Many or most people in Israel and Scotland have received inoculations of COVID-19 and reports from both confirm that vaccines prevent people from getting sick. But another question is arising: do they also block the infection?
Much depends on the answer, experts say.
If vaccines launched around the world prevent not only symptoms, but the virus itself, this could drastically slow the spread of the pathogen and accelerate the return to normal.
“If the real impact on infections were too high, it would be great news, because that’s what we need for collective immunity,” Marc Lipsitch, director of the Center for Communicable Disease Dynamics at the Harvard TH Chan School of Public Health, told AFP. .
Herd immunity is achieved when the majority of the population – estimates range from 60 to 80 percent – have acquired defenses against a virus, either by vaccination or because people have caught the virus and survived.
But if the Pfizer, Moderna and AstraZeneca vaccines – and perhaps others made in China, Russia and India – do not protect people from being infected, then even the injected people remain potential unintended carriers.
“The major concern is that vaccines prevent disease, hospitalization and death, but they do not sufficiently prevent transmission,” said Peter English, British consultant in communicable disease control and former chairman of the Public Health Committee of the British Medical Association AFP.
In this scenario, communities and economies that are already recovering from the pandemic face the prolonged use of masks, social distance and more or less rigid blockages until vaccination campaigns can be concluded.
“There is also a greater risk that the ‘escape variants’ of the vaccine will be launched while the virus continues to circulate,” said English.
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 has a harder time finding new hosts, a predictable phase in the evolution of a pandemic.
But recent and ongoing studies give reason for optimism.
The survey covering the entire Scottish population of 5.4 million – with a fifth inoculated with Pfizer or Oxford / AstraZeneca jabs – provides real-world validation that vaccines prevent COVID symptoms and diseases in more than 90 percent of times.
A study published on Wednesday in the New England Journal of Medicine – comparing two groups in Israel of almost 600,000 people each, one vaccinated and the other not – also reported a reduction in the disease in line with clinical trials.
But, unlike Scotland’s research, Israeli findings also showed that infections declined dramatically in the vaccinated group – 92% among those who passed at least a week after the second of the two doses.
The true level of protection may not be as high because Israel does not systematically test for COVID among people without symptoms, the authors acknowledge.
“They probably haven’t been able to detect some asymptomatic cases and we know that people without symptoms can still transmit the infection,” said English.
But the results are still encouraging, he added.
“These findings give us hope that vaccination alone can bring the R number below 1,” said English, referring to the limit above which a virus continues to spread.
“If I could – and this is the big question – eventually we would no longer need to take behavioral measures like blocking or masking 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 well they block infection?
Lipsitch said one reason is that when the pandemic started its devastating march around the world last spring, it was not a priority.
“What most interested the global community, the question they wanted answered quickly, was how well vaccines prevent disease,” he said, noting that clinical trials were designed with this in mind.
“We received responses quickly,” he added. “But we wouldn’t have done it if we had tried to do many things at the same time – especially things like measuring the impacts on the infection.”
Another reason is the challenge of tracking a disease that affects millions of people, whose impact ranges from no symptoms to death.
“Trying to find out how many people are asymptomatic, but potentially infectious, is difficult,” said English. “How do you identify them, unless you’re testing everyone routinely?”
In addition, even the best measures of infection – so-called CRP tests – are only about 70% sensitive outside laboratory conditions, he added.
But the extent to which vaccines affect the infection is likely to come into focus soon.
“The limited data available suggest that vaccines will, at least partially, reduce transmission, and studies to determine this more clearly are ongoing,” wrote Angela Rasmussen, virologist at the Center for Global Health and Safety at University Medical Center at Georgetown. in The New York Times.
One of the most promising indications so far has come from clinical trials of the Modern vaccine developed in the United States.
“When people came for the second injection, they were tested for viruses in the nose,” commented Lipsitch, who is writing a study to interpret the data.
“There was a more than 60 percent reduction in the proportion of people with viruses on day 28 if they received the vaccine, instead of a placebo, on day zero.”
In fact, the effect was even greater, he added.