A COVID-19 vaccine candidate from United Kingdom and South Korean pharmaceutical companies is entering late stage trials in which it will be tested against the AstraZeneca jab, instead of against a placebo.

The vaccine, called GBP510, works by combining two main ingredients: a synthetic antigen which triggers an immune response when it detects the novel coronavirus, and an adjuvant, which is a substance sometimes added to vaccines in order to enhance the immune reaction.

The synthetic antigen was developed by South Korean pharmaceutical company SK Biosciences, along with researchers from Washington University in the United States.

The adjuvant is produced by London-based GSK. The company says its adjuvant also reduces the amount of antigen needed per treatment, which increases dose yield. GSK has deals in place to supply its adjuvant to several other COVID-19 vaccine makers, including China's Xiamen Innovax Biotech.

China's Clover Biopharmaceuticals was set to use the GSK adjuvant for its COVID-19 vaccine, but terminated the partnership with GSK in February, and announced it would move forward with an adjuvant from US company Dynavax.

Clover said both adjuvants performed well in testing, and did not elaborate on its decision to part ways with GSK.

In early testing, the GBP510 jab induced a strong antibody response, according to SK Bioscience. Phase 3 trials will involve 4,000 participants in Europe and Asia. Commonly, late-stage trials involve testing a vaccine against a placebo. However, this trial will test GBP510 against the COVID-19 shot from AstraZeneca. This decision was made due to ethical questions over administering a placebo during a pandemic, when several functional and approved vaccines are available.

It is the second time a COVID-19 vaccine candidate has been trialed in this way, after France's Valneva tested the performance of its vaccine against the AstraZeneca treatment in the so-called Cov-Compare trial.

The GBP510 vaccine, which will require two doses, could be ready for delivery in the first half of 2022, GSK said in a statement.

South Korea hopes the vaccine will help ease future reliance on treatments from the US and Europe, which have been hit by delays. The Asian nation is battling a fourth wave of infections, which reached record highs in July, as repeated disruption of Pfizer and Moderna vaccine deliveries over the last two months have pushed back planned roll-outs of second doses by several weeks.

As of Monday, just under 56 percent of the country's population had received at least one vaccine dose, and just over 29 percent had received a full course.

GSK said it aims to deliver the GBP510 vaccine at scale through COVAX, which is a global aid initiative for the equitable distribution of vaccines.

"While many countries have made good progress with vaccination, there remains a need for accessible and affordable COVID-19 vaccines to ensure equitable access and to protect people across the world," said Thomas Breuer, chief global health officer at GSK.