Are we seeing the effects of coronavirus herd immunity or is it dangerous to even suggest so?

• Video report by ITV News Correspondent Tom Clarke

“Herd Immunity” used to be a technical term rarely used beyond the disciplines of epidemiology or vaccine science.

But now it’s one of the most controversial concepts of the coronavirus pandemic.

It’s a simple concept: as a population is being exposed to a virus, people become immune and can no longer spread the bug to others.

Once a certain threshold is reached, there simply won't be enough susceptible people left for the virus to infect and that’s herd immunity.

The general rule of thumb is that around 70 to 80% of a population has to be immune for a virus like SARS-CoV-2 to stop spreading (for very transmissible viruses like measles the number can be even higher — around 95%).

But what if, in the real world, we reach herd immunity at much lower levels.

Back in March theoretical epidemiologist Sunetra Gupta published analysis suggesting herd immunity for COVID-19 could be more like 20% if you take into account many of us seem to be resistant to infection, possibly due to previous exposure to related coronaviruses.

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Her paper was widely criticised by other epidemiologists, particularly those advising governments.

It was based on a mathematical analysis, not on real-world observations.

What’s more, by the time it was published the virus had begun to spread rapidly among care home residents.

It became clear that a policy based on herd immunity — allowing natural resistance to build up among those at low risk while protecting the vulnerable — would have been a disaster based on our failure to shield those most at risk.

But what if she and other disease modellers are right? And what could it mean for how we manage the outbreak from now on?

In countries like Brazil and Sweden, and parts of the US that didn’t pursue aggressive lock-down strategies, there is evidence that their outbreaks have now reached some kind of natural peak, with cases starting to fall.

Yet tests on the population suggest 20% — at most— have antibodies to the virus.

All were hard hit, an estimated 1 in 500 people died in the city of Manaus in Brazil, but the outbreak seems to be declining with far lower than 70% herd immunity.

Given we now have a greater ability to test for the virus, and should be better prepared to shield the vulnerable, is it time to revisit the idea of herd immunity?

Ms Gupta said: “We cannot consider our response solely along the single axis of how to keep coronavirus under control.

“The strategy the welfare state should take at this point in time is to shield the vulnerable and allow the virus to spread among those not at risk of dying

“We have to think about who is paying the price for this.

"We put in place a system that doesn’t protect the vulnerable and now we’re asking those that are a economically vulnerable to pay for that.”

Her views have split the scientific community, while few disagree with the theory, they argue we don’t know enough to be sure that stepping back and allowing immunity to develop will work in practice.

The World Health Organization’s (WHO) chief scientist Dr Souimya Swaminathan said: “Even if 1% of people who get infected die, that adds up to a huge number of people if you look at the global population.

“We should talk about herd immunity in the context of a vaccine.”

Furthermore, it’s not exactly clear what “immunity” to SARS-CoV-2 looks like.

There’s strong evidence emerging that antibodies to the virus decline over time.

More cases are now emerging of people being infected more than once.

Some argue this is good evidence that lasting herd immunity won’t develop at all — and that it may even challenge development of a vaccine.

Other experts argue other coronaviruses cause repeat infections, but only a mild illness and that soon, this new coronavirus will behave in just the same way once more of us have been exposed to it.

For now, it is still a largely academic debate.

But as the true costs of aggressive lockdown start making themselves felt in not just the economy but also people’s long term health, it’s a debate we’re only going to hear more of.