Health authorities are updating their screening protocols after a series of novel coronavirus cases here revealed that infection can occur without symptoms, and in patients who have not visited high-risk areas of China.

South Korea confirmed four additional cases of the virus Thursday morning, taking the national tally to 23.

Three of the four new patients were Koreans with no recent China travel history, and were infected through person-to-person contact. The other patient was a Chinese tourist from Hubei who arrived Jan. 23.

This means 12 of the 23 cases so far involved domestic transmission.

Authorities said the 22nd patient currently exhibits no obvious symptoms, having tested positive while placed under home quarantine after his sister was diagnosed Tuesday.

The Korea Centers for Disease Control and Prevention said Thursday that its fifth revision of the response guidelines will come into effect at 9 a.m. Friday, closing the loopholes. 

Until now, only symptomatic individuals who had visited Wuhan -- the central Chinese city where the outbreak originated -- within the past 14 days were subject to testing.

The latest guidelines expand the definition of a suspected patient to include those who have visited countries outside China with reported cases of the disease. Patients with medical certificates attesting to potential infection will also be eligible for testing, even if they have not been to Wuhan.

According to the KCDC on Wednesday, four confirmed patients were denied testing despite complaints of symptoms because they did not meet the existing diagnostic criteria. State health centers as well as the KCDC hotline 1339 failed to guide them to test for infections.

The disease control centers’ director, Jung Eun-kyeong, said Thursday that the possibility of local transmission could not be ruled out, and that caution was warranted even when visiting countries other than China.

“Research indicates risks of further community spread,” she said. “There may arise cases where routes of contraction are uncertain and unpredictable.”

The director also urged those planning visits to regions with confirmed cases to stay away from crowds and observe basic hygiene practices such as hand-washing, in line with expert recommendations.

By Kim Arin (arin@heraldcorp.com)