People hold hands on Fifth Avenue amid the coronavirus pandemic on April 10, 2021 in New York City. Noam Galai | Getty Images Entertainment | Getty Images As Covid vaccines roll out across the world, many are looking forward to reaching “herd immunity” — when the disease no longer transmits rapidly because most of the population is immune due to vaccinations or after being infected. It’s seen as a pathway to normality, and something doctors and political leaders often discuss when talking about defeating Covid-19. While there have been doubts about whether herd immunity is possible, medical experts who spoke to CNBC say it can be achieved. However, they point to a tough road ahead as sustaining a high level of immunity will be a challenge. “I think that every part of the world will reach herd immunity sooner or later,” said Benjamin Cowling, head of the division of epidemiology and biostatistics at the School of Public Health in the University of Hong Kong. Different communities may get there by vaccinations, infections or a combination of both, he added. Not everyone agrees. An article last month in scientific journal Nature outlined five reasons why reaching herd immunity may not be possible. The report said barriers to herd immunity include: new variants, waning immunity and questions over whether vaccines actually prevent transmission. Shweta Bansal, a mathematical biologist, told the publication: “Herd immunity is only relevant if we have a transmission-blocking vaccine. If we don’t, then the only way to get herd immunity in the population is to give everyone the vaccine.” Herd immunity: ‘Complicated’ but possible Health experts who talked to CNBC acknowledged that the factors raised in the Nature article could hinder progress toward herd immunity — but they said they believe it is still within reach. “We’re not trying to eradicate it, we’re trying to stop out-of-control 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 advisor to President Joe Biden, has said that 75% to 85% of people need to be inoculated to create an “umbrella” of immunity that prevents the virus from spreading. Fisher estimates that the figure is around 70%. “Getting to … 70% is possible, but there’s lot of threats to it,” he said, explaining that the percentage of a population that’s immune to Covid-19 would drop if immunity wears off, render the vaccines less effective. “Herd immunity is something very nice and conceptual to aim for, but it’s more complicated than that,” he said during a call. “If you want to call a magic number of about 70%, then all I’m saying is that’s very difficult to attain and maintain.” Herd immunity may not be permanent, it may be something that’s relatively short term. Benjamin Cowling School of Public Health at the University of Hong Kong Cowling agreed there’s “no guarantee” that the level of immunity would remain high in the long term. “Herd immunity may not be permanent, it may be something that’s relatively short term,” he said. Still, it’s something the world can work toward, he added, highlighting that booster shots can help if there’s a loss of protection. Returning to ‘normal’ It could take three to five years before the world gets back to a “totally normal state,” said Carlos del Rio, a professor of medicine at Emory University School of Medicine. “There’s a lot of transmissions still occurring globally, and I think it’s going to take 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 were reported over the previous week. The agency’s technical lead for Covid-19, Maria Van Kerkhove, said the world had reached a “critical point of the pandemic.” “Vaccines and vaccinations are coming online, but they aren’t here yet in every part of the world,” she added. Fisher said the world is still “very vulnerable to huge outbreaks” — but cases could be sporadic in five or 10 years. In the meantime, there will be a period of transition. “Herd immunity is not a binary phenomenon,” he said. “Most people think you either have it, or you don’t have it — but there’s obviously gray in between.” Cowling said he thinks the greatest risk of Covid will be the next 12 months, but the threat will diminish after that as vaccines are rolled out. “What I would expect in the coming years is that the virus will still circulate, it will be endemic, but it won’t pose a major public health threat anymore,” he said. — CNBC’s Berkeley Lovelace contributed to this report.