Who should get a fourth COVID-19 vaccine dose — and when?

With a fourth dose of vaccine only available across the province to those deemed at higher risk for severe COVID-19 outcomes, some infectious disease experts are calling for expanded eligibility due to the population's declining immunity.

And as new Omicron subvariants are emerging, it's important the public gain an immunity boost before the next wave, as research shows protection dips about six months after the first booster shot, they said.

However, some other experts said the fourth dose is likely most helpful for immunosuppressed and older people, but it may not provide enough benefit for the population at large to offer it to everyone at this point.

We're hitting a period where the bulk of people who have had two shots or more are in a period of waning immunity," said Todd Coleman, an epidemiologist at Wilfrid Laurier University. It would be prudent to offer and open up the fourth shot to the general population."

The protection against COVID-19 that vaccines offer declines after about six months, said Coleman.

Currently, the National Advisory Committee on Immunization (NACI) recommends that provinces and territories prioritize those 80 and older, long-term-care residents, and seniors living in other congregate settings for second booster shots, those who are moderately to severely immunocompromised, and adds that considerations be made for Indigenous peoples.

Second booster dose immunization programs for other high-risk groups and the general public may be needed in the future if data suggest concerning trends in COVID-19 pandemic," Health Canada told the Star in a statement.

Supplies of doses are sufficient for the populations NACI recommends vaccinating, and provinces and territories have stocks, Health Canada said.

In Ontario, those 60 and older are eligible as well as those living in long-term care, retirement homes, elder care lodges, congregate settings; and First Nation, Inuit and Metis individuals 18 and older plus anyone in their household over 18.

Some provinces have lowered the age requirement, as New Brunswick and Saskatchewan allow residents 49 and older to receive the fourth dose. In Quebec it's 18.

NACI published a report Tuesday stating that for the Omicron variant specifically, the protection from the disease offered by the first booster shot of an MRNA vaccine is about 60 per cent and decreases over time.

The committee also explained that vaccine protection against hospitalization and severe illness has been more durable" and protection climbs by 10 to 20 per cent after a first booster.

One study published in the journal Science in the fall found that a two-dose series of COVID-19 vaccines from Moderna, Pfizer-BioNTech and Johnson & Johnson was less effective after six months.

However, NACI does stipulate that while the protection gained from a second booster shot helps prevent severe disease, the duration of that protection is unknown. There are also uncertainties around the effectiveness of the current vaccines on variants of concern.

Omicron variants including the fast-emerging BA.4 and BA.5 indicate that more of the population should receive that immunity boost that's gained from another shot, ideally before the start of another wave, said Coleman.

The booster shots of the current vaccines do offer important protection against new variants, especially severe outcomes like hospitalization and death, he said.

Moderna has developed a vaccine that takes aim at BA.4 and BA.5, which it says has been highly effective, according to its trial data. The company plans to submit data to regulators by August so that it can be available by the fall, it announced Wednesday.

Even with variant boosters in development, it's better not to wait, and to have the population be protected against severe outcomes ahead of the fall, said Coleman.

Dawn Bowdish, the Canada Research Chair in aging and immunity and a professor of medicine at McMaster University, agreed with Coleman.

It makes a lot of sense to get as many people vaccinated at the beginning of a wave as possible," she said.

However, other health experts aren't entirely certain the population at large should receive a fourth dose.

Dr. Sameer Elsayed, a professor of infectious diseases, microbiology, epidemiology and biostatistics at Western University, said while a second booster is recommended for older people and immunocompromised people, there's not enough evidence that a fourth dose boosts immunity to a degree that's worth a mass vaccination campaign.

It's a very marginal benefit against infection with some newer variants, which are more widespread," he said.

Elsayed points to the European Centre for Disease Prevention and Control, which concluded in April that a fourth dose isn't needed for the larger population, but it is recommended for those at high risk.

The European Medicines Agency also argued in January that COVID-19 boosters should not be given too close together and they are concerned about overloading the immune system - which Elsayed agrees is a possible issue. However, NACI's recent report states the safety of a second booster is comparable to previous doses and no new safety signal was identified." It will continue to monitor the evidence.

He also pointed to a recent trial published in the science journal Nature indicating that a fourth dose only offers a slight boost against Omicron.

He recommended diverting more resources to improving health-care infrastructure rather than engaging in a fourth-dose push.

I don't think that's a good use of health-care resources," he said, adding that he would place more importance right now on masking.

Now the incidence of COVID in the summer is less, cases are milder in general. Giving a booster now doesn't really make sense ... we should really target a high-risk population."

With files from May Warren

Olivia Bowden is a Toronto-based staff reporter for the Star. Reach her via email: obowden@thestar.ca