Buffalo Bills player Damar Hamlin collapsed on the field and suffered a cardiac arrest moments after taking a hit to his chest during a tackle. While it’s not yet known why his heart stopped, some experts say his condition is most likely due to a heart rhythm problem as a result of that impact. Still, people on social media have baselessly speculated that it was caused by a COVID-19 vaccine.

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Damar Hamlin, a 24-year-old safety for the Buffalo Bills, collapsed during a Jan. 2 game against the Cincinnati Bengals after being hit in the chest during a tackle. 

While Hamlin initially stood up after the play, he almost immediately fell over, and millions watched on live television as medical personnel attempted to resuscitate him. The Bills said early the following morning in a statement that Hamlin had suffered a cardiac arrest, meaning his heart had stopped beating, after the hit, although his “heartbeat was restored on the field.” The team added that Hamlin was “sedated and listed in critical condition” at a hospital in Cincinnati. Early Tuesday afternoon the Bills said Hamlin remained in critical condition. The game was postponed.

While the cause of Hamlin’s cardiac arrest is not yet known, some physicians say the most likely cause is an arrhythmia that occurred as a result of the blow to his chest, known as commotio cordis.