“A potentially unexplained illness” in a recipient of AstraZeneca’s COVID-19 vaccine candidate AZD1222 has halted the company’s phase 3 clinical trial, the AP reports.

According to an AstraZeneca statement issued to StatNews, the company voluntarily stopped the trial to allow a review of safety data.

In a statement issued Tuesday evening, the company said its “standard review process triggered a pause to vaccination to allow review of safety data.” AstraZeneca didn’t reveal any information about the possible side effect except to call it “a potentially unexplained illness.” The health news site STAT first reported the pause in testing, saying the possible side effect occurred in the United Kingdom.

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