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Many or most people in Israel and Scotland have received COVID-19 inoculations and reports from both confirm that vaccines largely keep people from getting sick. But another question arises: do they also block the infection?
Much depends on the answer, experts say.
If the vaccines deployed around the world prevent not only the symptoms but also the virus itself, it could significantly slow the spread of the pathogen and speed up the return to normalcy.
“If the true impact on infections were very high, that would be great news because that’s what we need for herd immunity,” Marc Lipsitch, director of the Center for Communicable Disease Dynamics at AFP, told AFP. Harvard TH Chan School of Public Health. .
Herd immunity is achieved when most of the population – estimates vary between 60 and 80 percent – have developed defenses against a virus, whether through vaccination or because people have caught the virus and survived.
But if the Pfizer, Moderna, and AstraZeneca vaccines – and possibly others made in China, Russia, and India – do a poor job of preventing people from getting infected, then even people who have been injected remain potential and involuntary carriers.
“The big concern is that vaccines prevent illness, hospitalization and death, but will not prevent transmission sufficiently,” said Peter English, UK communicable disease consultant and former chairman of the Medical Committee of British Medical Association Public Health. AFP.
In this scenario, communities and economies already reeling from the pandemic face prolonged mask wear, social distancing, and more or less strict lockdowns until vaccination campaigns can be completed.
“There is also an increased risk that ‘escape variants’ of the vaccine will be launched as the virus continues to circulate,” Mr. English said.
Several of these variants – more contagious, more deadly, or both – have already proliferated in England, South Africa and Brazil as the SARS-CoV-2 virus struggles to find new hosts, a predictable phase in the evolution of a pandemic.
But recent studies and more in the works give cause for optimism.
Research covering the entire Scottish population of 5.4 million – with a fifth inoculated with the Pfizer or Oxford / AstraZeneca jabs – provides real-world validation that vaccines prevent COVID symptoms and disease by more than 90% time.
A study published Wednesday in the New England Journal of Medicine – comparing two groups in Israel of nearly 600,000 people each, one vaccinated and the other not – also reported a reduction in the disease according to clinical trials .
But unlike research in Scotland, the Israeli findings also showed infections fell sharply in the vaccinated group – by 92% among those at least a week after the second of two doses.
The true level of protection may not be as high because Israel does not routinely test for COVID among people without symptoms, the authors acknowledge.
“They probably failed to detect some asymptomatic cases, and we know that people without symptoms can still pass the infection on,” English said.
But the results are still encouraging, he added.
“These results give us hope that vaccination alone can get the R number less than 1,” said Mr. English, referring to the threshold above which a virus continues to spread.
“If it is possible – and this is the big question – we would ultimately no longer need to take behavioral measures such as lockdowns or masks to stop the spread.”
But how is it that despite hundreds of studies, as well as rigorous clinical trials involving tens of thousands of people and resulting in more than half a dozen successful vaccines, we still don’t know how successful they are. block infection?
Lipsitch said one of the reasons is that when the pandemic began its devastating march across the world last spring, it was not a priority.
“What mattered most to the global community, the question they wanted a quick answer to, was how well vaccines prevent disease,” he said, noting that clinical trials had been designed with this in mind.
“We got answers quickly,” he added. “But we wouldn’t have done it if we had tried to do too many things at once – especially things like measuring the impacts on the infection.”
Another reason is the challenge of tracking a disease affecting millions of people whose impact ranges from zero symptoms to death.
“Trying to determine how many people are asymptomatic but potentially infectious is difficult,” English said. “How do you identify them if you don’t systematically test everyone?”
On top of that, even the best measures of infection – the so-called PCR tests – are only around 70% sensitive outside of laboratory conditions, he added.
But the extent to which vaccines affect the infection will likely be clear soon.
“The limited data available suggests that vaccines will at least partially reduce transmission, and studies to determine this with more clarity are underway,” wrote Angela Rasmussen, virologist at the Center for Global Health Science and Security at Georgetown University Medical Center. in the New York Times.
One of the most promising indications to date comes from clinical trials of the Moderna vaccine developed in the United States.
“When people came in for their second injection, they were tested for the virus in their nose,” commented Lipsitch, who is writing a study interpreting the data.
“There was a reduction of more than 60 percent in the proportion of people infected with the virus on day 28 if they received the vaccine, rather than a placebo, on day zero.”
If anything, the effect was even greater, he added.
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