The game was scheduled to be played at Minute Maid Park in Houston. The A's member has not been identified.
Major League Baseball officials say the game was postponed out of an "abundance of caution and to allow for additional testing and contact tracing to be conducted."
The A's will self-isolate in Houston with recommended safety precautions in place.
The Astros beat the Athletics 6-3 on Saturday night for a doubleheader sweep. They won the first game 4-2. The doubleheader was scheduled after the A's and Astros decided not to play Friday night to raise awareness of racial inequality in the wake of the police shooting of 29-year-old Jacob Blake, a Black man, in Wisconsin.
COVID-19 testing has now postponed 38 games. All 30 major league teams played Saturday the same day for the first time since July 26.
The AP has contributed to this report.
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