As thousands of people get ready to flock to Wrigley Field on Thursday for Chicago’s largest mass gathering in more than a year, city officials warned that they may again shut the venerated ballpark to fans if the number of COVID-19 cases keep climbing.

The warning from the city’s Office of Emergency Management and Communications also applies to the White Sox’s ballpark, as well as bars, restaurants and other businesses, and comes amid an increase in the number of cases in Chicago and Illinois, particularly among young adults. Just this week, state public health officials announced that the lifting of some restrictions was being delayed because of increasing numbers of COVID-19 infections and hospitalizations since mid-March.

Both the Cubs and the White Sox will be allowed to admit as much as 25% percent capacity. For Wrigley Field, that means a maximum of a little more than 10,000 fans in the stands. Many more are expected to watch the game from nearby bars and restaurants that are routinely crowded with fans during home games. Those establishments are limited to 50% capacity, and customers must wear masks, just like at Wrigley.

City officials already have said they are on the lookout for whether bars and restaurants are complying with restrictions. OEMC warned Wednesday that “everyone needs to abide by public health guidance, including wearing masks in public even if you’ve been vaccinated.”

A few hours after the news release, the Cook County Medical Examiner’s Office announced that the county that includes Chicago had recorded its 10,000th COVID-19 death.

Of particular concern, said Chicago Mayor Lori Lightfoot, are the North Side neighborhoods near Wrigley Field.

At Wrigley and across town at Guaranteed Rate Field — the South Side home of the White Sox — cash won’t be accepted at concession stands or in souvenir shops. Also, everyone from players to fans “can expect comprehensive screening and sanitization procedures based on the latest scientific guidance,” according to the city’s news release.

The food menu at Wrigley will be limited to packaged items. Wrigley and Guaranteed Rate Field, where the White Sox’s home opener is scheduled for April 8, will implement pod seating to keep groups of fans separated.