Last summer, as Major League Baseball reeled from coronavirus outbreaks and canceled games, its analog in South Korea became a poster child of how to successfully hold spectator sports in a pandemic.

Now the roles are reversed. As M.L.B. prepared to hold its All-Star Game, canceled last year, in front of a capacity crowd in Denver, the Korea Baseball Organization suspended its season on Monday after five players tested positive for the coronavirus.

The news from South Korea is another sign of backsliding in the Asia Pacific region, where vaccination rates are relatively low, the Delta variant is surging and several countries are reimposing lockdowns.

A year ago, American baseball fans and broadcasters looked to Asia, where the virus was largely under control, to fill a programming void. As dozens of M.L.B. players and staff members were infected and its season delayed by nearly four months,

K.B.O. opened its season last May, with live coverage on ESPN, cardboard cutouts in the stands and actual cheerleaders dancing to K-pop anthems. Pro baseball leagues in Taiwan and Japan also started their seasons.

K.B.O. teams played most of last season in empty stadiums, but this year they have been allowed to welcome back some fans. In and around Seoul, the percentage of fans allowed back started at 10 percent and rose to 30 percent last month.

As late as November, no major outbreaks had been traced to K.B.O. stadiums in South Korea, the league’s secretary general, Ryu Dae Hwan, told Bloomberg News at the time. But the league would be vulnerable, he warned, if the national caseload rose.

Now, cases in South Korea have surged to their highest levels of the pandemic. On Tuesday, the country recorded more than 1,000 new infections for the seventh straight day. More than 40 percent of them were in the Seoul area, and health officials estimated that about a quarter were driven by the Delta variant.

Health officials have warned that, by mid-August, the daily infection rate could double from its current level. The rate — 2.1 infections per 100,000 people — is about a third of what the United States is recording, according to a New York Times database. But because only about 11 percent of South Korea’s population has been fully vaccinated, compared with 48 percent in the U.S., people there are more vulnerable to severe infection and hospitalization.

The rise in cases has prompted a scramble for shots. On Tuesday, the Korea Disease Control and Prevention Agency said that it had paused vaccination appointments for more than 1.6 million people ages 55 to 59 for one week because available doses had been exhausted, Reuters reported.

After calling off two games last week, K.B.O. suspended its season indefinitely on Monday, just as the government tightened movement and gathering restrictions in Seoul and nearby regions to their severest levels yet. The season will be on hold until at least Aug. 9. Only a week of games has been officially canceled so far because the league had always planned to take a three-week break for the Summer Olympics.

Fans in South Korea expressed exasperation with the league’s decision. Others criticized the two teams that reported the infections, the NC Dinos of Changwon and the Doosan Bears of Seoul. (The former beat the latter last fall, live on ESPN, to win last year’s K.B.O. championship.)

The league has a 44-page manual of Covid protocols, including requirements that players must sanitize their hands before and after taking the field or entering a locker room. They are also advised to sit in a zigzag pattern if they eat at restaurants as a way of limiting airborne transmission of the virus.

In a statement, the Doosan Bears apologized to its fans and league officials for the cases among its players, and promised to be “even more thorough in the future.”