Korea recorded the sharpest rise of new Covid-19 cases in more than a week on Wednesday, as infections related to nightclubs rebounded and outbreaks occurred among nurses at one of the nation’s largest hospitals.

Seventy-five high schools in Incheon and surrounding Gyeonggi Province sent students home on the first day of reopening schools out of coronavirus concerns as some of the students were learned to have visited Itaewon and other entertainment quarters during the holiday. Two students showed a positive response to virus tests.

The nation reported 32 new virus cases as of midnight Tuesday, following the daily tallies of below 20 in the previous nine days.

Among the 32, 24 were local infections with the other eight being new arrivals. The accumulated total rose to 11,110, the Korea Centers for Disease Control and Prevention said. The nation saw zero death on Tuesday, keeping the death toll at 263. With 128 newly cured people, 10,066 patients were fully recovered from the virus and left quarantine. The country, with a population of 50 million, has carried out 776,433 tests since Jan. 3, including 10,859 the previous day.

The nation switched to an “everyday life quarantine” system on May 6 to allow citizens to conduct social and economic activities while following eased quarantine rules.

Seoul Mayor Park Won-soon said situations involving the four nurses confirmed of Covid-19 at Samsung Medical Center (SMC) are different from those of the Middle East Respiratory Syndrome in 2015. At a radio talk show, Park said that cohort isolation of the hospital is unnecessary at the moment as the infection can be controlled. “Five years ago, the secrecy and lack of communication aggravated the MERS outbreak, but the situation is different now,” he said.

The Seoul mayor, one of the presidential contenders in the next election, also emphasized that the city has come to relatively contain the spread from bars and clubs in Itaewon, the multicultural nightlife district in the capital city. “In this case, the infection route of the first confirmed person was unclear, but the thorough inspection slowed down the spread. The City Hall can prevent further infections, as the SMC cases have a limited range of spread compared to those of Itaewon clubs,” he added.

Despite Park’s show of confidence, the recently subdued infections from the Itaewon cluster are unlikely to disappear soon as they spread to hospitals and schools, epidemiological experts said.

The public health authorities are concerned because nurses have the most frequent contact with patients and other medical staff. Due to the closed nature of the hospital, the virus is easy to spread, even with only one infected person. According to a public health center in Seosan, South Chungcheong Province, a 27-year-old nurse who works at a branch of the National Health Insurance Service (NHIS) in the city was also confirmed of the coronavirus on Wednesday.

The newly confirmed nurse is known to be a friend of the confirmed SMC nurse and stayed at the latter’s house from May 9-10.

The Ministry of Education is also on high alert as the number of Covid-19 patients suddenly increased after the high schools reopened for the first time in 80 days for 12th graders.

The ministry decided to close some of the schools again and sent students back to their homes, believing there is a high risk of clustered infections with some students showing Covid-19 symptoms. Ministry officials said they were looking closely at the situation together with the KCDC.