The concept of ‘herd immunity’, or population immunity, has been ferociously debated since the Covid-19 pandemic began last year. While some believe it is the most effective way to achieve mass protection from the virus, others think it to be an “unscientific” and “unethical” experiment.

The World Health Organization (WHO) define herd immunity as “the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.”

The WHO say they support mass protection through vaccination, but not by allowing a disease to spread through any segment of the population as this would result in unnecessary cases and deaths.

Scientists have voiced their concerns over chasing herd immunity in the past, warning of its consequences. It therefore came as a shock to many when the UK government decided to go ahead with easing nearly all restrictions on the 19th July, despite the rapidly rising case numbers.

In a letter, more than 1,200 scientists and medics accused the government of “embarking on a dangerous and unethical experiment”. They advised the government to instead delay the re-opening until everyone, including adolescents, had been offered vaccination and a high level of uptake had been achieved.

Reaching herd immunity is “not a possibility”

Now, despite the fact that more than 75% of UK adults have been doubled jabbed, the head of the Oxford Vaccine Group has said reaching herd immunity is “not a possibility”.

Professor Sir Andrew Pollard recently explained in an evidence session with MPs that because the Delta variant is still infecting people who have received both vaccinations, reaching herd immunity is “mythical”.

“The Delta variant will still infect people who have been vaccinated. And that does mean that anyone who’s still unvaccinated at some point will meet the virus … and we don’t have anything that will [completely] stop that transmission,” he said.

Prof Pollard also said that there is every possibility a new variant may appear that is “even better at transmitting in [the] vaccinated population”. He said this provides even more of a reason “not to be making a vaccine programme around herd immunity.”

Unless action is urgently taken, Pollard warns millions more could die by the end of the year. “If that happens that will be an enormous moral failure, a failure of leadership politically and also one where I think most of those in the world that I work in will actually feel as if it’s a huge failure to humanity,” he said.

Does the vaccine prevent transmission of the virus?

While the vaccines have weakened the link between infection and serious illness and death, the link has not been completely broken.

This was demonstrated in a recent React study, conducted by Imperial College London. The study found that fully vaccinated people aged 18 to 64 have about a 49% lower risk of being infected compared with unvaccinated people.

The findings also indicated that fully vaccinated people were about half as likely to test positive after coming into contact with someone who had Covid (3.84%, down from 7.23%).

So, while the jab might slow the process of transmission, it cannot stop the spread completely. This means that the virus will continue to spread even if the vast majority of the global population were to be vaccinated.

Will we live with Covid-19 forever?

There is a general agreement among scientists that the most likely long-term outcome is that the virus will remain widespread across the world, constantly circulating among the population but causing far fewer cases of severe illness and death as time goes on.  

Professor Paul Hunter, from the University of East Anglia, told the APPG on coronavirus that the seasonal coronaviruses in circulation will infect people “repeatedly” throughout their lives, typically every four or five years, on average.

He said: “A quarter of the UK population will get infected on average every year, what that means is about 45,000 people will be infected every day with these other coronaviruses.” He added, “Ultimately what happened with these other coronaviruses is that although you get a gradual escape, because we are getting re-infected so frequently, we actually keep up.”

Prof Devi Sridhar, chair in global public health at the University of Edinburgh, agreed with Hunter, adding that vaccines had “transformed” the pandemic but “not solved it”.

Instead, we will have to learn to live with the virus and continue to discover ways to manage it, rather than expect it to vanish.

Will we all need booster jabs?

One of the suggested methods of management is booster jabs, which the health secretary, Sajid Javid, has said the government will start offering to the most vulnerable next month.

However, Prof Pollard has questioned whether boosters will be necessary. He said: “The time we would need to boost is if we see evidence that there was an increase in hospitalisation – or the next stage after that, which would be people dying – amongst those who are vaccinated. And that is not something we are seeing at the moment.

“But we have to also have an understanding scientifically about how the vaccines work, and they are providing very high levels of protection against that severe end of the spectrum. But also, even as the levels of immunity start to drop that we can measure in the blood, our immune system still remembers that we were vaccinated and we’ll be remembering decades from now that we have those two doses of vaccine,” he continued.

There is also a debate about whether giving the British public booster jabs is the best way to use high-in-demand vaccine doses. Some critics say that rich countries should not be hoarding vaccine supply and should instead prioritise giving doses to poorer countries who still have a large number of vulnerable people unvaccinated.

Prof Sridhar made a point of this during the APPG conference when discussing whether children should be prioritised for the jab. She said: “I think actually the real issue is not about vaccinating kids versus the world – it’s what do we do about boosters that is being discussed in rich countries, because those are the doses that could be going abroad.

“Luckily … we have a very effective vaccine in AstraZeneca that can be used for the world, but it’s not going to solve our children issue here.”