4th COVID-19 dose: What is the value of an additional booster?

As a seventh wave of COVID-19 takes shape in parts of the country, more Canadians are now being offered a fourth vaccine dose.

In Ontario, starting Thursday, anyone aged 18 and older who had their first booster at least five months ago can now book another one.

Other provinces and territories, like Quebec, Prince Edward Island, Nunavut and Yukon, have also opened up their eligibility for second boosters — which would be a fourth dose of the vaccine — to anyone over the ages of 18 and 12, who have waited a certain number of months since their last dose.

With questions and mixed messaging around the timing of an additional booster, many are wondering if they should get vaccinated now or wait for the fall when Pfizer and Moderna are expected to roll out updated bivalent vaccines designed to protect against both the original strain and Omicron variant of the virus.

Tania Watts, an immunologist and professor at the University of Toronto, said because we are vulnerable to the current wave now, there is no reason to delay.

“Giving that extra fourth dose, it’ll bring up the antibodies again for a while and then they’ll taper off,” she said.

“The protection against severe disease in healthy people stays more constant, but there is a transient increase in protection against infection if you get that fourth dose.”

The current booster shots that are being offered have exactly the same formula as the first three doses, based on the original Wuhan strain of COVID-19.

Even though the virus has mutated with a new subvariant of Omicron, BA.5, dominating COVID-19 spread, a fourth dose of vaccine is still better than not getting vaccinated, said Alberto Martin, a professor of immunology at the University of Toronto.

“There’s no doubt that antibody levels will increase, but it will largely be against the original variant,” he said.

“It might provide at least a passive protection from getting reinfected,” Martin told Global News.

New variants change the viral sequence where the antibodies bind to the spike protein, said Watts. So there is always some loss of neutralizing antibody, she explained, but “it’s not a complete loss.”

“An extra boost can bring up your level of protection again and also there’s other antibodies that don’t necessarily completely prevent infection, but stick to the virus somewhere else and help mop it up.”

A COVID-19 infection does boost the immune system so a fourth dose may not be as pressing at this time for those people who have caught the virus, Watts said.

Getting infected with COVID-19 does add to the “complexity of the issue” when considering vaccination, according to Dr. Howard Njoo, Canada’s deputy chief public health officer.

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But, because of the risk of long-term consequences of COVID-19, or long COVID, Njoo said it’s better to depend on the vaccines for immunity rather than multiple infections.

“What the science is showing us is that certainly with vaccination, it’s a more controlled way of getting your immunity as opposed to relying on natural infection,” he said during a virtual news conference on Thursday.

Watts also said vaccine-induced immunity is “somewhat better” than virus-induced immunity in the longer term.

“But the combination of vaccine and infection or infection and vaccines seems to be like having another boost,” she said.

There is limited but promising data on the effectiveness of a fourth COVID-19 dose, especially for the vulnerable population.

A new Canadian study published in the British Medical Journal in June found that a fourth COVID-19 vaccine dose was associated with strong protection against severe outcomes in long-term care residents, although the duration of protection remains unknown.

Researchers looked at long-term care facilities in Ontario during the Omicron-driven waves between Dec. 30, 2021, to April 27, 2022.

Their findings suggest that compared with a third dose of an mRNA COVID-19 vaccine, a fourth dose improved protection against infection, symptomatic infection, and severe outcomes.

Meanwhile, according to a study published in the New England Journal of Medicine in May, a fourth dose of COVID-19 vaccine given to people over 60 in Israel made them twice as resistant to Omicron infection as thrice-vaccinated people in the same age group.

Based on the available research, the WHO says there is a short-term benefit of an additional booster dose for the highest risk groups, including health workers, those over 60 years of age or with immunocompromising conditions.

But the data to support an additional dose for healthy younger populations is limited, according to the World Health Organization.

“Preliminary data suggest that in younger people, the benefit is minimal,” it said in a May 2022 statement.

Pfizer and Moderna are currently developing an updated version of their COVID-19 vaccines to better target the Omicron variant and its sub-lineages.

Both companies are hoping for their variant-adapted bivalent vaccines to be ready for approval in the fall.

Moderna submitted its application to Health Canada for approval of the bivalent COVID-19 vaccine last month.

If approved quickly, doses could be ready for Canadians as early as September, according to Moderna Canada.

So if people get their fourth dose now, they still have the option of getting boosted again — but after five to six months to get the sufficient immune response, Watts said.

“I think it’s not a bad idea to get it if you feel like you can’t avoid exposure easily,” she said.

“And I don’t think it’ll stop you getting a later booster in the fall when we get into the respiratory season,” she added.

Martin agreed, saying there are no risks to getting subsequent boosters.

“It doesn’t hurt to get the (fourth) vaccine dose now and then potentially get another Omicron-specific vaccine in the fall,” he said.

— With files from Global News’ Teresa Wright