What a 4th COVID shot can do for you — and what it can't

This is an excerpt from Second Opinion, a weekly analysis of health and medical science news. If you haven't subscribed yet, you can do that by clicking here.

Fourth doses of COVID-19 vaccines are rolling out to more Canadians as eligibility opens up across the country, but research suggests there are limitations to the impact they can have on the highly contagious BA.5 Omicron subvariant fuelling Canada's first summer surge.

After growing pressure from health-care workers and the general public for access to additional shots, some provincial governments have expanded eligibility from just immunocompromised and older Canadians to anyone over the age of 18.

But the move contradicts the National Advisory Committee on Immunization (NACI)'s recent recommendations that a second booster should only be given now to those at highest risk of severe COVID-19, while younger Canadians could wait until the fall.

Data shows that while a third dose offers a significant increase in protection against infection, hospitalization and death — a fourth dose gives only marginal benefit against severe COVID-19 for most, including short-lived protection against infection, even in vulnerable groups.

"That fourth dose does add a small, modest decrease to your risk of developing severe illness," said Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization (VIDO) at the University of Saskatchewan.

"But I think that also it's limited what we can expect from it — I don't think certainly that we can expect it to stop this wave."

BA.5 has sparked an unexpected seventh wave across much of Canada, accounting for close to 40 per cent of cases and leading to a significant rise in COVID-19 hospitalizations, due to its ability to evade protection from both vaccination and previous infection.

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A recent study from Israel published in the New England Journal of Medicine (NEJM) found that while protection against infection and severe COVID-19 increased after a fourth dose in those over 60, the effectiveness of the vaccine began to wane after just several weeks.

A second study from Israel published in the British Medical Journal (BMJ) found that while people aged 60 and over gained additional protection against hospitalization and death in the three weeks after a fourth shot — protection against infection "quickly decreased."

Health-care workers were looked at specifically in another Israeli study published in NEJM that found a fourth dose provided only a marginal difference in infection rates compared to those with a third shot, and only a slight difference in severity of breakthrough symptoms.

A new Canadian study published in the BMJ analyzed the effectiveness of a fourth dose on Ontario long-term care residents. It found the jab initially improved protection against infection and severe COVID-19, but the duration of that protection "remains unknown."

"The science around this right now only has clearly demonstrated benefits of that fourth dose of the vaccine for the groups that (NACI) has outlined," Dr. Fahad Razak, an internist at St. Michael's Hospital in Toronto and the scientific director of Ontario's COVID-19 Science Advisory Table, told The Current on July 7.