Pharmacists say there is a simple solution to the shortage of COVID-19 vaccine: Use a tried-and-true method of combining leftover vaccine material from used vials to assemble additional doses.

According to NBC News, this method is called “pooling,” and has been used for years to augment supplies of medicines ranging from flu vaccines to chemotherapy medications. Pharmacists say that pooling the wasted liquid can increase the amount of available COVID-19 vaccine up to 10% using the same supply of vials.

However, the Food and Drug Administration says that since neither the Moderna nor the Pfizer vaccines contain preservatives, pooling what is left in a vial could result in cross-contamination of the drug. Other experts agree.

According to the Centers for Disease Control and Prevention, there are 10 doses per vial of the Moderna COVID-19 vaccine. But the agency warns pharmacists to “discard the vial when there is not enough to obtain a complete dose.” The CDC goes on to add, “Do NOT combine residual vaccine from multiple vials to obtain a dose.”

Pharmacy experts say that if the leftovers are used immediately, the risk of cross-contamination is low.

“If that vial is not used right away, then the risk of contamination is higher,” Stefanie Ferreri, distinguished professor and chair at the University of North Carolina Eshelman School of Pharmacy, told NBC News. “If the vial is used right away, with a new vial with the same lot number, then the risk of contamination is very low.”

She said only vaccines from the same lot number should be pooled, according to NBC News, so that if there are issues or side effects, it would be easy to trace the source.

Experts say that when vaccines become plentiful, pooling will not be an issue, but for now, getting a few extra doses from a set of vials can help hundreds of people daily.