Shanghai's Center for Disease Control and Prevention has asked citizens to keep calm, avoid moving around and get ready to answer a phone call if their health code turns red.

Amid the new round of COVID-19 resurgence in Shanghai during the National Day holiday, many citizens received a short message of risk warning from the COVID-19 prevention and control authority, or found their health code had turned red for an unknown reason.

Wu Huanyu, deputy director of the Shanghai Center for Disease Control and Prevention, explained the measures aim to curb the spread of the coronavirus more swiftly.

"The city's COVID-19 prevention system is always being activated due to the more frequent movement of people during the holiday," said Wu.

Once a positive case is detected, the epidemiological investigator will work along with the police, telecommunication carriers and big data center to trace the at risk groups immediately and send them short messages.

People receiving the message must follow the orders from the authority to receive an immediate PCR test and take precautionary measures to prevent the spread of the virus, Wu said.

Some Shanghai citizens have found their health code turned red for an unknown reason during the recent COVID-19 resurgence.

Five groups of people will be rated with a red health code in Shanghai.

They include those who test positive in an initial or mixed tube PCR test, the confirmed, asymptomatic and suspected cases, as well as travelers from overseas who are under central or home quarantine.

The close contacts and secondary contacts of the positive cases, along with people who had been to high or medium-risk areas will also have a red health code, Wu said.

People should relax, avoid moving around, take self-protection measures and keep a distance from others if their health code has turned red.

They are also required to make sure their phone is available for calls and cooperate with the follow-up quarantine and control measures or PCR test, he added.

They can dial the city's service hotline 12345 or make an online appeal on the Suishenban app, where they can check the reason for the red health code.