KOCHI - Serum Institute of India has threatened to file a defamation suit against a participant who reported having adverse reactions after receiving a dose of the company's test Covid-19 vaccine during the clinical trial in India.

A 40-year-old trial participant was admitted in a Chennai hospital for 15 days in October after experiencing severe neurological reactions 10 days after getting a dose of Covishield, as the vaccine manufactured by the Pune-based Serum Institute is called.

The vaccine technology was developed by Oxford University and UK-based AstraZeneca, and Serum Institute is conducting a Phase II/III trial on 1,600 human volunteers in India. The privately-owned Indian company is the world's largest vaccine manufacturer.

The man, a marketing consultant, was diagnosed after multiple tests with acute encephalopathy, which alters brain function and causes personality change, memory loss, and declining ability to reason or concentrate. The patient's wife said he is now recovering at home but suffers from weakness and mild mood swings.

Dr Ramakrishnan said that after he reported the adverse event within 24 hours to the trial sponsors, they "unblinded the trial".

He admitted that the patient had not received a placebo, but the study vaccine, but maintained that illness was "unrelated to the vaccine" and that the local institutional ethics committee agreed with him in its initial assessment report.