Health authorities have tightened COVID-19 restrictions in Shenzhen, Guangdong province, after the city detected new cases.

The health codes for two groups of people were changed from green to yellow, officials said on Sunday at a news conference. One group includes people who were at the Shenzhen Bao'an International Airport for more than an hour from June 10 to 18. The second group comprises people who had visited the same places as the confirmed cases and their contacts.

In the coronavirus fight, a green code means that the person is healthy, and yellow or red indicates a risk of infection.

People with yellow health codes can go to the nearest testing site to get a nucleic acid test. If the result is negative, the code will turn green. To date, 55 test sites have been set up in the city for yellow health code individuals, the local health commission said.

Shenzhen has one airport, 13 railway stations, and 30 bus stations. Under the latest epidemic rules, people with red and yellow codes cannot enter these places.

In addition to green codes, passengers since June 20 are required to present a negative nucleic acid test result obtained within 48 hours to leave Guangdong via Shenzhen.

Those who reach Shenzhen by air with a yellow health code must get a nucleic acid test at the airport.

On June 10, Flight CA868 arrived in Shenzhen from South Africa. As of Saturday, 38 passengers on the flight were found to have been infected with the coronavirus.

The municipal center for disease control and prevention completed the genome sequencing of some patients. Results showed that the virus in 19 of the cases has the same origin as the Delta variant, a COVID-19 strain first found in India.

On June 14 and 18, a 30-year-old male customs officer and a 21-year-old waitress were identified. Both worked at the Shenzhen airport and were infected with the Delta variant. On June 18, a 35-year-old man was confirmed with the Delta variant as well. He had paid a visit to the airport on June 15.

All stores in the passenger terminal areas have been closed.