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Fourth dose of COVID-19 vaccines distributed to more Canadians Eligibility is open nationwidebut research suggests there are limits to the effect it can have on the highly infectious BA.5, the Omicron variant fueling Canada’s first wave of summer.
after later Growing pressure from caregivers and the the public To access additional snapshots, some provincial governments have expanded eligibility for Canadians and immunocompromised seniors to anyone over the age of 18.
But this decision goes against the National Advisory Committee on Immunization (NACI) Recent Recommendations That a second booster dose now be given only to those at risk of severe COVID-19, while younger Canadians could wait until the fall.
The data shows that even if a third dose Provides a significant increase in protection against infection, hospitalization, and death: The fourth dose provides only marginal benefit against severe COVID-19 for most people, including short-lived protection against infection, even in vulnerable groups.
“This fourth dose adds a small and modest reduction in the risk of severe disease,” said Angela Rasmussen, a virologist at the Vaccine and Infectious Diseases Organization (VIDO) at the University of Saskatchewan.
“But I think what we can expect from him is also limited, I certainly don’t think we can hope to stop that wave. »
BA.5 triggered a seventh unexpected wave across most of Canada, accounting for roughly 40% of cases leads to a Strong increase in hospital admissions due to COVID-19Relationship The possibility of escaping protection. Vaccination and previous infection.
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Fourth dose to protect against infection for a short time
A recent Israeli study published in The New England Journal of Medicine (NEJM) It found that while protection against infection and severe COVID-19 increased after a fourth dose in people over 60 years of age, the vaccine’s effectiveness against infection began to decline after only a few weeks.
A second Israeli study published in British Medical Journal (BMJ) It found that if people aged 60 and older had additional protection against hospitalization and death within three weeks of the fourth episode, protection against infection “rapidly” declined.
Healthcare workers were specifically tested in another country israeli study published in the NEJM that found that the fourth dose provided only a marginal difference in infection rates compared to those who received the third dose, and only a slight difference in the severity of symptoms at onset.
A new Canadian study published in BMJ They looked at the effectiveness of a fourth dose in long-term care residents in Ontario. It found that the vaccine initially improved protection against infection and severe COVID-19, but the duration of this protection “is still unknown.”
Fahad Razak, an internist at St. Michael’s Hospital in Toronto and chief scientific officer for Ontario, told the COVID-19 scientific advisory timeline. Present July 07.
“Who are the elderly and immunocompromised or high-risk people. »
Research suggests that while some people would benefit from a fourth dose, including long-term care residents, the elderly, and other vulnerable groups, data showing a clear benefit for people under 60 are not compliant.
Immunocompromised older Canadians may benefit from a fourth dose, but younger groups get “very little benefit,” said Dr. Zane Shagla, an infectious disease physician at St. Joseph’s Healthcare in Hamilton and an associate professor at McMaster University.
“I don’t think there are many advantages here. It certainly won’t change the dynamics of the epidemic in terms of disease reduction. Really, to get an extreme effect, you would need almost everyone to get the fourth dose almost immediately. »
“And until then, look, your immunity will wane in six to eight weeks and you’ll notice they’re not really protected from infection anyway and BA.4 and BA.5 will still be there in six to eight weeks. ”
Still lacking absorption of third doses in Canada
Alors que l’extension des quatrièmes doses à plus de Canadiens pourrait maintenant avoir un effet limited sur la trajectoire de la vague induite par BA.5 dans la plupart des régions du pays, le plus gros problème est la mauvaise absorption des troisièmes doses à travers the country.
Less than half of Canadians I received a third dose, which research suggests should be added great protection against severe COVID-19, a number that has stagnated for several months amid mounting pressure to extend the fourth dose.
“The biggest problem right now is the large number of Canadians who haven’t gotten a third dose,” said Dr. David Naylor, who led the federal investigation into Canada’s national response to the 2003 SARS outbreak and who now co-chairs the federal investigation. COVID-19 working group.
“It is becoming increasingly clear that the table ration for this vaccine is three shots, and we need to stop referring to people who have received two doses as full shots. »
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But Naylor hopes that expanding the fourth dose to more Canadians will have an impact on the prevalence of BA.5 across the country, although the data does not yet show a clear benefit for younger age groups.
“For someone who is younger, healthy, has had three injections and has a recent infection, I can see the logic of waiting for the fourth injection,” he said.
“There is no question that the marginal returns from a fourth dose are small at the individual level for this population, but they do present a risk of reinfection and broader coverage may help slow population-wide spread. »
Expanding the fourth dose to Canadian youth now in violation of NACI guidelines could do more harm than good by encouraging more people to come back for a third dose, Chagla said.
“There will be people who say: ‘Okay, I’m on the second dose, they’re already saying I need a fourth dose, what use are these vaccines if they don’t protect me against Omicron?’ » «
“Every one of these doses that come out, the whole picture of what vaccines can do changes and we need a lot of people to come back for their third dose. »
Rasmussen, of the University of Saskatchewan, said his concern is that many Canadians might view the boosters as “optional” with limited immune benefit. He said messages about the third dose should be “home delivered” to increase uptake.
“This third dose provides a significant increase in protection against serious disease and Omicron BA.1 infection,” he said.
“That fourth dose is probably additional, maybe just a modest benefit, but I think the third dose is very obvious. At least with BA.1, it offers significant advantages. »
Updated vaccines on the horizon
Timing boosters to provide robust immune protection against COVID-19 are becoming increasingly crucial with the emergence of highly pathogenic sub-variants of Omicron BA.4 and BA.5, as well as a disturbing new variant that is causing a boom in called india Baccalaureate 2.75.
While Canadians who choose to receive a fourth dose of the original strain of the vaccine may only be protected from infection for several weeks, updated bivalent vaccines that combine the original vaccine with Omicron’s horizon protection are also available.
Modern bivalent vaccine He is the one on whom many people pin their hopes. But experts fear that pointing to the original Omicron bloodline is not enough because of Search It shows a deficit of cross-protective immunity against the very different variants BA.2.12.1, BA.4 and BA.5.
“The usual argument for waiting is that we might get better vaccines in the fall. However, the data on bivalent vaccines is less compelling or complete than many of us expected,” Naylor said.
Bivalent vaccines target the viral spike protein. It differs from newer and more innovative platforms with new goals or delivery methods, such as nose lifters Where coronavirus vaccines.
“It has been a guessing game to guess what the virus will do next, and those who receive a fourth dose in the near future will still be eligible for any new combination starting in mid-October or later. »
These new vaccine technologies could induce a more robust immune response by stimulating another arm of the immune system or by targeting different coronaviruses entirely.
“We don’t know when these model vaccine variants will be available, but it may take some time, especially if we wait because it seems prudent to get really good data on their efficacy and safety,” Naylor said.
“To me, this reality and the continued prevalence of BA.5 together argue for a fourth hit, even if similar, with modest marginal returns. »
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But Chagla said Canadians need to start thinking about the protection offered by COVID-19 vaccines differently than they used to. He said there will be “limits” to what they can and cannot do at an individual and population level in the future.
“The first two doses did amazing things for the world and really changed the disease and really changed the dynamics of the disease, and the third dose was able to save a lot of people from severe Omicron disease,” he said.
“But the way this virus has evolved and the way our vaccines…will not be a long-term solution to prevent transmission, and there is no guarantee with an Omicron vaccine that it will offer anything different. »
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