SEOUL (REUTERS) - South Korea on Thursday (May 20) said it will conduct a clinical trial that mixes Covid-19 vaccine doses developed by AstraZeneca with those from Pfizer and others.

The decision comes as a growing number of countries look into using different Covid-19 vaccines for first and second doses amid supply delays and safety concerns that have slowed their vaccination campaigns.

A health official said the trial will include around 500 military personnel who were vaccinated with a first dose of AstraZeneca's Covid-19 shot.

The study will examine T cells - immune cells that can destroy virus-infected cells - and neutralising antibodies in those who were given a combination of doses, the health official told reporters.

The timing and the size of the study have not been decided, the health official said, but will involve a diverse age group.

South Korea's vaccination campaign has been hampered by global shortages and shipment delays, deepening public scepticism over Seoul's goal of reaching herd immunity by November.

It has given first doses to just over 7 per cent of its 52 million population, the Korea Disease Control and Prevention Agency (KDCA) data showed on Thursday.

KDCA reported 646 new confirmed cases of the coronavirus as at Wednesday midnight, bringing the total infection to 134,117, with 1,916 deaths.