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Federal health officials on Friday said U.S. trials of the COVID-19 vaccine from AstraZeneca and Oxford University can continue after they were halted last month because two trial participants developed neurological symptoms, according to The Wall Street Journal.

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Citing an unidentified source, the Journal reported that the U.S. Food and Drug Administration has finished examining two cases in which vaccine trial participants developed a possible neurological side effect while taking AZD1222. Officials could not determine whether the vaccine caused the neurological issues, according to the newspaper.

The FDA plans to require that researchers tell trial participants about the cases in which study participants developed neurological issues and monitor them for any signs of related events, the Journal reported.

AstraZeneca paused its Phase 3 vaccine trials last month after a person in the U.K. fell ill without explanation. At the time, officials with the pharmaceutical company noted that illnesses sometimes happen by chance in large clinical trials and that such a pause is routine “whenever there is a potentially unexplained illness.”

>> Related: Johnson & Johnson temporarily halts late-stage COVID-19 trial after participant falls ill

In a private call with investors last month, AstraZeneca’s CEO, Pascal Soriot, said that a woman developed symptoms which appeared to be consistent with transverse myelitis, STAT News reported. Symptoms of the disorder, which is caused by an inflammation of the spinal cord, typically include weakness in the arms or legs, sensory symptoms like numbness or tingling, pain and bladder or bowel dysfunction, according to Johns Hopkins University.

The pause was not the first for clinical trials of AZD1222. Soriot said trials were paused in July after a person developed neurological symptoms but that they resumed after the person was diagnosed with unrelated multiple sclerosis, according to STAT News.

Officials with AstraZeneca began Phase 3 clinical trials of its vaccine candidate in the U.S. in August. The trial was expected to include as many as 30,000 adults from various racial, ethnic and geographic groups.

>> Related: Coronavirus: Pfizer vaccine could be distributed by year’s end, CEO says

Health officials in the U.S. have said they expect to see a COVID-19 vaccine by the end of the year.

The United States leads the world with the most coronavirus cases and the highest death toll. Since the start of the pandemic, officials have confirmed more than 8.4 million infections and reported more than 223,000 deaths nationwide, according to numbers compiled by Johns Hopkins.

As of Friday, more than 41.9 million COVID-19 cases have been reported worldwide and more than 1.1 million people have died of the viral infection, according to Johns Hopkins.