Red Sox players take batting practice Thursday at Yankee Stadium following the announcement that their game against the Yankees was postponed because of positive COVID-19 tests among New York players. Frank Franklin II/Associated Press

NEW YORK — The post-All-Star break opener between the Boston Red Sox and New York Yankees on Thursday night was postponed because of positive COVID-19 tests among New York vaccinated pitchers Jonathan Loaisiga, Nestor Cortes Jr. and Wandy Peralta.

“It’s a fluid situation that could spread. It has spread to some degree,” Yankees General Manager Brian Cashman said. “We have three positives and we have three pending that we’ve had rapid tests on. … We’ll wait now for the lab tests to come back, which I’m assuming is going to be positive, as well. So that would increase our number to six, but we’re not at six yet. We’re at three confirmed.”

Loaisiga went on the COVID-19 injured list Saturday, when the Yankees were in Houston, and he did not travel home with the team Sunday. Cortes and Peralta went on the COVID-19 IL on Thursday.

Cashman said all three were fully vaccinated, as are most of the players on the team. Among the three, two received the Johnson & Johnson vaccine, and the other was either Pfizer or Moderna, according to Cashman.

“Those players are doing well thus far,” he said. “And that would speak again to the belief that those vaccinations are working and ultimately they’re to protect us from severe illness and/or death.”

The three players awaiting lab results are in quarantine. Cashman would not say whether they include the Yankees’ All-Stars who were in Denver: Aaron Judge, Gerrit Cole and Aroldis Chapman.

MLB was conducting contact tracing under its protocols.

“The vaccines that we encourage everybody to get guarantee not getting hospitalized and not getting death coming from COVID, which is important, but it doesn’t prevent you from contracting COVID,” Cashman said. “It just obviously protects you from the severe worst-case scenario effects from COVID.”

Cashman said Major League Baseball had not yet decided whether to postpone Friday’s game.

“The last year, year and a half, has kind of in some ways kind of prepared you for this kind of stuff,” Yankees Manager Aaron Boone said. “Certainly disappointing and frustrating.”

New York was among the first major league teams to reach the 85% vaccination threshold to lessen coronavirus protocols such as dropping mask use in dugouts and bullpens.

Despite all those vaccinations, the Yankees had more than a half-dozen positive COVID tests in May, and third-base coach Phil Nevin became seriously ill with a kidney infection that kept him away from the team for more than three weeks.

New York players were on the field taking early batting practice about 3 1/2 hours before Thursday’s scheduled start when the Yankees asked media to leave the field while the team conducted COVID testing.