How many people actually need to get the COVID-19 vaccine for everyone to be safe?

If there's one thing that's true about how many people need to get a COVID-19 vaccine to protect us all, it's this: The goalposts have changed in 2021.

Early in the pandemic, top scientists at the World Health Organization (WHO), as well as Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in the U.S., said between 60 and 70 per cent of the population must acquire resistance to the virus - either through vaccination or infection - for life to return to some degree of normality."

But now, as the fourth wave bears down and the highly-transmissible Delta variant has muscled out all other strains to become dominant, those numbers are far less reliable. And what the new threshold should be is less than clear as experts wrestle with what will be enough protection to stave off Delta.

We talk about herd immunity like it's an absolute number and that's probably not entirely fair to do," said Dr. Isaac Bogoch, an infectious diseases specialist at University Health Network and associate professor at the University of Toronto. It's more accurate to think in terms of community-level protection where our daily lives are less impacted by the virus.

COVID is not going away, but you will certainly be able to prevent our health-care system from becoming overwhelmed and stop major disruptions in Canadian society if you have a greater number of people vaccinated."

For its part, the Ontario Ministry of Health says that with Delta circulating, the province needs to achieve the highest immunization rates possible - 85 to 90 per cent or above - to truly reach herd immunity." The Infectious Diseases Society of America recently revised its threshold estimate to well over 80 per cent, maybe approaching 90 per cent."

Meanwhile, Fauci has in recent weeks shied away from saying what percentage of the U.S. population would need to be vaccinated to reach a level of pre-pandemic normalcy, noting instead that it's important to vaccinate as many people as possible.

Herd immunity refers to the protection a population gets from a disease through widespread vaccination or infection and recovery. Of course, in the case of the coronavirus, while infection confers immunity on those who recover, it has also resulted in millions of deaths.

Dr. Peter Juni, epidemiologist and scientific director of Ontario's COVID-19 Science Advisory Table, says it's time to stop using the term herd immunity" altogether.

We should instead be talking about the extent of protection afforded by the vaccine to the population and the health-care system," he said. We can use that protection to slow down the pandemic and soften its impact on hospital- and ICU capacity."

And that means a focus on getting as many people as possible over the age of 50 vaccinated, as they remain at high risk of ending up in hospital and the ICU if they contract the Delta variant.

The other priority should be to protect children under the age of 12 who are not eligible for the vaccine, especially in light of the fact that Ontario has a limited number of pediatric ICU beds, Juni said.

We should focus on at least getting as many people in every other age group that have contact with children vaccinated to protect the children," he said.

COVID will continue to spread in Ontario until the majority of people are immune either through vaccination or infection," said Dr. Anna Banerji, an infectious disease specialist at the University of Toronto's Temerty Faculty of Medicine.

It's only a matter of time," she said, adding that even then, there will likely continue to be sporadic cases in remaining unvaccinated individuals or in people with weakened immune systems.

It will be smouldering along," she added.

More than 80 million American adults remain unvaccinated at a time when COVID deaths are surging across that country, with an average of more than 1,500 per day, along with over 140,000 new daily cases in recent weeks. Canada is faring comparatively better thanks to a high proportion of vaccination - about 84 per cent of the eligible population has received at least one shot and about 77 per cent are fully vaccinated. Still, that means about five million adults in Canada remain unvaccinated.

Those who haven't had shots are not only putting everyone at risk with further spread; they're also adding to the possibility that more variants like Delta could surface, said Wilfrid Laurier University virologist Stephanie DeWitte-Orr.

Simply put, the longer unvaccinated populations remain, the more chances the virus will have to replicate, a process that can introduce mutations. Most mutations have no effect on the virus, but, occasionally, gain-of-function" mutations can occur that will make a virus stronger and more transmissible, she added.

Viruses are tricky buggers," DeWitte-Orr said. When there are lots of chances to replicate, and therefore lots of chances to mutate, it's difficult to predict what could come next. Best case scenario is we achieve herd immunity with Delta before the next variant comes."

The Last Shot is an occasional series examining what it will take to reach the unvaccinated and move us past the pandemic.

Kenyon Wallace is a Toronto-based investigative reporter for the Star. Follow him on Twitter: @KenyonWallace or reach him via email: kwallace@thestar.ca