12.1, BA.4 and BA.5.
That experimental vaccine combines Moderna's original shot with protection against the BA.1 Omicron strain, and while the company released new data Wednesday showing the formulation still generated a significant immune response against BA.4 and BA.5, it elicited a much stronger response to BA.1.
WATCH | Moderna's vaccine targeting Omicron shows promise:
"I don't think we're going to see a major advantage of a boost by an Omicron-specific booster than by just a regular boost with the (original) strain," Otto said.
"Either way, the boost helps because it raises the antibody levels, but I don't think it's raising it that much more in a way that will help us neutralize Omicron."
'We need better long-term strategies'
University of Toronto immunologist Jennifer Gommerman says Omicron-specific boosters may be effective against Omicron subvariants in the real world — but only if the virus doesn't throw something else at us in the months ahead.
"If Omicron is basically all the tricks that the virus has left, then sure, I think it makes sense to be doing Omicron-based vaccinations because we have a very different virus now than we had at the beginning of the pandemic," she said.
"The concern would be if the virus still has enough real estate to make a new version of itself that's really different than Omicron — then that would be not what we'd want to take."
Alyson Kelvin, a virologist at the Canadian Center for Vaccinology and the Vaccine and Infectious Disease Organization in Saskatoon, said whether or not a bivalent vaccine is going to protect against the Omicron subvariants circulating now is a "huge question."
"At this point, I think we need to expect new variants, whether it's a new sublineage of Omicron or a new variant altogether," she said.
"We need better long-term strategies and personally I don't think a bivalent vaccine is, from what we've seen of the virus, a sustainable strategy. It's figuring out how we can cover more variants at once to anticipate what the next variant is going to be."
Gommerman said nasal boosters, which aren't yet available, will likely better protect against infection by targeting a different arm of the immune system — but while several are under development, including from a team at McMaster University, they could still be years away.
WATCH | McMaster University developing nasal COVID-19 vaccines:
New variants could shift timing of boosters
Canada now needs to decide whether the goal of its strategy with current generation vaccines is to protect against severe COVID or to try to prevent transmission altogether, Gommerman says, until a new vaccine or variant changes the game.
"If your goal is to prevent infection, we're just gonna have to keep boosting forever," Gommeman said. "But if the goal is to protect the vulnerable, then we've already been doing that."
Gommerman said she would not get a fourth dose unless she knew there was a compelling public health reason to do so — like if COVID levels were surging, vulnerable groups were at risk and the timeframe lined up against waning immunity against infection.
"But I know that my immune protection in the form of immune memory is going to keep me out of the hospital," she said. "It's not going to necessarily keep me from being at home for a week, but it will keep me out of the hospital and that's what the vaccines were designed to do."
Otto said new variants will likely continue to emerge, given the "huge amount of diversity of the virus that is being maintained globally," and the fact that Omicron emerged independently of other variants like Alpha, Beta and Delta.
"We're already seeing that substantial evolutionary change in Omicron — and all I'm saying is, don't discount those other strains that are still circulating globally," she said.
"We're in a game of Whac-A-Mole, and we don't know where the next mole is going to come up."
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