Several countries, including Canada and Spain, have already approved such dose-mixing mainly due to concerns about rare and potentially fatal blood clots linked to the AstraZeneca vaccine.
A Spanish study found that giving a dose of the Pfizer shot to people who already received the AstraZeneca vaccine is highly safe and effective, according to preliminary results.
Some 835,000 doses of the AstraZeneca vaccine from COVAX were scheduled to arrive by the end of June, which South Korea planned to use mainly as a second shot for around 760,000 health and frontline workers who had received their first dose in April.
The shipment is delayed to July or later, while the country had used up available AstraZeneca reserves to meet stronger than expected participation in its vaccination campaign which helped the country meet its first-half inoculation target ahead of schedule.
Over 27% of its 52 million population have been inoculated with at least one dose of a coronavirus vaccine as of Thursday, and the country is on track to meet a target of 70% by September and reach herd immunity before November.
The government expects to receive 80 million doses of Covid-19 vaccines in the third quarter including 10 million in July. They are from AstraZeneca, Pfizer, Johnson & Johnson and Moderna.
South Korea said last month it will conduct a clinical trial that mixes Covid-19 doses from AstraZeneca with those from Pfizer and others.
It has so far run a trial in 100 health workers to examine the formation of antibody and other immune effects, Korea Disease Control and Prevention Agency (KDCA) director Jeong Eun-kyeong told a briefing on Thursday.
South Korea reported 507 new infections by midnight on Thursday, for a tally of 150,238 cases, the KDCA said, with a death toll of 1,996 since the pandemic began.