Chinese company Sinopharm began delivering its Omicron-specific vaccine candidate to fully vaccinated people in a major clinical trial which launched in Hunan province on Monday.

The trial, involving over 4,000 participants, is the world's "largest, earliest and most comprehensive" one to test a vaccine specifically designed to tackle the Omicron variant of COVID-19, the company said in a statement released on Monday.

The upgraded vaccine was developed by a Beijing unit under China National Biotech Group, a subsidiary of Sinopharm.

It received approval to launch human trials in Hong Kong on April 13 and then gained permission for trials on the mainland on April 26.

The company first began administering the vaccine to unvaccinated people in a separate trial rolled out in Hangzhou, Zhejiang province on May 1.

Zhang Yuntao, vice-president of China National Biotech Group, said during an earlier interview it will take at minimum three to four months for researchers to complete trials.