More than half a million people have signed up to a newly launched service called Dr. B in an attempt to snag a COVID-19 vaccine — and reduce the number of doses that might end up in the trash. Led by ZocDoc founder Cyrus Massoumi, Dr. B is a website that aims to function as a kind of emergency alert system for thawed coronavirus vaccines, which typically need to be injected within six hours of being thawed.

“This vaccine is now the scarcest resource on earth,” says Massoumi. “We were concerned about the fact that a lot of the vaccine ends up in this last-minute shuffle at the end of the day.”

A longtime drag on conventional health care, missed appointments have become an unusually vital issue for the coronavirus vaccine. Massoumi estimates as much as 20-30 percent of vaccine appointments are missed, leaving a thawed vaccination dose that must be used within six hours or be permanently lost. State reports suggest that few of the doses are allowed to expire. Instead, vaccination sites distribute them to employees or whoever happens to be standing outside. But that confusion has led to chaotic lines and a haphazard approach to priority lists — something Massoumi delicately describes as “suboptimal.”

Dr. B is designed to serve as a kind of standby list for those situations, giving providers an easy way to summon willing patients in a matter of hours. More than half a million people in the US have signed up to be on the waitlist, each giving their home zip code and filling out a catchall version of their state’s questionnaire. (Each state has a slightly different list of who is eligible for vaccines first. In New York, the first tier, 1a, is people over 65 or with co-morbidities. The next tier, 1b, includes grocery store employees and other essential workers, and so on.)

When a provider ends up with thawed doses, Dr. B sends out texts to participants from nearby zip codes, prioritizing them according to the state health department’s tier list. Once texted, a participant has fifteen minutes to confirm that they can get to the provider, then two hours to reach the location and get their shot. If you say you can’t make it, you go to the back of the line (within your priority group).

The system counts on providers to schedule a follow-up shot, which both the Moderna and Pfizer / BioNTech vaccines still require — but the hope is that simply getting patients in the door for their first dose will be a significant improvement.

This isn’t the first project aiming to create a standby list for coronavirus vaccines, but it’s quickly become the largest one, absorbing the smaller Vaccination Standby project in February. What’s most impressive is how quickly the effort has grown. Still limited to America, Dr. B’s waitlist currently has more than half a million people, up from 300,000 just two weeks ago.

Providers are coming on board more slowly, since each one needs to be vetted to ensure it’s legitimate, but there are already two sites providing shots through the system — in Little Rock, Arkansas and Queens, New York. The company won’t say how many doses have been given out through those sites, but they do say alerts have been sent out each day that the two test sites were open, suggesting at least a handful have been administered through the system. More than 200 other providers are in the pipeline, spanning 30 states.

Massoumi says signing up those providers is still the biggest bottleneck — but it’s an impressive record for what is still mostly a volunteer effort. There’s no plan to monetize the system and much of the hosting and other back-end services have been donated by companies like Amazon, Twilio, and others. Some staff are on loan to the project from their regular jobs; others are being paid out of Massoumi’s pocket. There’s no clear sign of what will happen to the project after COVID-19 is defeated.

“We’re just trying to get this moving as fast as we possibly can,” Massoumi says. “We have patients and provider sites that want to use this and we just want to make sure that it’s ready for prime time.”

As with any vaccination problem, there are concerns about who will get access and who won’t. Black and Latino communities have lagged behind in vaccination rates, a reminder of long-standing patterns of discrimination in US health care. The new standby list runs the risk of making those inequities worse: the nature of the project means it will only work for patients with phones who can drop what they’re doing and head to a vaccination site at a moment’s notice.

The problem isn’t lost on medical ethicists. Gabriel Lázaro-Muñoz, a professor at Baylor’s Center for Medical Ethics and Health Policy, told The Verge he applauded the effort behind Dr. B, but he worried the project could deepen inequality in how vaccines are distributed.

“If Dr. B sends out a text, most of the people who are going to be able to drop what they’re doing and get the vaccine will be people who have access to emergency childcare, people with cars, people who can probably get out of work more easily than most underserved populations,” Lázaro-Muñoz said. “So they’re trying to address the problem, but it also exacerbates the equity issue.”

Many of the fixes the team has tried only address parts of the broader problem. The project has already launched a Spanish-language version of the site and is in talks with Uber and Lyft to provide free rides in connection with the shots — an effort that would make a significant dent in the transportation problem.

But Massoumi believes Dr. B’s queueing system will be fundamentally more equitable than what most states are doing, particularly as more tiers open up. Most state systems have focused on giving the elderly and vulnerable first access to the vaccine — but as more people become eligible to receive the shot, vulnerable patients will find themselves in a larger and larger pool of potential recipients. Under the current system, a first-tier patient who waited to get their shot would have no advantage over anyone else in an open tier. Massoumi compares it to an airplane boarding call, when a Group 1 passenger who arrives late has to wait in the same line with all the other groups. But because Dr. B has granular information on each person’s tier, the system can send the most vulnerable patients to the front of the standby line automatically — a kind of express lane for late arrivals.

Behind it all, there’s the steady march of vaccinations. More than 20 percent of the US population has received at least one dose of the vaccine at this point, and President Biden has said he expects the government to have secured enough doses for every adult in America by the end of May. Even small lags in rate or distribution can compound to huge gaps over the coming months.

Dr. B’s plans are still limited by providers’ willingness to use the system, which has kept them limited to the US rollout and places where the scheduling system fails. But like any startup founder, Massoumi isn’t afraid to dream big, noting that there’s nothing stopping the system they build from being applied for non-thawed doses or in other countries where the rollout is proceeding more slowly.

“It’s not just this country. The entire world is not going to be vaccinated until 2023,” Massoumi says. “It’s a global problem, both the pandemic and the scheduling problem, and what we’re creating is a global solution.”