AstraZeneca has reportedly stopped its global Phase 3 clinical trial of its investigational Covid-19 vaccine AZD1222, following an unexplained illness observed in a participant.

The UK-based drugmaker said that it has voluntarily paused the clinical trials to ensure the review of safety data by an independent committee, and is working to speed up the review to mitigate the possible impact on the trial timeline.

According to health-oriented news agency Stat, the suspension is due to a suspected serious adverse reaction, and the participant is expected to recover.

The New York Times reported that a participant in the UK has developed transverse myelitis, an inflammatory syndrome affecting the spinal cord. According to the report, the illness observed in the participant is not clearly linked to AstraZeneca’s vaccine candidate. The company has refused to comment on the report.

AstraZeneca spokesperson said: “This 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. The company is working to expedite the review of the single event to minimize any potential impact on the trial timeline.”

AstraZeneca is conducting AZD1222 clinical trials in the UK, Brazil and South Africa

AZD1222 was developed by the University of Oxford and its spin-out company, Vaccitech. The vaccine leverages a replication-deficient chimpanzee viral vector to deliver the genetic material of the SARS-CoV-2 virus spike protein, and induce an immune response against the coronavirus.

AstraZeneca has undertaken the AZD1222 development programme across the world, with late-stage clinical trials being conducted in the UK, Brazil and South Africa.

The Biomedical Advanced Development Authority (BARDA), and the National Institute of Allergy and Infectious Diseases (NIAID) are providing funding support to conduct the clinical trials.

The company has recently expanded the AZD1222 clinical development programme in the US, by commencing a Phase 3 clinical trial, dubbed D8110C00001, designed to evaluate the safety, efficacy and immunogenicity of the vaccine candidate in 30,000 participants, aged 18 years or above.