Like the Covid-19 virus itself, the idea of herd immunity has surged back into public life having been suppressed for months. It was initially touted as a way to hold back the pandemic – by allowing sufficient numbers of infections to occur and so reduce numbers of non-immune potential hosts for the virus. The disease would then stop spreading, it was argued.

The notion quickly fell out of favour when researchers highlighted the high death toll that would have to occur in the UK before herd immunity was achieved. Nevertheless, the idea has now bubbled back and is again making headlines.

According to signatories of the Great Barrington Declaration which was published last week, it is now time to remove lockdown restrictions for most of society and to allow the population to get on with their lives while still protecting the vulnerable and old. Herd immunity would build up and soon the scourge of Covid-19 would disappear.

It is an enticing argument. But is herd immunity really a panacea whose time has come? Can it lift the curse of Covid from the world? Many UK scientists counsel caution.

As they point out, only 8% or so of the British population has been infected with the Covid-19 virus. “To get to herd immunity we would need that to reach around 70%,” said Sir Robert Lechler, president of the Academy of Medical Sciences. “Not only are we a huge way off this but we now know that immunity to Covid-19 decreases over time, and that people can be re-infected with the virus.”

For this reason, scientists argue that it is very unlikely that herd immunity could be sustained without a vaccine or regular reinfection. More to the point, if attempts were made to achieve herd immunity by lifting lockdown restrictions, there would be a vast increase in excess deaths, mainly among the old and the vulnerable. NHS services would be overwhelmed while high numbers of “long Covid” cases would have long-term consequences even for those who suffer only mild initial symptoms.

Scientists also point to the example of Manaus in Brazil. The city suffered a devastating wave of Covid-19 cases that killed more than 3,000 people earlier this year. Then virus levels subsided and claims were made that the city had achieved herd immunity. However, cases have now started to surge again in Manaus, suggesting the city failed to achieve herd immunity despite its high death toll.

In fact, the concept of herd immunity is simply not palatable, added Michael Head, senior research fellow in global health, Southampton University. He describes the Great Barrington Declaration as little more than an online petition and points to groups of experts – from Independent Sage to the Academy of Medical Sciences – who have spoken out against the idea.

“In the UK, there was a complete national lockdown for around three months,” he added. “During this time, despite very little UK life running as ‘normal’, thousands of vulnerable people – who overall make up one in four of the population – died of the pandemic coronavirus and yet we still only have 8% of the population with some level of immunity.”

To raise that level to 60 to 70% in a bid to achieve herd immunity would therefore have devastating consequences, Head concluded.

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