Dr. Anthony Fauci, President Joe Biden’s chief medical adviser, warned about the risks of China’s planned retreat from its zero COVID policy—and reiterated that the pandemic isn’t over.

On Wednesday, China eased some of its quarantine requirements in a move away from its zero COVID policy that has, at times, included lockdowns of major cities. The move comes after anti-government protests that erupted throughout China over the strict anti-COVID measures.

But to “reopen” without the widespread adoption of vaccines— particularly western vaccines that are considered more effective than Beijing's—would be a huge risk, Fauci said.

“If they don’t do things like mount and implement a proactive vaccination campaign, and you open up, you are going to have a wave of infections which are certainly going to be associated with a certain degree of severity of disease,” Fauci told the Financial Times’ Global Boardroom conference on Wednesday.

Experts previously told Fortune that because of China’s zero COVID approach, its population has limited immunity from infection due to an overall lack of exposure. That coupled with low vaccination rates among its older residents and less effective vaccines—as Fauci said—could trigger a huge wave of infections.

If that happens, there’s an increased chance of mutations, Fauci said, that could create new variants. New variants can sometimes spread more quickly than the original virus, as seen with Omicron.

“Whenever you have a large wave of transmissions of a virus, you give it ample opportunity to mutate,” he said. “And when you give a virus opportunity to mutate, that allows it to form potentially new variants. And once you get a brand new variant that could have an impact on the rest of the world.”

Still Fauci said that China’s strategy was “flawed” from the beginning because the country didn’t use its lockdowns to vaccinate its population. In an interview with the Washington Post last week, Fauci said policies like China’s zero COVID should come with an “end game” and a purpose.

“In the era of vaccination, you might want to restrict until you can get your population in totality vaccinated, particularly the highly vulnerable elderly individuals, but as it turned out, that's not what happened,” he said. “The vaccination of the elderly has not been well performed, and the vaccine they have has been not a particularly effective vaccine.”

Fauci advised that China’s citizens be given more effective vaccines.

“I would suggest that they import western ones, particularly the highly effective mRNA vaccines that are used in most countries in the world,” he said. “Unfortunately, the original Chinese vaccines did not have the level of efficacy of some of the other vaccines.”

This story was originally featured on Fortune.com

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