IN a handful of South Korean hospitals, designated nurses are using specially designed syringes to squeeze extra doses of coronavirus vaccine out of each vial in a bid to stretch the still limited number of vials to cover more people.

The practice has raised debate over medical safety and commercial concerns from the manufacturers who charge by the dose.

But at Seoul's National Medical Center, healthcare workers say it's actually a safe and easy process that should be a no-brainer for countries struggling to provide enough vaccines quickly.

"Two designated nurses take shifts to extract the doses and each of us had no trouble getting seven doses from each vial, vaccinating everyone," said Kim Eun-suk, an intravenous therapy specialist who was taking a shift extracting doses of Pfizer's COVID-19 vaccine from vials that officially only hold six.

On Tuesday, Kim said the centre vaccinated 629 people with 90 vials of Pfizer vaccine co-developed by its German partner BioNTech, compared with the 540 people possible had they only extracted six doses from each vial.

It takes about five minutes to extract the doses using "low dead space" syringes designed to minimise the residual volume, she said.

“Extraction itself is not difficult. It requires squeezing the exact amount with the syringe. The most important is sterilization and I think any nurse would be able to pull through."

At the suggestion of frontline nurses, the Korea Disease Control and Prevention Agency (KDCA) said it was up to providers whether to use of remaining doses, but did not make it a new standard or mandatory because it said it could burden the healthcare workers on site.

National Medical Center president Chung Ki-hyun said that the contracts with manufacturers - who sell by the dose - should not be a roadblock for on-site healthcare workers to use the remaining doses when they can save lives.

"With care and precision, the extra dose isn't as hard to extract," he said. It's not clear how many South Korean clinics are using the extra doses, but Chung said following the official limit means throwing away potentially life-saving vaccines.

Experts were divided about the decision to extract extra doses, as pooling leftover vaccine from multiple vials can lead to contamination. With the specialised low dead space syringes, however, a full extra dose can typically be pulled from a single Pfizer vaccine vial, and as many as two extra doses from AstraZeneca's vaccine vials.

Urging caution over imprecise extraction, the Korean Medical Association (KMA) has advised its members to discard the remaining doses in the vials, according to a statement seen by Reuters.

KDCA said 446,941 people were given first doses of AstraZeneca and Pfizer shots by Tuesday midnight. South Korea has reported 470 new cases on Tuesday, adding to the total coronavirus cases of 93,733, with 1,648 deaths.