People hold hands on Fifth Avenue amid the coronavirus pandemic on April 10, 2021 in New York City.

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As Covid vaccines roll out around the world, many look forward to “herd immunity” – when the disease stops spreading quickly because the majority of the population is immune from vaccination or infection.

It is seen as a path to normalcy and something doctors and political leaders often discuss when talking about defeating Covid-19.

While there have been doubts as to whether herd immunity is possible, medical experts who have spoken to CNBC say it can be achieved. However, they point to a difficult path, as maintaining high levels of immunity will be a challenge.

“I think every part of the world will sooner or later reach herd immunity,” said Benjamin Cowling, director of the epidemiology and biostatistics department at the University of Hong Kong’s School of Public Health. Different communities could get there through vaccinations, infection, or a combination of both, he added.

Not everyone agrees.

An article last month in Nature identified five reasons herd immunity might not be possible. According to the report, the barriers to herd immunity include: new varieties, dwindling immunity, and questions about whether vaccines actually prevent transmission.

Shweta Bansal, a math biologist, told the publication, “Herd immunity is only relevant if we have a vaccine that blocks transmission. If we don’t, the only way to get herd immunity in the population is by to give everyone the vaccine. “”

Herd immunity: “Complicated” but possible

Health experts who spoke to CNBC have recognized that the factors raised in the article on nature could hinder progress toward herd immunity – but they believe that is still within reach.

“We’re not trying to eradicate it, we’re trying to stop the runaway community transmission. In that sense, we can achieve (herd immunity),” said Dale Fisher, professor of infectious diseases at the National University of Singapore’s Yong Loo Lin School of Medicine.

Dr. Anthony Fauci, chief medical officer to President Joe Biden, said 75% to 85% of people need to be vaccinated to create an “umbrella” of immunity that will prevent the virus from spreading. Fisher estimates the number is around 70%.

“Reaching 70% is possible, but there are many threats,” he said, explaining that the percentage of a population immune to Covid-19 would decrease as immunity wears off. make the vaccines less effective.

“Herd immunity is something very nice and conceptual, but it’s more complicated,” he said during a call. “If you want to call a magic number around 70% then all I am saying is very hard to come by and maintain.”

Herd immunity may not be permanent, but rather short-term.

Benjamin Cowling

School of Public Health at the University of Hong Kong

Cowling agreed that there is “no guarantee” that immunity levels would remain high over the long term. “Herd immunity may not be permanent, but rather short-term,” he said.

Still, it’s something the world can work towards, he added, emphasizing that refresher shots can help when protection is lost.

Back to the “normal”

It could take three to five years for the world to return to “completely normal,” said Carlos del Rio, a professor of medicine at Emory University School of Medicine.

“There are still a lot of broadcasts around the world and I think it will be some time before that changes,” he told CNBC’s Street Signs Asia on Monday.

The World Health Organization warned this week that the pandemic is “growing exponentially” and more than 4.4 million new Covid-19 cases have been reported in the past week.

The agency’s technical director for Covid-19, Maria Van Kerkhove, said the world has reached “a critical point in the pandemic”.

“Vaccines and vaccinations are going online, but they are not yet available in all parts of the world,” she added.

Fisher said the world is still “very susceptible to large outbreaks” – but cases could sporadic in five or ten years. In the meantime there will be a transition period.

“Herd immunity is not a binary phenomenon,” he said. “Most people think you either have it or you don’t – but it’s obviously gray in between.”

Cowling said he thinks the greatest risk for Covid will be in the next 12 months, but the threat will decrease afterwards as vaccines are introduced.

“What I would expect in the years to come is that the virus will still circulate, it will be endemic, but it will no longer be a major threat to public health,” he said.

– CNBC’s Berkeley Lovelace contributed to this report.