Antivirus will be taking a break over the holidays. We’ll be back on January 9th.

When healthcare workers started administering the first shots of the Pfizer/BioNTech vaccine in the US this week, they noticed something strange. There was an extra dose lingering at the bottom of the small glass vial.

“They initially thought that they had incorrectly done it because there was so much left in the vial after they pulled up the five doses,” Erin Fox, Senior Pharmacy Director at the University of Utah told The New York Times. “They sent us a picture and were like, can we use the extra?”

On Wednesday, the Food and Drug Administration announced on Twitter that yes, the pharmacists can use the extra. They can use every single full dose in a vial, but, the FDA added, they can’t cobble together a dose from multiple vials.

Slipping extra vaccine into vials might seem weird for a company to do — especially with something as precious as this. But it’s a standard practice for injectable drugs and products like vaccines. Just like molasses gets stuck on the sides of a measuring spoon, liquid can cling to the sides of the sterile vials. The syringe used to give the shot can also cause tiny bits of a vial’s contents to be lost — those losses can be on the order of microliters, but every little bit adds up. Putting in exactly enough for, say, five doses runs the risk of leaving some vials short, and a patient without the shot they need.

But leaving too much in can also be a problem in certain circumstances. If there’s too much of a drug in a vial, there’s the potential to give a patient the wrong amount of a drug, and it presents a temptation to misuse the leftovers. It also gets extremely expensive for pharmaceutical companies to include a lot of leftovers in their products.

That’s partially why back in 2015, the FDA came up with guidelines for the pharmaceutical industry to help them figure out how much ‘overfill’ was acceptable for their products. The guidance is pretty vague — since there are lots of different types of drugs or vaccines that can be injected, there’s no one prescription for how much extra can be included. For single-dose vials, they say it’s probably best not to include enough for a full second dose, but for multi-dose vials (like the Pfizer / BioNTech vaccine) the recommendation is simply that they “should contain no more than 30 mL of drug product except under specific circumstances.”

The vials of the Pfizer / BioNTech vaccine easily fall within that range. Each one holds just 0.45 mL of vaccine material, which then gets diluted with 1.8 mL of a saline solution. Since each dose is only 0.3 mL, that’s definitely enough for an extra dose or two, but still far less than the 30 mL limit that the FDA recommends.

The fate of the extra vaccine doses is still being decided. Between Monday, when the first vaccines were distributed in the US, and Wednesday, when the FDA said they could be used, many extra doses were simply discarded. Health care systems are also worried about the two-dose nature of the vaccine. Shipments of the vaccine across the US are still uncertain, and health care providers want to have enough on hand to give everyone who gets a first dose the recommended second dose three weeks later.

There still aren’t a lot of answers yet. The FDA says it is “working with Pfizer to determine the best path forward.” Pfizer is asking institutions to figure it out for now: ”Vaccinators need to consult their institution’s policies for the use of multidose vials,” a spokesperson for Pfizer told STAT.

The US has ordered 100 million doses of the Pfizer / BioNTech vaccine, enough to vaccinate 50 million people. More than 200 million doses of the just-authorized Moderna vaccine are on the way. But with a population of more than 328 million, and a pandemic still spiraling out of control, every extra dose counts.

Here’s what else happened this week.

Research

Does COVID-19 spread on buses, subways? New York aims to find out

A study into how well the coronavirus spreads on public transit will soon start in New York City. The researchers plan to use out-of-service buses and train cars to see how different types of aerosols move around the various modes of transit. (Paul Berger/The Wall Street Journal)

A wild mink in Utah has COVID-19. Veterinarians fear this is just the beginning.

Animals can get COVID-19 too. Check out this deep dive into which wildlife is also vulnerable to the disease. (Brian Resnik/Vox)

Yes, COVID penis is a thing

Some people are noticing erectile dysfunction as they recover from the virus. How often this problem occurs is still anecdotal, but researchers are trying to track this symptom. (Wudan Yan/Elemental)

How effective is the mask you’re wearing? You may know soon

For the most part, masks these days don’t have standards. Some government agencies are trying to change that. (Sheila Kaplan/The New York Times)

Development

Moderna COVID-19 vaccine second to get FDA sign-off in US

On Friday evening, the FDA authorized a second COVID-19 vaccine for use in the US. This vaccine will start to be distributed in the US in the coming week. (Nicole Wetsman/The Verge)

Vaccines are here. we have to talk about side effects

There are side effects to both vaccines authorized by the FDA. That fact will need to be communicated clearly as vaccination campaigns begin, and people start getting both the shots and the side effects that go with them. (Maryn McKenna/Wired)

‘I haven’t even told my wife’: Inside the frantic and secretive sprint to name the Covid-19 vaccines

COVID-19 vaccines won’t get a name for a long time, but companies have already started trying to craft the perfect name for their precious product. (Damian Le Garde/STAT)

Has COVID-19 permanently altered the development timetable for other vaccines?

The timeline of vaccine development shrank dramatically over the past year. Some of those changes will last — others won’t. (Annalisa Merelli/Quartz)

Perspectives

“I almost could cry talking to you now… I feel like I didn’t just get a vaccine, I got a shot of hope. It’s hope that this is the beginning of the end of this terrible pandemic that we’ve all been experiencing — but us on the front line have really seen the suffering and the tragedy associated with it.”

—Dr. Maggie Hagan, the director of infection prevention at Ascension Via Christi hospitals in Kansas tells The New York Times.

Dr. Jeff Toll, who has admitting privileges at Cedars-Sinai Medical Center, one of the first hospitals to stock the vaccine, recalled a patient asking: “If I donate $25,000 to Cedars, would that help me get in line?’” Toll said no.

—Laura J. Nelson and Maya Lau report on the wealthy people trying to squeeze to the front of the vaccine line in The Los Angeles Times.

More than Numbers

To the more than 75,508,468 people worldwide who have tested positive, may your road to recovery be smooth.

To the families and friends of the 1,671,772 people who have died worldwide — 313,246 of those in the US — your loved ones are not forgotten.

Stay safe, everyone.