With the need for COVID-19 vaccines continuing to outstrip the U.S. supply, pharmacists point to two ways to immediately increase it by an estimated 10%.

Vaccinators could give the extra dose that occasionally remains in a vial after the official number of shots has been administered. And they could combine partial doses — the remnants in two or three vials — to make a whole dose of a manufacturer’s immunization.

But both of those ploys are discouraged or outright forbidden by federal vaccine overseers.

“I just don’t get why they’ve been so hard-nosed,” said Michael Cohen, president of the Institute for Safe Medication Practices (ISMP) in Horsham. “We’re trying to vaccinate people as quickly as possible. That 10% increase in supply would go to people who could be vaccinated immediately.”

As with so many aspects of the pandemic, federal guidance on using residual vaccine is evolving as events unfold. The result, however, is that the directives are sometimes ambiguous, inconsistent, or puzzling. For example, the U.S. Food and Drug Administration condones using a potential extra dose from Moderna’s vials but not Pfizer’s vials, while the U.S. Centers for Disease Control and Prevention strictly forbids using such windfall doses, no matter the manufacturer.

“Only the number of doses indicated in the manufacturer’s package insert should be withdrawn from the vial,” says the CDC, the nation’s public health authority. “After the maximum number of doses has been withdrawn, the vial should be discarded, even if there is residual vaccine or the expiration date has not been reached.”

The Philadelphia Department of Public Health, for one, is hewing to that hard line. This month it directed all city vaccine providers to stop stretching their supply.

“The practice of pulling extra doses should be discontinued effective immediately,” said the email from the department’s immunization program. “Per the CDC, ANY doses beyond the approved [number] should not be pulled for administration. This means only the following should be pulled/administered per vial: 10 doses/vial for Moderna and 6 doses/vial for Pfizer and 5 doses/vial for J&J.”

At the Mazzoni Center, a health care provider for the LGBTQ community, medical director Nancy Brisbon lamented the waste. “Yesterday, we had to very regretfully say goodbye to about five or six doses,” she told CBS3 TV last week.

James Garrow, spokesperson for the city Health Department, said, “Our hands are tied. It’s not a matter of whether we think it’s a good policy.”

Why do some vials yield more doses than the authorized number?

One reason is that all medications have a tiny bit of overfill built into the manufacturing, said Michael Ganio, senior director of pharmacy practice at the American Society of Health-System Pharmacists (ASHP).

Another reason is that certain needle and syringe combinations reduce the “dead space” loss — the vaccine remaining in a needle and the top of a syringe after an injection. Doses are such minuscule amounts — Moderna’s 0.5 milliliter shot, for example, is the equivalent of a tenth of a teaspoon — dead space droplets are important.

ASHP and ISMP partnered with the U.S. Pharmacopeia on a guide that explains how to optimize COVID-19 vaccine preparation and safety by using low dead space injectors.

“We see no reason that additional dose should not be used,” Ganio said. “There are logistics to work through. The syringes and needles provided by the government may need to be supplemented. But there’s nothing different about that extra dose that makes it less suitable.”

Nothing medical, at least. But manufacturers are paid by the dose, not by the vial, so extra doses have economic implications.

That’s why in December, after vaccinators discovered they could draw an extra dose or two out of Pfizer’s five-dose vials, Pfizer executives successfully lobbied the FDA to update the vaccine’s authorization language to specify that each vial contains six doses.

Now, Pfizer and the FDA cite that update, without explicitly saying don’t try to get a seventh dose. In contrast, the FDA website says that if vaccinators manage to get an eleventh dose from a Moderna vial, “this may be used if it is truly a full dose.”

Moderna is now negotiating with the FDA to put as many as 15 doses in each vial, according to news reports.

While the guidance on pulling extra doses is murky, both the CDC and FDA say residual vaccine from several vials must never be combined.

“This is an infection control measure,” an FDA spokesperson emailed. “Cross-contamination of multi-dose medications through the use of the same needle and syringe has occurred with other medications when this practice was utilized, causing serious bacterial infections.”

Actually, the CDC’s periodic reports of disease outbreaks related to unsafe injection practices in health care settings mostly involve reusing a needle or syringe, or both, to inject multiple patients.

Expert groups agree that basic safeguards could practically eliminate any infection risk from pooled COVID-19 vaccine: Use a sterile injector to withdraw remnants from no more than three vials, all with the same lot number and manufacturer. Use pooled doses soon after the vials are first punctured. That would mean within six hours for a dose pooled in a contamination-free “clean room,” or within one hour if a clean room is not available. (Drugs such as chemotherapy are often pooled in clean rooms.)

“There’s not enough time for bacteria to grow in six hours,” Cohen said. “So this whole thing doesn’t make sense to me.”

Colorado Gov. Jared Polis and Oregon Gov. Kate Brown have formally asked the FDA to allow pooling, noting that “we are not able to locate any data to substantiate why this practice is prohibited.”

At least one place in the world, the province of Saskatchewan, Canada, is allowing pooling of vaccine outside a clean room. The policy will be reevaluated “when ample vaccine supply is available.”

“The potential risks can be minimized ... with the intended benefit of protecting more individuals from COVID-19 disease,” explains the provincial vaccine management team.