Will all Ontarians — not just those 50 and up — need a third COVID-19 shot?

With the news that Ontario is opening up third doses of the COVID-19 vaccine to everyone aged 50 and over, many may be wondering if all Ontarians with two shots will need another jab.

In short, it's likely - we just don't know when. It's also not clear whether a third dose will be considered necessary to become fully vaccinated or serve as a booster - and how it will be extended to those under 50 down the road.

What we do know is that the research to date shows those aged 50 and over - who are more at risk of hospitalization and death if they contract COVID - benefit from a significant boost in protection with a third shot.

Do we all need a third dose now? No. Do some of us need a third dose now? Yes. Do we need to expand eligibility for third doses now? Yes, which is what we're doing today," said Dr. Isaac Bogoch, an infectious diseases specialist at University Health Network and a former member of Ontario's now-disbanded vaccine distribution task force.

And more importantly, that eligibility can expand and should expand with time because it's probably a three-dose vaccine for most adults. It probably is."

Ontario's chief medical officer of health, Dr. Kieran Moore, announced Thursday that booster shots will be opened up to all adults 50 and over roughly six months or 168 days after their second dose.

They join those 70 and over, health-care workers and essential caregivers in congregate settings, those who received two doses of the AstraZeneca vaccine or one dose of the Johnson & Johnson shot, as well as First Nations, Inuit and Metis adults, who are already eligible.

An additional 250,000 immunocompromised people, transplant recipients, patients with hematological cancers, and seniors living in congregate settings became eligible to get their boosters in late August.

Bookings for Ontarians 50 and over will start on Dec. 13, with dates for younger age groups coming in the new year. The expanded eligibility for third doses comes as countries attempt to stay ahead of the new Omicron variant. But Moore said we will need to wait for emerging data to know how well boosters will protect against it.

Asked why he was not opening up third doses to everyone now - the U.S. Centers for Disease Control and Prevention has recommended everyone 18 and up get a third dose - he said the priority for now is first doses for kids and boosters for older adults.

But we absolutely will be following the science," he said.

One large Israeli study recently published in the Lancet found that compared with two doses of the Pfizer COVID vaccine at least five months before, a third dose was about 93 per cent effective in preventing hospitalization, 92 per cent effective in preventing severe disease and 81 per cent effective in preventing death, a week or more after the third dose.

Our findings suggest that a third dose of the (Pfizer) vaccine is effective in protecting individuals against severe COVID-19-related outcomes, compared with receiving only two doses at least five months ago," the authors wrote.

Last month, the BMJ (formerly the British Medical Journal) reported that a preprint study by the UK Health Security Agency found significant increased protection" from a booster dose against symptomatic COVID for people over 50.

The journal noted that two weeks after receiving booster doses, protection for those who received the Pfizer vaccine increased to 94 per cent, while protection for those who received the AstraZeneca shots increased to 93.1 per cent.

Closer to home, a recent report from Public Health Ontario of breakthrough cases, or people who get COVID despite being fully vaccinated, found the majority of those who need hospital care are adults over the age of 60, with the highest proportion in their 80s.

Kelly Grindrod, a pharmacy professor at the University of Waterloo, said while two doses still provide good protection, you see with waning immunity, hospitalizations in older populations."

So if and when we start seeing that with the under 50s, then we would know that this is a third dose vaccine for young people as well," she said.

Many vaccines take three doses, she added, so it would not be surprising if the COVID vaccines turned out to need the same. Or people could require a booster every year, even every few years.

This is not unusual ... giving your immune system a vaccine and then reminding it periodically about how to fight something off," she said.

Strengthening that memory is really, really useful."

Bogoch noted that emerging data from countries like Israel, the U.S. and Qatar indicate evidence of waning immunity over time, which may have resulted in some overinterpretation on the need for boosters. He said even with that overinterpretation, however, there is clearly a need for third doses in some people.

He tempered his comments by pointing out that the emerging data on the need for boosters is largely coming from countries that stuck to the manufacturers' recommended intervals between dose one and dose two, such as Israel, which followed Pfizer's instructions to administer the doses three weeks apart.

In Canada, we did things a little bit differently. Many people had their first and second doses separated by two months and even more in some cases," he said.

What we know about multi-dose vaccines, including everything we've learned about the COVID-19 vaccine, is that you get more robust immunity by separating dose one and dose two."

Dr. Jeff Kwong, a family doctor and an epidemiologist at the Dalla Lana School of Public Health, said when it comes to offering third doses for all age groups, the writing's on the wall," pointing to other jurisdictions such as the U.S. where adults 18 years old and over are already eligible and British Columbia, which announced this fall that it would begin offering third doses to everyone 12 years old and over in January."

I think it's just an inevitability," he said.

Manitoba and the Northwest Territories have expanded third doses to all adults, while Nunavut recently made everyone 12 and up eligible.

So far we've framed it as two doses with a booster, but maybe it really is three doses. Because who knows what's going to happen after the third dose? Nobody knows," Kwong said. Is it going to wane again in six months after that? We have no idea."

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

May Warren is a Toronto-based breaking news reporter for the Star. Follow her on Twitter: @maywarren11