(CNN)The Omicron coronavirus variant may be reaching around the world, but different places are seeing significantly different effects.
In the United States, Covid-19 case numbers have been falling since January. They may have hit a plateau as a subvariant of Omicron, BA.2, becomes the main cause of infections.
But in China, an area of the world that has had few spikes during the pandemic, there has been a dramatic increase in cases as BA.2 rips through the country.
The difference, experts say, is part policy and part population-level immunity. What's happening in China doesn't necessarily mean the US is in for another huge spike in cases.
All health is local -- at least now
"Public health is very much a local thing," said Andy Pekosz, a virologist at the Johns Hopkins School of Public Health.
With the first Covid-19 cases in 2020, he said, trends were similar around the globe because most people's bodies had no experience with the coronavirus, and there were no vaccines. They had no protection against it.
Since then, different countries have used different vaccines and have had different success with vaccination campaigns. There's also a different level of immunity among populations in different regions. All of that affects the trajectory of Covid-19 cases.
"You would probably expect to see more variation from country to country in terms of case numbers and surges and fatality rates from here on out," Pekosz said. "All of that means it's just difficult to make blanket statements about how things are going to go forward."
Different case management strategies
Lockdowns in the US are a thing of the past and will probably stay that way, public health experts say. Though the decline in case numbers seems to be leveling off in the US, it's rare to see someone wearing a face mask in many places.
By comparison, China has a zero-Covid policy. As cases have been on the rise in more than two dozen provinces, restrictions have gotten much tougher.
In Shanghai, a city of 26 million, people are confined to their homes, able to go out only for essential supplies. Regular tests are mandatory. Officials throughout China check color-based "health code" systems to monitor people's movements. Everyone has a smartphone app that shows their personal QR health code: green for healthy, yellow for close contacts and red for confirmed or suspected cases.
If someone tests positive for Covid-19 -- even if they don't have symptoms -- they must isolate in large temporary facilities set up in stadiums and convention centers. They can't leave until they test negative twice. Pets are even sent to other centers for monitoring. Public health officials cordon off the person's home and disinfected everything.
With the highly contagious Omicron variant, lockdowns can't totally stop the growth in cases, but the strategy has helped, experts say.
China's seven-day daily new case average is more than 14,000, according to Johns Hopkins University. By way of comparison, the country has about four times as many people as the US -- but as of Thursday, the US is averaging more than 30,000 new Covid-19 cases per day. This is one of the lowest daily case rates since mid-July.
"That's a stark contrast," said Abram Wagner, a research assistant professor of epidemiology at the University of Michigan. "We're in different trajectories, with the US case numbers going down and kind of plateauing at this point in time, so the direction is different. But I think seeing the smaller number of cases in China, I think the policies that China has in play with restrictions on some level have had an effect."
Whether strict lockdowns are too heavy-handed and infringe too much on personal liberties is a different conversation, he said. But China has kept cases much lower overall than in the US, even during a spike.
What's made things more difficult is exactly how easy it is to catch Covid with Omicron.
"Omicron has changed the equation in China," Wagner said. "It's just hard to maintain a dynamic zero-Covid policy that stamps it out completely."
'Their firewall has broken down with Omicron'
"Omicron, to compare it to other variants before it, has really changed an awful lot of the playing field," Pekosz said. "Its transmissibility is just off the charts."
Omicron is three times as likely to spread as the Delta variant. None of the vaccines made to protect against the original strain works as well, especially in populations in China that haven't seen spikes over the past two years.
"Sort of their firewall has broken down with Omicron, and they have a massive amount of people who are susceptible to this virus," said Justin Lessler, an epidemiologist at the University of North Carolina's Gillings School of Global Public Health.
BA.2, which is playing a role in the spike in cases in China and is now the dominant strain in the US, is especially contagious. Some epidemiologists have said BA.2's basic reproduction number may be as high as 12, meaning each sick person can infect an average of 12 others. That puts it on par with measles.
Though it's difficult to predict the trajectory of the pandemic in the US, Lessler said, community immunity could keep the country from seeing an across-the-board spike. Instead, we should see a "mild blip."
"I expect to see some resurgence here, but it's a very different picture, even with the increase in the Omicron subvariant BA.2," Lessler said.
The US population has built up immunity that China does not have. China has seen relatively few cases over the course of the pandemic, but the US Centers for Disease Control and Prevention estimates that 37 million Americans -- about 1 in 11 -- got Covid-19 in the past winter alone.
"The [BA.2] subvariant of Omicron is not an entire leap ahead of Omicron, whereas Omicron was really quite different than Delta," said Dr. Davidson Hammer, a professor of global health and medicine at Boston University. "I think that there are a lot of people, at least the United States, that have had Omicron, and there's growing evidence that there's they're close enough in terms of their immunologic profile that if you've had 1, you're unlikely to get 2."
In other words, there's a more limited pool of people who might become infected in the US than in China.
Vaccine strategy differences
Many in the US have some coronavirus immunity from vaccination and boosters. More than 81% of the population has at least some protection with at least one dose, according to the CDC. Boosters have been less popular, but they do work when people get them.
Another problem for China may be the kind of vaccines it's used, according to Dr. Ian Lipkin, a professor of epidemiology at the Mailman School of Public Health at Columbia University.
"The increase in the number of cases reflects a combination of factors. Your population that's immunologically naive, they haven't seen much of the virus in the past and because they haven't been vaccinated effectively to resist them," Likpkin said.
China has mostly used the Chinese-made Sinovac and Sinopharm vaccines. Both companies said their vaccines were more than 78% effective against Covid-19, but studies suggested otherwise. Late-stage trials of the Sinovac candidate in Brazil showed an efficacy rate of 50.38%, barely above the World Health Organization threshold for approval.
Other studies have suggested that immunity from two doses of these vaccines wanes rapidly and that the protection may be limited, especially among older people and especially compared with mRNA vaccines used in the US. And against Omicron, all vaccines have been found to offer less protection.
Wagner thinks the difference between US and Chinese vaccines may be more of a wash when it comes to Omicron, but Lipkin says he has been telling colleagues in China for months that they need better protection.
"A way out of the surge would be to rapidly vaccinate everyone with more effective vaccines. That's not what they're doing," Lipkin said. "The notion that you can somehow conquer this with lockdowns is not going to work unless you've coupled the lockdown with an effective vaccine strategy.
"It's so disheartening and worrisome," he added. "I spend enormous amounts of time in China, and I've been advising them to switch vaccines for many months, and they just won't do it."
It matters who's vaccinated
The US has done a pretty good job of vaccinating the elderly, who are more vulnerable to severe disease and death, Lessler said. Almost 90% of people 65 and older have gotten two doses of the vaccine, according to the CDC, and 70% have gotten a booster.
"In China, my understanding is, the vaccine rates are sort of inverted somewhat, and the elderly have lower vaccination rates," he said.
With the most at-risk population unprotected in parts of China, Hong Kong has seen the world's highest death rate by population size this month.
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In the US this week, the Biden administration expanded eligibility for second boosters to people who are 50 and older, but one element that could limit this strategy is money. Congress failed to pass legislation that would fund Covid efforts. The uninsured may lose access to free treatments, tests and vaccines, and that could affect case numbers and deaths in the US.
With so many people getting sick globally, what also could change the direction of the pandemic in the US is another variant. If more people get sick, there's more of a chance that another strain could come along -- one that escapes the protection of current vaccines and treatments.
"A new variant could certainly come along in the future that will bring another wave, and we will be forced to respond," Lessler said. "That's why it still makes sense to get vaccinated and boosted."
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