Many or most people in Israel and Scotland have received COVID-19 vaccinations, and reports from both confirm that vaccines largely prevent people from getting sick. But another question arises: do they also block the infection?
Much depends on the answer, experts say.
If vaccines being rolled out worldwide ward off not only the symptoms but also the virus itself, it could greatly slow the spread of the pathogen and speed it back to normalcy.
“If the actual impact on infections were very high, it would be great news because that’s what we need for herd immunity,” said Marc Lipsitch, director of the Center for Communicable Disease Dynamics at Harvard TH Chan School of Health. Public Health. .
Herd immunity is achieved when most of a population – estimates range between 60 and 80 percent – have defended themselves against a virus, either through vaccination or because humans caught the bug and survived it.
But if the Pfizer, Moderna, and AstraZeneca vaccines – and perhaps others made in China, Russia, and India – don’t protect people from infection, even those injected remain potential, unwitting carriers.
“The major concern is that the vaccines prevent illness, hospitalization and death, but not sufficiently prevent transmission,” said Peter English, a UK-based consultant in communicable disease control and former chairman of the Public Health Medicine Committee. British Medical Association. AFP.
In that scenario, communities and economies already reeling from the pandemic face prolonged mask wearing, social distancing, and more or less harsh lockdowns until vaccine campaigns can be completed.
“There is also a greater risk of vaccine ‘escape variants’ being spread while the virus continues to circulate,” said Engels.
Several such variants – more contagious, more deadly, or both – have already spread in England, South Africa and Brazil, as the SARS-CoV-2 virus has a harder time finding new hosts, a predictable stage in the evolution of a pandemic.
But recent studies and others in the pipeline give cause for optimism.
Research covering the entire Scottish population of 5.4 million – a fifth of which are vaccinated with the Pfizer or Oxford / AstraZeneca jabs – provides real-world validation that the vaccines prevent COVID symptoms and disease more than 90 percent of the 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 reduced disease in accordance with clinical studies.
But unlike the study from Scotland, the Israeli findings also showed that infections were greatly reduced in the vaccinated group – with 92 percent among those who were at least a week after the second of two doses.
The actual level of protection may not be that high because Israel does not systematically test for COVID in people without symptoms, the authors acknowledge.
“They probably haven’t detected some asymptomatic cases, and we know that people without symptoms can still transmit the infection,” said Engels.
But the results are still encouraging, he added.
“These findings give us hope that vaccination can only get the R-number below 1,” said Engels, referring to the threshold above which a virus continues to spread.
“If we can – and this is the big question – we should eventually stop using behavioral measures such as lockdowns or masks to interrupt the spread.”
But how is it that, despite hundreds of studies and rigorous clinical trials involving tens of thousands of people and leading to more than half a dozen successful vaccines, we still don’t know how well they block infections?
Lipsitch said one reason is that when the pandemic began its devastating march across the world last spring, it wasn’t a priority.
“What the global community cared most about, the question they wanted to answer quickly was how well vaccines prevent disease,” he said, noting that clinical trials were designed with that in mind.
“We got answers quickly,” he added. “But we wouldn’t have done that if we had tried to do too many things at once – especially things like measuring impact on infection.”
Another reason is the challenge of detecting a disease that affects millions of people and varies in impact from zero symptoms to death.
“Trying to figure out how many people are asymptomatic but potentially contagious is difficult,” said Engels. “Unless you routinely test everyone, how do you identify them?”
What’s more, even the best measures of infection – called PCR tests – are only about 70 percent sensitive outside of lab conditions, he added.
But the extent to which vaccines affect infection will likely become apparent soon.
“The limited data available suggests that vaccines will at least partially reduce transmission, and studies to establish this with greater clarity are underway,” wrote Angela Rasmussen, a virologist at the Center for Global Health Science and Security. the Georgetown University Medical Center, Wednesday. in The New York Times.
One of the most promising indications to date comes from the clinical trials for the Moderna vaccine developed in the United States.
“When people came in for their second injection, they were tested for virus in the nose,” said Lipsitch, who writes a study interpreting the data.
“There was a reduction of more than 60 percent in the number of people who had a virus on Day 28 when they got the vaccine, instead of a placebo on Day Zero.”
The effect was even greater, he added.