A vial of a COVID-19 candidate vaccine
© John Cairns / University of Oxford / AFPA vial of a COVID-19 candidate vaccine, known as AZD1222, co-invented by the University of Oxford and Vaccitech in partnership with pharmaceutical giant AstraZeneca.
A participant in a COVID-19 vaccine trial has been sued for $13.5 million for defamation.

The world's largest vaccine producer, Serum Institute of India, has sued a COVID-19 vaccine trial participant who alleged that the trial caused him "serious side effects".

In filing a INR 1 billion ($13.5 million) defamation case, the firm rejected the charges by the 40-year-old businessman, from the southern Indian city of Chennai, as "malicious". The firm has also said the man's illness had nothing to do with the vaccine trials.

"The volunteer is falsely laying the blame for his medical problems on the COVID vaccine trial," said a statement issued by the Serum Institute, the world's biggest vaccine manufacturer by the number of doses produced and sold globally.

The lawyers of the vaccine volunteer have called the defamation suit an "intimidation attempt."

The participant alleged that undergoing the trials caused a "neurological and psychological" breakdown that impaired his cognitive abilities.

"Our client states that the severe trauma he went through from 11th October 2020, because of the 'Acute Neuro Encephalopathy' that he suffered, is an extreme side-effect of the test vaccine that he took on 1st October 2020," said the anonymous participant's legal representative in a notice. The man is demanding a compensation of INR 50 million ($676,000) from the Serum Institute, and has asked for the trials to be halted immediately.

According to the wife of the businessman, he "lost the ability to do simple tasks" such as making online payments, as well as lost his "creativity and confidence".

Serum Institute's trial isn't the only COVID-19 vaccine trial that has raised safety questions.

Bharat Biotech International, another Indian drug maker conducting COVID-19 vaccine trial, said a volunteer was hospitalised after experiencing adverse effect during a phase-one trial in August, Live Mint reported.

This also isn't the first time the vaccine developed by Oxford University and AstraZeneca has come under scrutiny. While the vaccine initially showed an efficacy of 70 percent, it later claimed that the vaccine was actually 90 percent effective, citing a "dose error".

The Serum Institute has produced millions of doses of the vaccine, and expect to ask for emergency-use authorisation within the next two weeks.