China is at risk of a second wave of coronavirus outbreak as it gradually eases restrictions while handling a large number of arrivals from foreign countries, warned a study on Thursday.

Aggressive social distancing measures implemented in various provinces have helped China contain the spread of the virus outside central Hubei Province, the epicenter of the outbreak in the country. The efforts have helped in keeping the reproduction rate – a measure of the number of people sickened by an infected person – below one.

"While these control measures appear to have reduced the number of infections to very low levels, without herd immunity against COVID-19, cases could easily resurge as businesses, factory operations, and schools gradually resume and increase social mixing, particularly given the increasing risk of imported cases from overseas as COVID-19 continues to spread globally," said Professor Joseph T Wu from the University of Hong Kong who co-led the research.

Educational institutions have yet to resume their normal schedule, but commercial establishments have started operation in most parts of China. Businesses have been instructed to avoid overcrowding, maintain social distancing and promote hand washing.

The peer-reviewed study published in medical journal The Lancet, uses modeling to analyze data of confirmed cases between mid-January and February 29. Researchers calculated the transmission rate and severity of COVID-19 in four major cities – Beijing, Shanghai, Shenzhen, Wenzhou – and 10 provinces, which had a high number of coronavirus cases.

China has already imposed a series of measures, including a "health code" system, which gives users color-coded designations based on their health status with a QR code. A person with a green code means they had no exposure to the virus and they're allowed to travel; those with an orange code can visit only selected places.

Compulsory temperature scanning of customers at the entrance of restaurants, shopping malls and offices has been imposed strictly as a part of a bevy of precautionary measures. Mandatory quarantine is in place for people arriving from foreign countries.

However, researchers suggest these preventive measures have to continue until an effective antiviral drug or vaccine for COVID-19 becomes commercially available in the country.

"Although control policies such as physical distancing and behavioral change are likely to be maintained for some time, proactively striking a balance between resuming economic activities and keeping the reproductive number below one is likely to be the best strategy until effective vaccines become widely available," added Joseph.

The study also warned once the cases start rising, then "simply tightening control interventions again would not reduce the burden back to its original level."

It would require extra effort to drive the reproductive number below one in order to revert to the pre-relaxation levels – likely resulting in both higher health and economic loss, researchers warned. Real-time monitoring of the virus and continuous implementation of preventive measures could help maintain the flattened epidemiological graph, they suggested.

"We are acutely aware that as economic activity increases across China in the coming weeks, local or imported infection could lead to a resurgence of transmission," said Dr. Kathy Leung from the University of Hong Kong and co-lead author of the study.

"Real-time monitoring of the effect of increased mobility and social mixing on COVID-19 transmissibility could allow policymakers to fine-tune control measures to interrupt transmission and minimize the impact of a possible second wave of infections."

(Cover: Workers assemble cars at the Dongfeng Honda Automobile Co., Ltd factory in Wuhan, central China's Hubei Province, April 8, 2020. /AP)