AstraZeneca COVID-19 Vaccine Phase 3 Trials Halted After Suspected Adverse Reaction in UK Participant

AstraZeneca (Cambridgeshire, England) has halted a large Phase 3 study of its COVID-19 vaccine at dozens of sites across the US has due to a suspected serious adverse reaction in a participant.

AstraZeneca is conducting trials of its adenovirus vector-based COVID-19 vaccine candidate, AZD1222, developed by the University of Oxford (Oxford, UK). According to STAT News, a number of different reactions can qualify as suspected serious adverse reactions, symptoms that require hospitalization, life-threatening illness and even death. An individual familiar with the matter told STAT News that the nature of the adverse reaction and when it happened were not immediately known, although the participant was expected to recover. It was also not clear in which clinical trial the adverse reaction occurred, although it appears to have taken place in the Phase 2/3 trial underway in the UK.


An AstraZeneca spokesperson told STAT News that the company’s “standard review process triggered a pause to vaccination to allow review of safety data.” The spokesperson termed the pause as “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 is “working to expedite the review of the single event to minimize any potential impact on the trial timeline,” according to the spokesperson.

An individual familiar with the development told STAT News that a hold was placed on the trial out of “an abundance of caution.” Another individual familiar with the matter told STAT News that the finding was impacting other AstraZeneca vaccine trials underway, as well as the clinical trials conducted by other vaccine manufacturers.

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