Herd immunity occurs when a disease runs out of room and can no longer spread because enough of the population have been exposed to it, either because they’ve already had it or have been vaccinated.
If nobody is immune to an illness – as was the case at the start of the pandemic – it can spread like wildfire. But if, for example, half of people have developed immunity there are only half as many people the illness can spread to.
As more and more people become immune the pathogen finds it harder and harder to spread, until its pool of victims becomes so small it can no longer spread at all.
The threshold for herd immunity is different for various illnesses, depending on how contagious they are – for measles, around 95 per cent of people must be vaccinated to it spreading. For polio, which is less contagious, the threshold is about 80-85 per cent.
But because there is no vaccine for Covid-19, it means actively hunting down herd immunity through natural exposure is controversial because it would mean tens of thousands of people would die.
Government advisors have previously said around 60 per cent of Britain would need to be infected to achieve herd immunity — around 40million people. But, in theory, it would mean around 240,000 Britons would die, given that the SARS-CoV-2 virus is estimated to kill around 0.6 per cent of everyone it infects.
And scientists still do not have any firm proof as to how long immunity actually lasts once a person has fought off Covid-19, and doctors around the world have warned of re-infections — even though the evidence suggests they are less serious.
Some research has suggested the herd immunity threshold could actually be as low as 10 per cent, if it spreads more rampantly among the most socially active. This is because they are into contact with others more regularly and are, therefore, more likely to spread the illness.
Herd immunity without a vaccine is considered a controversial route for getting out of the pandemic because it gives a message of encouraging the spread of the virus, rather than containing it.
No 10 was even forced to deny herd immunity was the strategy after Boris Johnson’s chief aide Dominic Cummings reportedly confirmed the plan at a private event back in February, allegedly saying it was ‘too bad’ if it meant ‘some pensioners die’.
And leaked emails published last month showed that both Sir Patrick Vallance and Professor Chris Whitty faced backlash from academics over the controversial ‘herd immunity’ approach that was further discussed in March.
Meanwhile, unlike most European nations, Sweden never imposed a lockdown and kept schools for under-16s, cafes, bars, restaurants and most businesses open when the disease hit Europe in February.
Researchers have even suggested that the Scandinavian nation has since built up a degree of immunity to the virus, with one academic claiming that the virus may now have run out of steam in Sweden.
But data compiled by Our World In Data — a website that has tracked the pandemic since it began — suggests cases have began to rise again over the past few weeks. For instance, Sweden’s seven-day average of daily infections stood at 560 on October 1, up from 250 at the start of September.