AstraZeneca COVID-19 vaccine trial paused

AstraZeneca's COVID-19 vaccine trial has been put on pause after researchers suspected an adverse reaction in a participant in the United Kingdom, a spokesperson for the company told Stat News.

Why it matters: There are presently nine vaccine candidates in Phase 3 trials. AstraZeneca’s is one of the most promising candidates, and is the first known Phase 3 vaccine trial to be halted.

Of note: It is unclear what the unfavorable reaction was and when it occurred, but the participant is expected to recover. It's also not known who paused the study, how severe or rare the reaction may be and how long the hiatus could last.

• A variety of responses may be flagged for possible adverse reactions, including hospitalization, life-threatening illness and death.

What they're saying: The halt is “a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials, while it is investigated, ensuring we maintain the integrity of the trials," AstraZeneca said in a statement per Stat News.

"In large trials illnesses will happen by chance but must be independently reviewed to check this carefully. We are working to expedite the review of the single event to minimize any potential impact on the trial timeline."

The big picture: AstraZeneca started its Phase 3 trial in the U.S. in late August. The process is taking place across 62 sites throughout the country.

• Phase 2 and 3 trials previously began in the U.K., Brazil and South Africa.