When COVID-19 hit and the Irish government needed a technology company with a reputation for being quick and nimble to build a contact tracing app in March, NearForm jumped at the chance to help.

The company’s software engineers, who were all used to working remotely even before the pandemic, created COVID Tracker, a decentralized app that keeps users anonymous but alerts them if they’ve crossed paths with someone who has tested positive for coronavirus in the past two weeks. With a 35% adoption rate, it has been such a success story in Ireland that versions are now being used in four U.S. states, with many more expressing interest. 

“From getting back to life as we would like it to be, we have to get the transmission rates down so we can control and ultimately quash the virus. That’s our primary aim,” Larry Breen, NearForm’s chief commercial officer told Fortune. “The sooner we reduce those numbers, we get to open up our economies, our businesses, shops, and sports.”

In the past month, Pennsylvania, Delaware, New Jersey, and New York launched COVID Alert apps built in partnership with NearForm. The entire process, from building to deploying the app for each new government client, takes less than 30 days, according to the company. 

It’s also relatively cost effective, which has caught the attention of other state governments, according to Breen. New York’s app cost $700,000 to build and deploy. By comparison, the German government spent $22.5 million to fund the development of its own contact tracing app, the Associated Press reported.

How contact tracing apps work

Contact tracing apps use the Bluetooth signal from a person’s smartphone to send anonymized keys to other people who also have exposure notification apps. 

“If another app user spends 10 minutes within six feet of you, your phones swap random codes to remember the contact. These codes don’t say anything about you or your location,” NearForm explained during the setup process for the COVID Alert NY app. 

Those keys, which are randomly generated numbers, are stored in a national server by the Association of Public Health Laboratories. If someone using the New York app reports he tested positive for coronavirus, anyone running the app who was near him recently will receive an alert with guidance to quarantine and get tested. NearForm said each anonymous key deletes itself after 14 days as an added layer of privacy protection.

Each state has had a patchwork of regulations, from reopening timelines to mask mandates, but the technology the app is built on seeks to make contact tracing apps interoperable across state lines.

In April, Google and Apple formed a rare partnership to create an open source API for exposure notifications, allowing companies such as NearForm to build apps using the API that don’t track a person’s movements, but are effective when it comes to breaking the chains of transmission.

Since NearForm’s app runs on the Gapple API, that means it can work with any other app built using the same code, even if a competing company makes it. 

“The actual open source code sits underneath [the app] and the reports and findings and makes it publicly available,” Breen said. “Anybody with concerns is free to look.”

While NearForm initially focused on creating a centralized app, Breen said it pivoted to a decentralized approach using the tech giants’ API so the company could ensure its app would have the most profound impact on public health.

Can NearForm’s success abroad be replicated in the U.S.?

Getting people to wear a mask is still hard enough in some states, but encouraging them to download and use an app that tracks their movements, even if it’s anonymously, is an entirely new obstacle.

“Citizen adoption is one of the biggest barriers. There’s this trepidation of, ‘Am I giving away data?’” Breen said. “The more we can get consistent messaging out there about how this can affect change and save lives and start moving the country or the state back to some level of normality and take away the confusion, we can get people to more actively engage.”

Last week, more than 105,000 people downloaded New Jersey’s app, just days after it launched. In New York, there were at least 500,000 downloads days after the state’s app was available to the public. Pennsylvania reported 300,000 downloads since its app launched on Sept. 24.

“By utilizing this technology, we can quickly notify more people who have been exposed to COVID-19. This innovative solution enhances our COVID-19 response and gives residents another tool to stay calm, stay alert, and stay safe all in the palm of their hands,” Pennsylvania Department of Health Secretary Dr. Rachel Levine told Fortune. “This is a simple, secure way that each and every one of us can unite together to help protect our communities from COVID-19.”

While it’s solid progress, the downloads represent just a small percentage of each state’s population. By comparison, NearForm’s app was downloaded by 25% of the Irish population 36 hours after its launch in July and now has more than 35% adoption across the country’s population of 4.9 million people.

“We have people now who are doubting the advice they hear from public health agencies. Will people trust a contact tracing app?” said Alok Patel, a hospital physician in San Francisco and a medical journalist. “These apps have to be very clear, not just about privacy and data, but by also sharing quality information about why the app is important and how contact tracing can help break the chains of transmission.”

Even if the contact tracing apps are only as good as the portion of a population using them, a study published last month in MedRxiv found they can make a difference. If just 15% of the population of Washington State downloaded contact tracing apps and actively reported their symptoms, digital alerts could reduce COVID infections by 8% and deaths by 6%, according to the study, which was written by tech researchers and a team of academics.

Breen said NearForm has been receiving calls from governments around the world, including more U.S. states, that want to explore the possibility of launching their own contact tracing apps. As of Friday, a review of the app stores found there were just 14 states with contact tracing apps, including pilots from Arizona and California.

“Contact tracing with an app is also one extra part of all the other mitigation strategies we have,” Patel said. “If you have an app, manual contact tracing, masks, physical distancing, testing strategies, all those together will help prevent future outbreaks and ultimately help us reopen our economy, get back to the office and some sense of normalcy.”

More coronavirus coverage from Fortune:

• WHO director calls herd immunity “scientifically and ethically problematic”
• Empathy is an underrated weapon in fighting vaccine skepticism
• “A tale of two Americas”: How the pandemic is widening the financial health gap
• How major conventions like SXSW and CES are working around the extended pandemic timeline
• COVID-19 can survive up to 28 days on phone screens and money, new research shows