• The theory goes that if 60-70% of a population have Covid antibodies, then the transmission of the virus would gradually decrease.
• Vaccine hesitancy, the role of children in transmission and the emergence of new Covid variants could prevent herd immunity being achieved.
• "We may never hit herd immunity on a global stage and fully eradicate the virus," one expert told CNBC.

When the coronavirus pandemic started to sweep around the world in 2020, a number of governments and health authorities appeared to pin their hopes on "herd immunity."

This approach would see the virus spread though society and cause infections, but also provoke an immune response in those who have recovered.

If enough people gained these antibodies — say, around 60-70% of the population — then the transmission of the virus would gradually decrease, and those who had not yet been infected would be protected by the increasingly small opportunity the virus had to spread.

That was the theory.

In reality, Covid-19 swept through the Asia, Europe and the Americas prompting millions of infections — from which millions of people recovered — but also hundreds of thousands of hospitalizations and deaths. To date globally, the virus has caused over 164 million infections and 3.4 million deaths.

The strategy of targeting herd immunity was quickly abandoned by most countries — with some notable exceptions such as Sweden — and lockdowns became the primary way of trying to prevent the spread of Covid as vaccines were rapidly developed.

Now we have highly effective vaccines and immunization programs are continuing apace across the world. This has sparked hope that once enough people in populations have been vaccinated, herd immunity could be achieved — that is, once enough people are vaccinated, the virus will have nowhere to go and will die out.

But yet again, Covid-19 is proving to be unpredictable, and we still don't know how long protection from vaccines, or natural immunity acquired by previous infection, lasts.

Vaccine hesitancy, the role of children in transmission (young children are not eligible for vaccines) and, most importantly, the emergence of new Covid variants around the world are also unknowns that could also prevent herd immunity, experts warn.

Most of them believe Covid-19 will become endemic like the flu (meaning it will continue to circulate in parts of the population, likely as a seasonal threat) while hoping it will become less dangerous over time.

'Nowhere near herd immunity'

Epidemiologist Lauren Ancel Meyers, director of the University of Texas Covid-19 Modeling Consortium, described herd immunity as "the idea that if we vaccinate enough people around the globe, the virus will have nowhere to spread, and the pandemic will completely fade out." 

"Unfortunately, we are very far from that reality on a global scale," she told CNBC.

"The virus continues to spread rapidly on many continents, more contagious variants that can possibly break through immunity are continually emerging, and many countries lag far behind the U.S. in rolling out vaccines." 

She noted that even in U.S. cities there are critical pockets of low immunity: "Where I live in Austin, Texas, we estimate that vaccination coverage ranges from under 40% to over 80% depending on which neighborhood you live in. Everywhere, children under age 12 cannot yet get vaccinated. As long as there are pockets of low immunity, this stealthy virus will continue to spread and produce new variants."

Nonetheless, Meyers noted that even if we don't achieve full herd immunity, "vaccines may help us get to a place where Covid-19 is a significantly less lethal threat."

There has been a lot of misinformation and misunderstanding about the herd immunity threshold, according to Meyers. "Simply speaking, the herd immunity threshold is the fraction of the population that must be immunized before the virus will fade away. But in the real world, it's complicated."

"With emerging variants and pockets of low vaccination coverage, there is no guarantee we'll get there," she said, noting that it's important people realize: "The more people vaccinate, the faster the threat will fade."

"We may never hit herd immunity on a global stage and fully eradicate the virus. But that doesn't mean we won't get back to a sense of normalcy soon. We are already seeing the numbers of new cases and hospitalizations beginning to decline," Meyers added.

'Challenging' strategy

After a year, the coronavirus has undergone some significant mutations and a number of variants have become dominant due to their increased transmissibility — such as those first detected in the U.K. and South Africa last year.

Now a variant first detected in India in October 2020 is rampaging through the country and beyond. As with previous mutations, experts are investigating whether it is more transmissible (early evidence suggests so), more deadly (early evidence suggests not) and could render Covid vaccines less effective (early evidence suggests not).

Professor Lawrence Young, a virologist at the University of Warwick's Medical School in the U.K., told CNBC that the pursuit of herd immunity was likely unachievable when it comes to Covid-19.

"Pursuing herd immunity in terms of allowing people to be infected and then recover is not great because obviously with Sars-Cov-2 people are getting sick — but also, the issue is what is the herd immunity threshold and what proportion of the population would you need to be protected? And that so much depends on the transmissibility of the virus," Young noted.

"We're dealing with variants that have different abilities to spread and I think that's what makes achieving herd immunity, or indeed relying on herd immunity, quite challenging."

He stressed that there were still lots of "unknowns" about Covid-19.

"And I think trying to get herd immunity through vaccination is going to be impossible. Variants and the fact that you don't necessarily get immune protection for life once you're vaccinated makes it more difficult," Young said.

Asked whether there's a possibility that the coronavirus could be eradicated, Young said: "it's not going to happen."

"We're going to have to live with it, like flu, and we just need to get as many people vaccinated in order to stop them getting sick."