(CNN)For weeks, Nick Muerdter had been hearing about his coworkers' concerns and frustrations as they tried to navigate complicated and evolving Covid-19 vaccination systems to land an appointment for their parents.
He built a website showing open Covid-19 vaccine appointments across the US. Some call it a lifesaver
Some sent around emails with helpful articles and brainstormed for a solution.
"It seemed like a lot of people were just spending a lot of time on these pharmacy websites," said Muerdter, a 34-year-old software engineer for the National Renewable Energy Laboratory in Colorado. "And those were particularly frustrating just because it was, you know, you go to the website, you enter a zip code, you select a store and then nope, no, no appointments. And then you need to try a different store or try a different zip code."
In other words, the kinds of repetitive tasks that Muerdter knew computers can do better -- and faster. And that gave him an idea.
In mid-February, Muerdter began creating a tool in his free time that scanned local pharmacies' vaccine appointment availability and gathered all that information in one place -- allowing users to view nearby available appointments just by plugging in their zip code and how far they were willing to drive.
Coworkers that had been searching for weeks found appointments for eligible family members in just days, he said, with the help of the new website.
"This just really sort of tried to automate what you would have to do if you were to go the websites and enter every single zip code or ... check every single store in your area," he said.
And soon, what started off as a small side project became an "all consuming" job, outside his full-time job. Muerdter's website now covers all 50 US states, plus Washington, DC, Puerto Rico and the US Virgin Islands.
He calls it: Covid-19 Vaccine Spotter.
While Muerdter says his lists are not exhaustive and still recommends that Americans check other locations as well -- like county and state websites -- dozens of grateful users have posted success stories on social media.
Since its launch nearly two months ago, the website has been viewed by millions. It's been shared by Colorado Gov. Jared Polis, who thanked Muerdter and urged residents to look through the site if they're looking for appointments, and Arkansas state Sen. Greg Leding, who also recommended the tool. Muerdter's creation also earned a mention in New Jersey's new Covid-19 vaccine appointment finder website.
And then, there's the stream of messages of thanks pouring into Muerdter's Twitter profile.
Among them is a comment left by Erin Walton, who says she felt so grateful for the tool's help, she simply had to reach out.
Walton, who lives in Illinois, began frantically looking for Covid-19 vaccine appointments for her parents who live in Iowa, right after the state expanded its vaccine-eligibility guidelines to include them. Her parents are both in their 60s, she says, with pre-existing conditions putting them at higher risk for severe Covid-19.
"I just was looking on every website I could, their county, the state, I was looking at the CDC's website and nothing was coming up," she said.
Until she heard about Vaccine Spotter and "I immediately found them appointments."
"I started speaking the gospel about it to everyone I know," Walton said, helping a friend's mother book an appointment in Arizona, two good friends in Ohio and finally finding an appointment for her husband and herself when they became eligible in their own state.
"It was a huge lifesaver," she said.
"Just the comfort of knowing my parents are going to be OK is huge, after last year," Walton later added, speaking through tears on the phone.
Roughly 27% of all US adults have been fully vaccinated against Covid-19, and while every adult in the country is expected to become eligible for a shot by April 19, finding an appointment is still, for some, no simple task.
Kira-Lynn Ferderber, an essential worker at a rape crisis center in Florida, also struggled to find an appointment last month after becoming eligible for the vaccine.
Without a car and unable to travel far, Ferderber says the search -- going back and forth between multiple websites and lists -- became overwhelming.
Until someone on Twitter posted about Vaccine Spotter. And Ferderber found an appointment two days later.
"I take the bus or I take Ubers a lot and in Florida a lot of people don't wear masks, even in businesses I can't avoid going in... and I work with the public as well," Ferderber said. "So it was just such a relief to have my first vaccine."
Keeping the website functioning -- and bettering it -- means a lot of "late nights and weekends" for Muerdter.
Sometimes it's adjusting the tool to changes on pharmacies' websites or coming up with new mechanisms to navigate sites that block access to their data. Other times, it's adding new features (like filtering appointments by vaccine type) or expanding to more pharmacies.
"The feedback has really been sort of amazing and positive and so many people seem to have found this useful," he said. "That's just sort of what makes me happy."
And knowing he's helping is reward enough, Muerdter says.
But flooded with offers of donations, Muerdter created an option to contribute to his efforts, money he says goes toward covering website costs. Whatever is leftover, he gives to charities like UNICEF and Direct Relief.
"I'm not trying to make money off of this," he says. "I'm just happy to help people."
As for how long he'll keep the website going?
"My main goal is really just to keep it up as long as it's useful," he said. "In my mind, the sooner it's not useful then the better because that either means we're all vaccinated or ... there's enough appointments out there that it's not this huge sort of game and issue to try to find them."