Asian countries risk new waves of coronavirus infections when they lift lockdowns. The same could happen in the rest of the world.

Travelers are bringing the virus back into Asian countries that have controlled domestic spread, leading to new restrictions in Hong Kong, Singapore, and Taiwan.
To control new waves of infections, countries and cities may need to impose more lockdowns in the future.
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The two-month coronavirus lockdown in Wuhan, China is set to end on April 8 , but the city could see yet another devastating wave of infections after that.

Italy has imposed similar measures its outbreak is now the largest outside China, and its death toll is far higher than China's. Other countries have followed suit, too.

Since Wuhan's historic shutdown began on January 23, China has seen a sharp drop-off in its rate of new cases. The country reported no new local infections for the first time on March 19. On Wednesday, it began allowing residents of the Hubei province outside the region's capital city of Wuhan to start leaving if deemed risk-free. Elsewhere in China, classes are resuming and businesses are reopening.

But some experts fear that lifting those restrictions could start the whole process over again.

Lockdowns merely delay the outbreak's peak by about three months, Dr. Ben Cowling, an epidemiologist at the University of Hong Kong who researches influenza transmission and control measures, told Business Insider.

"What happened in Wuhan and now what's happened in north Italy is not the peak of an epidemic. That's about a month away from the peak," he said. "They are still facing now, most likely, a second wave in one to two months' time. So are they going to shut down again?"

Travelers and asymptomatic people spread new waves of infection

hong kong airport coronavirus protective hazmat suit Tyrone Siu/Reuters

There are two main ways the virus can make a resurgence as residents emerge from their homes, return to work, take their children to school, and go shopping.

First, a small number of residents who were under lockdown could still have the virus when restrictions lift but not know they're sick. Those people could then spread it, starting a new wave of infections.

Second, international travelers could bring the virus back into the country.

The latter already seems to have led to a surge of new infections in Hong Kong.

Hong Kong's outbreak consisted of just 100 cases at the beginning of March; the city had implemented widespread social distancing, work-from-home rules, public-information campaigns, and high-tech case mapping. On March 2, civil servants went back to their offices. Two weeks later, the city reported a jump to 160 COVID-19 cases .

Then last week, as residents who had been abroad began returning home, Hong Kong's cases more than doubled. As of March 25, it has reported 410 cases of the new coronavirus.

"This is a pattern playing out across parts of Asia mainland China, Singapore, Taiwan that were among the first to tackle the outbreak," CNN analyst James Griffith wrote Monday. "All are now introducing new restrictions as a sudden wave of renewed cases begins to crest."

singapore coronavirus Getty Images

Hong Kong Chief Executive Carrie Lam said the city has already weathered its first two waves of the virus.

"The first wave was the worries of transmissions from mainland (China), so we have put in a lot of measures," Lam said Saturday, according to CNN. "The second wave was the local transmissions, with those clusters arising from dinners and other things. Now we are facing the third wave."

Hong Kong's newest restriction, announced Monday, is that non-residents are not allowed to enter the territory. The city also sent civil servants back home and implemented testing requirements for anyone entering the city. Lam also requested that bars and restaurants stop selling alcohol.

Singapore and Taiwan announced their own new restrictions to curb imported cases over the weekend.

New waves could lead to multiple shutdowns

Wuhan Getty Images

Almost all new coronavirus cases in mainland China are now coming from people traveling from abroad, including Chinese students returning home .

"Those [cases] are now seeding the second wave," Cowling said. "They need to decide whether to do another shutdown."

Preventing the importation of new cases will be key to containing the next wave.

A study published in the journal Science found that travel restrictions can be effective once a country or territory has controlled the virus' spread within its own communities.

"Chinese provinces and other countries that have successfully halted internal transmission of COVID-19 need to consider carefully how they will manage reinstating travel and mobility to avoid the reintroduction and spread of the disease in their populations," Moritz Kraemer, a lead author on the study, said in a statement.

wuhan coronavirus quarantine lockdown hubei China Daily via Reuters

Chinese scientists and health experts have downplayed the threat of another domestic wave of coronavirus infections, however.

"For me, a second outbreak (of coronavirus), a domestic outbreak in China, wouldn't be a great concern," Cao Wei, deputy director of the Department of Infectious Diseases at the Peking Union Medical College Hospital, told Reuters on March 18.

But experts have pointed out that the 1918 flu pandemic killed more than 50 million people in three waves. The second was the deadliest.

Once COVID-19 cases begin to rise again in any area, authorities would most likely have to mandate social distancing again a second lockdown.

"What happened in Wuhan could happen repeatedly to a city," Cowling said.

Holly Secon and Bill Bostock contributed reporting.

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