Yes, vaccines slow transmission of COVID-19 – but it’s not enough to protect those who don’t have a vaccine
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Even though highly contagious coronavirus variants are circulating, virologists and immunologists say major vaccines not only prevent serious infections, but likely slow transmission as well.
However, this does not mean that the vaccinated can fully protect the unvaccinated – who total billions of people worldwide – especially in areas with low vaccination rates.
“Most vaccines work very effectively to prevent transmission from the vaccinated infected person to an unvaccinated person,” said Akiko Iwasaki, professor of immunobiology at Yale University School of Medicine.
In various countries where immunization efforts are widespread, other clear trends are emerging: severe COVID-19 cases are on the decline as vaccination rates increase; “Revolutionary infections” among fully vaccinated individuals remain rare and generally benign; and cases of serious illness – resulting in hospitalization or death – are now occurring widely among unvaccinated populations.
All of this reinforces the protective power of this first crop of vaccines at a time when much of the world is reopening, including Canada.
But with so many unvaccinated people in the world and due to the transmissibility of variants such as delta, this virus continues to spread, Iwasaki said.
“The whole concept behind herd immunity is to provide this protection to unvaccinated people by having enough vaccinated people around them,” she said.
“But I think we can’t be complacent, because this type of herd immunity really requires the vaccination of a large number of people within a given population. And many places have yet to achieve it. “
Few cases of COVID-19 among fully vaccinated
In Canada, COVID-19 cases have fallen sharply in recent months, and the data shows only a small percentage of these are among fully immunized Canadians.
But with more than half of the population not yet fully immunized, millions of people remain at risk of infection.
While several experts who spoke to CBC News say those vaccinated help curb transmission to those who are not yet protected, the precise mechanism behind why COVID-19 vaccines likely reduce transmission is not entirely clear.
Angela Rasmussen, a virologist at the Vaccines and Infectious Diseases Organization at the University of Saskatchewan, said vaccines act less like a wall or shield that completely blocks a virus from entering your body, and more like a complex army which fights it once these invaders arrive.
For example, she said, imagine someone coughing in your face. You will inhale virus particles whether or not you are vaccinated. But for those who are protected by a vaccine, it’s what happens next that makes the difference.
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The immune system of vaccinated people fights the virus faster, giving it few opportunities to replicate and minimizing symptoms such as coughing or sneezing that would help transmit it, Iwasaki said.
“This kind of activity, we know, propels and expels the virus much better than if you just breathe,” she said.
Since fully vaccinated individuals are usually able to clear infection so quickly, it is “incredibly unlikely” that they will ever produce enough virus to infect others, Rasmussen said.
More research needed to assess impact on transmission
While these are the principles behind how vaccines can prevent both infection and transmission, it is difficult to get real-world data to determine exactly how well they are slowing the spread of the virus. Rasmussen said, with more research needed as SARS-CoV-2 continues to evolve.
The team behind a recent study on vaccine protection in Israel recognized the challenges, saying their research had several key limitations. Individual behavior, political decisions such as lockdowns, and factors such as post-infection immunity all played a role in scrambling the waters, making it more difficult to determine how well vaccines were stopping the spread of the disease. virus.
Yet after taking all of this into account, the researchers found “observational evidence that vaccination not only protects individuals who have been vaccinated, but also provides cross-protection to unvaccinated individuals in the community,” according to their peer-reviewed briefing note in a June issue of the scientific journal Nature.
The team analyzed immunization records and test results collected during a rapid vaccine rollout in 177 communities, and found that immunization rates in each region were associated with subsequent declines in infections among unaccompanied youth. vaccinated.
“On average, for every 20 percentage points of vaccinated individuals in a given population, the positive test fraction for the unvaccinated population decreased by about two-fold,” the researchers wrote.
Another recent study by Public health England, which was published as correspondence in the New England Journal of Medicine last month, found that three weeks after people received a single dose of Pfizer-BioNTech or AstraZeneca-Oxford vaccine, transmission of the virus in households was reduced by 40 to 50%.
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Spread of the virus through variants
But while those vaccinated can help protect those who have not received their full series of vaccines, those efforts do not go far.
In much of Africa, where vaccination rates are among the lowest in the world, variants such as delta, alpha and beta spread rapidly, increasing infection rates and putting pressure on hospital systems. various countries.
Across the continent, the number of deaths from COVID-19 rose 43% week-on-week, the World Health Organization said on Thursday, with at least six countries facing a shortage intensive care beds.
“We really need to get it right quickly, because the simple fact is that the areas that may have received some respite from the pandemic so far are now being hit very hard,” said Jason Kindrachuk, professor. viral pathogenesis assistant. at the University of Manitoba.
“South Africa is a perfect example where they have low vaccination coverage. It is the most prosperous country in Africa, and they were once affected by the gamma variant, now they are affected by the delta again. . “
The delta variant, which is considered to be significantly more transmissible than other widely circulating variants and the original strain of SARS-CoV-2, is also establishing itself even in highly vaccinated countries like Israel, the UK, the United States. United and Canada, where it now accounts for almost 75 percent of cases in Ontario alone.
However, in all of these countries, daily deaths have declined dramatically since vaccination efforts began.
Unvaccinated “driving transmission”
Yet pockets of unvaccinated individuals remain – whether due to eligibility, reluctance, or lack of access – and there is a clear division in how this virus now affects the population. people’s lives, with unprotected individuals paying the price for its spread.
In the United States, for example, almost all deaths from COVID-19 are now among those who are not fully vaccinated, according to an Associated Press analysis.
And we fear more and more that increased number of cases in states with low vaccination rates – like Missouri, Arkansas, Nevada, Louisiana and Utah – could be the harbinger of more hospitalizations and deaths in these specific regions, even if the United States as a whole is avoiding a massive peak.
“If there is a significant proportion of people who are not vaccinated – and this is exactly what we are seeing in the United States – it will be the people who primarily drive transmission, it will be the majority of cases, these will be. be the vast, vast majority of hospitalizations and deaths, ”said Rasmussen.
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It’s a similar situation in Canada as well.
In Saskatchewan, COVID-19 patients recently admitted to intensive care units across the province have been overwhelmingly unvaccinated. In June, 15 people died from the virus and authorities said none of them were fully vaccinated.
A gamma-fueled epidemic that swept through the heavily vaccinated Yukon also offers a cautionary tale. As of June 6, the territory had recorded fewer than 90 cases, but it has since reached more than 460 – the largest outbreak the North has seen – and they are largely among those who have not been fully vaccinated.
Uneven immunization coverage could mean that these disparities will continue in the months to come, Kindrachuk said, as communities with lower immunization rates “don’t have that protective pad” against more transmissible variants even though overall vaccination increases.
“In the end, the vaccines worked really well,” he said.
“It’s more a question of, how can we distribute them globally to the areas where we see high transmission and reduce that toll?”