HONG KONG - On the first day of unimpeded travel between mainland China and Hong Kong, Olivia Gai was one of the first in line.
Crossing the border from the Chinese territory of Hong Kong to the mainland had been a hassle since the start of the Covid-19 pandemic, requiring weeks of quarantine. But this time, it was smooth and swift, said Gai, 27, who works in wealth management.
As she entered the mainland city of Shenzhen, "I saw two extremely festive display boards and the staff said I could write a wish and put it on - so many people were writing and taking pictures," Gai said via Xiaohongshu, China's version of Instagram. "I was about to cry at that moment."
China reopened to the world Sunday after three years of isolation, ending quarantine requirements for inbound travelers and issuing its citizens passports for tourism again. The hurried lifting of "zero-Covid" restrictions after mass protests has accelerated what appears to be China's worst outbreak of the pandemic, leading the United States and many other governments to impose restrictions on travelers from the world's second-largest economy.
The U.S., the World Health Organization and others have said China is not being transparent about the extent of its outbreak, and that the lack of information raises the risk of a new variant of concern emerging undetected.
China, in turn, has criticized the new testing and other requirements as unscientific and discriminatory, and threatened to take countermeasures. On Tuesday, the Chinese embassies in South Korea and Japan said they would stop issuing short-term visas for travelers to China.
"We once again call on relevant countries to make sure that their Covid response measures are fact-based, science-based and proportionate," Foreign Ministry spokesperson Wang Wenbin said at a regular news briefing Tuesday. China itself still requires all arriving passengers to have tested negative for the virus within 48 hours of departure from their points of origin.
Social media users in China had expressed anger over the strict border measures in South Korea, sharing videos of travelers from China wearing yellow cards around their necks and being escorted for testing on arrival by South Korean soldiers in uniform. The Korea Disease Control and Prevention Agency told NBC News that the soldiers were asked to assist in the rush to implement the new arrival procedures and that trained quarantine staff would help travelers from China find the testing area starting Wednesday.
The South Korean Foreign Ministry says its restrictions on travelers from China are based on science. Japan said Wednesday that it had lodged a protest to China over the suspension of visas.
The U.S. and other countries imposing restrictions have cited a lack of data from China, which is a "black box" when it comes to its current Covid outbreak, said Ho-fung Hung, an associate professor of sociology at Johns Hopkins University.
The country, which narrowly defines Covid-related deaths, has reported only about three dozen in the last month, a number wildly out of sync with anecdotal reports and social media images of overwhelmed crematoriums and funeral homes. According to some estimates, China's Covid death toll could reach 1 million or more in the coming months.
The government's low official case numbers are also at odds with local reports. A spokesperson for the provincial government in Henan said Monday that as of Jan. 6, 89% of the province's 99 million people had been infected. The number of patients visiting "fever clinics" for treatment has declined continuously since peaking Dec. 19, the spokesperson said.
Experts including Hung say it's possible that the Chinese government itself doesn't have exact numbers, especially since mass testing programs have ended and many people with mild infections are recovering at home without reporting them.
"It's challenging to collect these data with accuracy in the context of an acute epidemic," said Karen Grépin, an associate professor in the School of Public Health at the University of Hong Kong.
Chinese officials reject claims that they have not shared information transparently. They say the outbreak is predictable and under control, and that life is returning to normal in cities and provinces where cases have already peaked.
Nonetheless, more countries have begun requiring negative Covid tests for travelers from China, and at least one country, Morocco, has banned arrivals entirely.
Public health experts have questioned the effectiveness of such restrictions, which many countries also introduced after the omicron variant emerged in late 2021.
Those restrictions "had basically no impact whatsoever in terms of slowing the spread of the omicron variant globally," Grépin said.
Foreign governments' actions may be driven in part by memories of China's lack of transparency about the virus at the start of the pandemic, said Hung, who called pre-flight testing requirements "cautious and reasonable."
"It's just like in January 2020, when the world started to notice something serious is going on in Wuhan, in China, and the Chinese government is in denial and saying everything's under control," he said.
In Hong Kong, a recent poll by the Hong Kong Public Opinion Research Institute found that respondents were split on the idea of reopening the mainland border. While many Hong Kong residents welcome quarantine-free travel after years of being separated from their families and businesses on the mainland, concerns about an influx of visitors who may have the virus have sent people flocking to vaccination centers that were almost empty a few weeks ago.
"The mainland says that only a few thousand people are diagnosed a day, but this is not the case," Eileen Cheung, 59, a retiree, said Tuesday while waiting in line to be inoculated.
GISAID, a public database based in Munich, said Tuesday that analysis of the latest genomic sequencing data from China showed that the variants in its outbreak "closely resemble" those already circulating around the world.
Countries in Southeast Asia, which are eager for the return of high-spending Chinese tourists, have largely refrained from imposing new restrictions. Chinese tourists arriving at the airport in Bangkok on Monday were welcomed with garlands by three Cabinet ministers.
The health minister of Singapore, a country experts have praised for its pandemic response, said Monday that pre-departure testing would not be required for travelers from China, because severe cases "can originate from any country, any region in the world." Other exceptions include South Africa and New Zealand, whose health ministers have said that entry restrictions on travelers from China are unnecessary.
The virus is expected to spread further in China in the coming weeks as families gather for the Lunar New Year, which before the pandemic was known as the world's largest annual human migration. The Chinese Transport Ministry said last week that it expected more than 2 billion passenger trips during the 40-day holiday period, double the number last year and 70% of what it was in 2019.
Experts say holiday travel will bring the virus to rural areas that have so far avoided the major outbreaks seen in China's metropolises.
Even as China's outbreak stirs fears abroad, Chinese social media has been awash with anxiety over XBB.1.5, a highly immune-evasive omicron subvariant that is becoming dominant in the U.S. but has not been reported domestically in China.
One commenter on Weibo, a social media platform similar to Twitter, demanded action from the WHO: "Why don't they recommend for all Americans to do Covid tests?"
This article was originally published on NBCNews.com