“If the true impact on infections was very high, it would be great news because that is what we need for herd immunity,” said Marc Lipsitch, director of the Centre for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health.

Herd immunity is achieved when most of a population – estimates vary between 60 to 80 per cent – have acquired defences against a virus, whether through vaccination or because they caught the bug and survived.

“The big concern is that the vaccines prevent illness, hospitalisation and death, but won’t sufficiently prevent transmission,” said Peter English, a Britain-based consultant in communicable disease control and former chair of the British Medical Association’s Public Health Medicine Committee.

In that scenario, communities and economies already reeling from the pandemic face prolonged mask wearing, social distancing and more or less hard lockdowns until vaccine campaigns can be completed.

“There is also a greater risk that vaccine ‘escape variants’ might be thrown up as the virus continues to circulate,” English said.

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Several such variants – more contagious, more deadly or both – have already proliferated in England, South Africa and Brazil as the Sars-CoV-2 virus finds it harder to find new hosts, a predictable phase in the evolution of a pandemic.

But recent studies along with others in the pipeline give reason for optimism.

Research covering the entire Scottish population of 5.4 million – a fifth inoculated with the Pfizer or Oxford/AstraZeneca jabs – provides real-world validation that the vaccines prevent Covid symptoms and illness more than 90 per cent of the time.

But unlike the research from Scotland, the Israeli findings also showed infections had sharply declined in the vaccinated cohort – by 92 per cent among those at least one week past the second of two doses.

The true level of protection may not be that high because Israel does not systematically test for Covid among people with no symptoms, the authors acknowledged.

“They are likely to have failed to detect some asymptomatic cases and we know that people without symptoms can still transmit the infection,” English said.

But the results were still encouraging, he said.

“These findings give us hope that vaccination alone may get the R number below 1,” English said, referring to the threshold above which a virus continues to spread.

“If it can – and this is the big question – we would eventually no longer need to take behavioural measures such as lockdowns or masks to interrupt 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 do not know yet how well they block infection?

One reason, Lipsitch said, was that when the pandemic began its devastating march across the globe last year, that was not a priority.

“What the global community cared about most, the question they wanted answered quickly, was how well vaccines prevent disease,” he said, noting that clinical trials were designed with that in mind.

“We got answers quickly, but we wouldn’t have if we had tried to do too many things at once – especially things like measuring impacts on infection.”

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Another reason is the challenge of tracking a disease affecting millions that ranges in impact from zero symptoms to death.

“Trying to work out how many people are asymptomatic but potentially infectious is difficult,” English said.

“How do you identify them unless you are routinely testing everybody?”

On top of that, even the best measures of infection – so-called PCR tests – are only about 70 per cent sensitive outside laboratory conditions, he said.

But to what extent vaccines impact infection is likely to come into focus soon.

“The limited data available suggests that vaccines will at least partly reduce transmission and the studies to determine this with more clarity are under way,” Angela Rasmussen, a virologist at the Centre for Global Health Science and Security at Georgetown University Medical Centre, wrote on Wednesday in The New York Times .

One of the most promising signals so far has come from the clinical trials for the Moderna vaccine developed in the United States.

“When people came in for their second shot, they were tested for virus in the nose,” said Lipsitch, who is writing a study interpreting the data.

“There was a more than 60 per cent reduction in the proportion who had virus on day 28 if they got the vaccine, rather than a placebo, on day zero.”

If anything, the effect was even larger, he said.