Just four days after beginning a truncated coronavirus-delayed season, Major League Baseball ran into a serious obstacle on Monday with the postponement of scheduled games due to a COVID-19 outbreak among Miami Marlins players.

The postponement of the games in Philadelphia and Miami was a potentially ominous development for MLB and other major professional sports leagues in the United States and Canada hoping to forge ahead during the pandemic. The National Basketball Association and National Hockey League are set to resume play this week after a hiatus of more than four months, while National Football League training camps are opening.

While it was unclear whether the MLB season has been placed in jeopardy, some public health experts urged the suspension of play.

“They need to suspend games, do aggressive contact tracing, and see how bad this outbreak is,” Dr Ashish Jha, director of the Harvard Global Health Institute, wrote on Twitter. “I don’t know if MLB can resume the season. But, without aggressive action and vigilance, there is little hope we’ll see more baseball without more outbreaks.”

MLB Commissioner Rob Manfred told the MLB Network on Monday that the health of the players and their families was the league’s “first concern.”

“We’re making sure we do everything possible to minimize the spread of the virus among our employees,” he said, adding that the league has done tens of thousands of tests and the positive rate had been .4%.

“So we feel like the protocols have worked pretty well,” he said.

Manfred said he would only consider halting a team’s season if the club lost so many players that it was rendered “completely non-competitive.”

The United States leads the world in coronavirus cases and deaths, with worrisome infection rates in numerous states including the Marlins’ home state of Florida.

According to an ESPN report, at least 13 Marlins players have tested positive in recent days.

The Marlins opened their season on Friday in Philadelphia and were scheduled to return to Florida on Sunday for their scheduled Monday home opener against the Baltimore Orioles but put off traveling after the positive tests.

The team’s Tuesday afternoon game against the Orioles in Miami will also be postponed.

The Philadelphia Phillies were scheduled to host the New York Yankees on Monday but that game was postponed because the Marlins players had used the stadium.

MLB said in a statement the games were shelved while it conducts “additional COVID-19 testing,” with the Marlins self-quarantining in Philadelphia awaiting the results.

If the tests are negative, the team will play the Orioles on Wednesday in Baltimore, Manfred said.

Marlins CEO Derek Jeter said in a statement that the postponement “was the correct decision to ensure we take a collective pause and try to properly grasp the totality of this situation.”

Pandemic Havoc

The pandemic has roiled sports worldwide, delaying and interrupting seasons, forcing the cancellation and postponement of major events and pushing the Summer Olympics planned for Japan back a year amid questions about health risks for athletes and others. U.S. collegiate sports also are in flux.

MLB had planned to open its 162-game regular season in March but postponed it because of the pandemic, opting for a condensed 60-game schedule that began last Thursday in Washington.

“Remember when Manfred said players health was PARAMOUNT?!” Los Angeles Dodgers pitcher David Price, who opted out of the season due to the pandemic, asked on Twitter. “Part of the reason I’m at home right now is because players health wasn’t being put first. I can see that hasn’t changed.”

The NBA has assembled its teams in a restricted campus at Disney World in Florida. The NHL has brought its teams to two sites in Canada. But most MLB teams are playing in their usual stadiums. MLB, NBA and NHL games have no fans present.

“They are kind of at the razor’s edge at this point,” Bob Dorfman, a sports marketing expert at Baker Street Advertising in San Francisco, said of MLB. “The last thing they would consider doing is cancelling the season, and I think they will exhaust every other possibility, whatever that might be. There is just too much money on the line.”

The NHL is due to resume play on Saturday with an expanded 24-team playoff format in Edmonton and Toronto. The NHL said it registered no positive COVID-19 tests out of the 4,256 administered between July 18 and July 25. The NBA is scheduled to resume its season on Thursday.

With the NFL season scheduled to begin in September, Commissioner Roger Goodell said in an open letter to fans that teams must follow rigorous health and safety protocols, with “strict regulations” for isolating players who test positive.