COVID-19 is about to explode in China. What that could mean for the United States.

China is likely to see an explosion of COVID-19 cases in coming weeks, experts say, as the country lifts its long-standing and highly unpopular zero-COVID-19 policy.

China is extremely vulnerable right now because its population – especially older adults, who are the most likely to suffer severe disease – is undervaccinated, has no natural immunity from infection and a limited supply of treatments.

Experts predict hundreds of millions of infections and as many as 1.5 million to 2 million deaths.

"I think China is going to blow in the next six to 12 weeks," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, in a Thursday webinar. "Instead of falling off a 5-foot cliff, we're going to watch them fall off a 1,000-foot cliff."

A raging epidemic in China could be bad news for controlling the virus in the U.S., he and other experts said, because travelers will arrive sick and the chances of mutation increase anytime a virus infects a lot of people.

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Any time a virus rages out of control, especially in a population as large as China's, there is a good chance that new variants will develop, said Dr. Jeremy Luban, an expert in viruses at the UMass Chan Medical School.

The variants now circulating in China seem to be the ones that have been most prevalent here, including the omicron subvariants BA.5 and BQ.1.

"There's no specific reason to be concerned other than that a lot of infections are bad for evolution of new things that we can't predict," Luban said Wednesday on a Massachusetts Consortium on Pathogen Readiness media call. "The more the rate of infection can be controlled in China, the better."

The U.S. carefully monitors for infections and variants among travelers, using wastewater and other means, said Dr. Ashish Jha, the White House coronavirus response coordinator, in a Thursday news conference.

"If new variants emerge, I'm confident we'll be able to address them," he said.

Opinion: COVID-19 isn't over. How to help everyone, everywhere endure the next stage.

What's likely to happen in China?

One model predicts China will see 100 million symptomatic cases, 5 million hospital admissions and up to 1.6 million deaths, just from COVID-19, not counting the strain on the health system that will lead to even more tragedies, said Jennifer Bouey, a RAND epidemiologist.

She said there are not enough intensive care units in the country to handle this level of demand and she expects the health care system will be overwhelmed. Blood banks are already seeing a shortage of donations, she said.

If mainland China were to see the same death rate as Hong Kong did during an outbreak in February and March, more than 2 million Chinese people would be predicted to die in the coming months, said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, on the call with Luban.

Why is China so vulnerable to COVID-19 right now?

• Natural immunity: Because of the zero-COVID-19 policy, few Chinese people have been infected so they lack natural immune protection, Bouey said.

• Vaccine protection has waned: About 90% of the Chinese population got a first round of vaccinations but far fewer received boosters and most of those shots were more than six months ago, she said.

• Lack of trust: Early in the pandemic, there were some scandals where patients were given fake or adulterated COVID-19 vaccines, reducing public faith in them, said Timothy Heath, a senior international defense researcher at RAND.

• Vaccine misconceptions: The government also prioritized vaccinating young, healthy people, which fostered a public belief that the vaccine could be dangerous for older people.

'A very hard road ahead' for China

Following widespread public protests, the Chinese government lifted its zero-COVID-19 restrictions on Dec. 1.

Bouey said she's seen little evidence that the Chinese government was prepared to suddenly lift its restrictions. A booster campaign this summer would have made a big difference, along with a substantial pre-purchase of antiviral treatments.

"We see the government just starting the last couple of days talking about boosters and antivirals," she said.

Heath said reversing course now "will further fuel distrust and skepticism among the Chinese people who are not sure how much truth the government is really telling."

Both he and Bouey said they don't think the government is accurately reporting COVID-19 cases right now. Official reports say infections are falling, while social media, empty streets and shops, medication shortages, and long lines at hospitals tell a different story.

Still, Hanage predicts China will have fewer deaths per capita than the U.S. because it delayed its outbreak until after the arrival of vaccines. "China has a very, very hard road ahead of it in the coming months, don't get me wrong, but in the absence of vaccination, it would be much, much worse."

Dig deeper

• In Shanghai earlier this year: Explaining the ongoing citywide lockdown

• From November: White House weighs in as China gripped by protests

• Coronavirus tracker:  A look at the state of COVID in the US 

Contact Karen Weintraub at kweintraub@usatoday.com.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

This article originally appeared on USA TODAY: COVID outbreak likely in China post policy: US impact, explained