(NEXSTAR) – Looking for extra COVID vaccine doses? A new website can help.

Dr. B is an alert system that lets you know when there are spare COVID-19 vaccines nearby.

The Pfizer and Moderna vaccines must be used within six hours of thawing, or they get tossed. Some vaccine centers will give out the soon-to-expire doses to lucky people in the area. Dr. B can alert you when that happens.

Nearly 750,000 have already signed up for the service.

To sign up, go to hidrb.com and click “I want the COVID vaccine” on the website landing page. From there, you’ll be asked to enter identifying information that includes your phone number, first and last names, zip code, date of birth, email, profession and qualifying health conditions.

When a provider in the Dr. B network has leftover doses in your area, the website sends you a notification — even if you don’t qualify for the vaccine under your state’s current vaccination guidelines.

From there, you must respond to the text and claim the dose within 15 minutes before traveling to the provider.

After your first dose, the provider will work with you to schedule a second one.

The service is free and available to anyone 18 years and older who resides in the U.S. and is legally and medically qualified to receive the COVID vaccine. You must also have a cellphone to use the service

Already, two vaccine sites have started testing the program and about 200 other providers have applied, the company told the New York Times.

Per the Times, Dr. B sorts people by local rules pertaining to vaccine priorities, so those in higher risk categories receive the vaccine first.