A second Covid-19 vaccine trial has been called off over unexplained illness caused by the vaccine.

A study conducted by Johnson & Johnson’s COVID-19 vaccine candidate has been paused while the company investigates whether a study participant’s “unexplained illness” is related to the shot.

According to the statement released by the company on  Monday evening, the illnesses, accidents and other so-called adverse events “are an expected part of any clinical study, especially large studies,” but that its physicians and a safety monitoring panel would try to determine what might have caused the illness.

The pause is at least the second such hold to occur among several vaccines that have reached large-scale final tests in the United States (US).

The company declined to reveal any more details about the illness, citing the participant’s privacy.