As the United States and Europe reel from their first waves of the novel coronavirus, many Asian nations anxiously look ahead to the not-so-distant future for signs of a second surge.

The governments of China, South Korea, Hong Kong, Taiwan and Singapore successfully flattened their first infection curves since the start of 2020 through tough measures both praised and criticized — from the unprecedented multi-million person lockdown in China to the quick "test, trace, treat" method of South Korea, the reignition of the SARS-born National Health Command Center in Taiwan and the quick shutdown of Singapore's borders.

With domestic cases mostly tamped down, imported infections are now the biggest danger with citizens or permanent residents returning to their home countries — and serving as viral Trojan horses. Infections brought home from abroad account for much of the recent uptick in Asia's new cases. Health experts are calling for vigilance, encouraging the public to fight coronavirus fatigue.

"I am nervous," Dr. Jerome Kim, director general of the International Vaccine Institute in Seoul, South Korea, told CBS News Asia correspondent Ramy Inocencio. "What I hope is that what we see instead are little blips, so it's like putting out a fire. You know that there are going to be embers that are glowing or smoking. You want to stamp those out before the fire starts again."

Propelled by imported infections, China's officially reported daily coronavirus cases hit a six-week high on Monday. The National Health Commission reported 108 new infections with 98 originating from beyond the country's borders. On Tuesday, China reported 89 new cases — just three were domestic.

One of China's newest battlegrounds against coronavirus is the northeastern province of Heilongjiang and the 80,000-person city of Suifenhe on the Russian border. Dozens of the country's newest cases returned through the remote, frontier crossing. The border is now closed to travelers, Suifenhe is on lockdown and a new 600-bed hospital for the expected infected is near completion.

Meanwhile, in China's central Henan province, reinfection is the worry. In one county, 600,000 people are now on lockdown after a doctor returning from Wuhan tested positive. Despite 14 days of self-isolation, she reportedly passed the virus to colleagues and a classmate. Similar to Suifenhe, all businesses in Jia county are closed except for essential services. Social media images show blocked roads and empty streets, echoing the 76-day lockdown of Wuhan which was lifted less than a week ago.

In South Korea, the Korean Centers for Disease Control is looking into viral reactivation: more than 100 people who fully recovered from coronavirus have since tested positive again. Similar cases have been found in other countries, including China, Kim said. However, he cautioned about jumping to conclusions, stressing tests need to be conducted to confirm if the coronavirus found a way to hide in a "reservoir" inside the infected or if the issue is linked to testing sensitivities.

In Japan, the northern island of Hokkaido and the country's biggest prefecture by area redeclared a state of emergency after lifting its initial declaration because of double-digit infections each day over nearly the past week.

"We are facing a crisis of a second wave in the spread of infections," said Hokkaido Governor Naomichi Suzuki.

Japan's capital, Tokyo, the country's third-largest city, Osaka, and several prefectures are under their own states of emergency with residents encouraged to stay home. Tokyo Governor Yuriko Koike has told all of the city restaurants to close by 8 p.m. Japan's coronavirus infections have topped 7,500 confirmed cases, three times as many as April 1, after an initial lag behind other Asian countries.

"I don't think the virus has been eliminated," Dr. Ben Cowling at the University of Hong Kong School of Public Health told CBS News on March 20. At that time he already warned if measures were relaxed, countries might see a resurgence in infections.

"One of my concerns is that we're going to get into a nasty cycle of having to have repeated lockdowns maybe every two or three months," he said.