In clinical trials, several leading COVID-19 vaccines have been tested as a two-shot regimen. In the real world, three doses have been shown to provide strong protection against serious disease. And now, in several countries, fourth doses are being explored as a means of warding off waning immunity.
Does that mean you should rush out and get another chance if the opportunity arises? Not necessarily.
The human immune system is an extensive, multi-faceted defense network. It starts out rather immature in your childhood, usually sharpens with age, and tends to struggle more to fight off pathogens in your prime.
It’s partly for this reason that there’s no one answer to getting a fourth dose – since your age is a major factor in how your immune system reacts to the training provided by the vaccination.
For anyone who is at high risk of contracting severe COVID — including the elderly, people with comorbidities and immunocompromised people — a fourth vaccine is likely a “really good idea” and provides significant additional protection, said Angela Rasmussen, virologist and researcher at the Vaccine and Infectious Disease Organization at the University of Saskatchewan.
“However, for many people who don’t fit into these categories, it’s hard to say that the fourth hit will bring much benefit, especially in the long run, over a third hit,” she said. .
Study shows 4th vaccine boosts protection for older people
In the United States on Tuesday, federal regulators authorized a fourth dose of Pfizer-BioNTech’s COVID-19 vaccine for Americans 50 and olderdue to concerns about waning immunity in this age group and for anyone aged 12 and over with a weakened immune system.
Here in Canada, there is a disparate approach between the provinces. Many are following the lead of the National Advisory Committee on Immunization, which recommends a fourth vaccine for moderately to severely immunocompromised people six months after their third dose.
Fourth doses are also recommended in Ontario for certain vulnerable older adults, including residents of long-term care facilities, retirement homes and other congregate assisted living facilities, while Quebec now offers access to anyone aged 80 and over as well as.
And in Israel, where hundreds of thousands of people have already received a fourth blow from Pfizer-BioNTech, a study recently showed that it offers more protection for the elderly – including a nearly 80% lower COVID-19 death rate than elderly people who received just three doses.
The country’s largest healthcare provider, Clalit Health Services, said the 40-day study included more than half a million people between the ages of 60 and 100. (The results have not yet been peer reviewed.)
Nearly 60% of participants had received two booster shots in addition to the two-shot background regimen studied in the clinical trials. The others had only received a third dose.
The researchers recorded 92 deaths among the first group and 232 deaths among the second, smaller group.
“The main finding is that the second booster saves lives,” Ronen Arbel, health outcomes researcher at Clalit and Sapir College, told Reuters recently.
New variants can mean new boosters
But several outside experts have pointed out that for most younger people, three doses work very well to avoid hospitalization and death.
“If you’re a healthy, young person, I wouldn’t necessarily be pushing very hard for a fourth shot right now,” said Melanie Ott, director of the Gladstone Institute of Virology and professor of medicine at the University of California at San Francisco.
That could change, she added, if a new variant emerges capable of causing more severe disease or evading vaccine-induced immunity. If that happens, a new booster for the general public could become “urgently needed,” she said.
For now, however, most of the population can rest easy knowing that the original vaccines offer high levels of protection against serious disease – even against the dominant Omicron variant, including the highly contagious BA.2 subvariant.
And for those who would benefit from another encore, the question is: which move makes sense? Another round of a vaccine designed to fight the ancestral virus, or a vaccine designed for Omicron and its offshoots?
First results of a study on monkeys which has not yet been peer-reviewed – which pits the existing Moderna mRNA-based shot against an Omicron-specific booster – found no major difference in the level of protection, this which suggests that a radical overhaul might not be warranted.
“I think we all thought that [an Omicron booster] would be the best, but that doesn’t necessarily seem to be the case,” Ott said. “We could do well enough with a fourth shot of the same.
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Future of vaccination efforts ‘complicated’
There is also the question of how long vaccine-induced immunity lasts, and that is difficult to answer just over a year after the global rollout of the COVID-19 vaccine.
At present, it’s unclear whether strong protection spans years, decades — or far from it.
This potential for waning immunity to infection or even severe disease, coupled with the possibility of unpredictable new variants emerging as this virus evolves, means this is a “very complicated picture for the world.” ‘future,’ said Matthew Miller, an immunologist and researcher at McMaster University.
Coronaviruses tend to evolve at a slower rate than influenza viruses, he noted, although the large number of SARS-CoV-2 infections around the world has given him plenty of opportunities to mutate dramatically – and he agreed that bespoke reminders might be needed at some point. line.
However, it is still unclear if this would be on a yearly or less frequent basis, and if these additional injections would benefit everyone, or only those most at risk.
Encouraging the continued use of reminders could also be a challenge, if the current climate in Canada is any indication. The most recent federal data shows that less than half of eligible Canadians received three or more doses, compared to over 80% who received two.
“There is a lot of work going on to think about how to operationalize a seasonal COVID vaccine program in concert with our seasonal flu program…although I think there is still considerable uncertainty,” said Miller.
Inhaled vaccines that could prevent infections, Miller is looking for as part of a Canadian teamcould also enter the picture, as well as the possibility of so-called “universal” vaccines that would target all coronaviruses or variants of SARS-CoV-2.
Two shots ‘not as good as three’
But while the future of global vaccination efforts remains unclear — with even a fourth dose not necessarily needed for most of the population — several experts who spoke to CBC News agreed that one thing is clear: it’s worth the bother to be Three strokes.
“We actually have quite a bit of evidence now over the BA.1 and BA.2 waves around the world showing that a third hit really does significantly increase protection against infection and serious illness caused by the Omicron variant” , Rasmussen said. .
During the Omicron wave, vaccine efficacy against hospitalization was over 90% in the two months following a third dose, and fell to about 80% by month four.
“Two hits aren’t as good as three hits,” Ott said. “So at this point I would definitely encourage everyone to get three shots because you get a huge boost and advantage.”