Why the timing of your next COVID shot is so important

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But the latest available federal data is weeks out of date and modelling experts CBC News spoke to estimate the true proportion of BA.4 and BA.5 cases is more than 20 per cent — and could be as high as 50 — with one of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." 

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.

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."