This week, California residents started receiving push notifications on their smartphones prompting them to opt-in to CA Notify, a system that can anonymously alert users that they may have been exposed to the Covid-19 coronavirus.CA Notify, which officially launched on Thursday, is based on technology built by Apple and Google earlier this year called exposure notifications that uses Bluetooth to measure when people have been close to each other for extended periods of time, suggesting possible coronavirus transmission, without collecting personal information.The app is designed to alert users that they may have been exposed to the coronavirus early in a potential infection, so that they can get tested or quarantine before they spread the virus.California is the most populous of 19 U.S. states so far to roll out an exposure notification system and is the state where both Google and Apple are headquartered. The app is being released as 80% of California’s population has been instructed to stay home for three weeks.Because of the way the system was designed, California’s Department of Public Health (CDPH), which operates CA Notify, says it doesn’t have exact statistics about the number of activations. But the department estimates that 4 million Californians may have activated the system on their phone as of Friday, based on the number of times a specific URL on its server has been accessed.California has just under 40 million residents, so it’s reached about 10% of the state’s population so far.CA Notify is the biggest test so far for Exposure Notifications Express, an update to the Google-Apple framework announced in September intended to boost adoption. It enables governments to send push notifications to users that the apps and system are available in their area.How to get it and how it worksCA Notify works with both iPhones and Android phones, although it requires a device that can run up-to-date software. On iPhones made starting in 2015 with up-to-date software, CA Notify can be turned on in the Exposure Notifications section in the Settings app. People with eligible Android phones can download an app from from the Google Play store. The app is available in 15 different languages.California says the app is a complement to other public health measures such as wearing a mask, social distancing, and hand washing. Exposure notifications are also distinct from contact tracing, a practice where public health departments manually contact people who have tested positive to find other people who may be at risk.Exposure notifications instead exchange random strings of letters and numbers between phones using Bluetooth. When someone with the app tests positive, they get a text message from California’s public health department with a code. They input that code into their app or system, and it send an alert to other people who had been within six feet for more than 15 minutes informing them of possible exposure to someone with Covid-19 without revealing the identity of the person who tested positive.The alert doesn’t have information about the location, time, or identity of the close contact, but it tells the user the date it was recorded.Earlier this year, one major concern with exposure notification apps in the United States was that they did not work across state lines. In the United States, the development of these apps happen on the state level, instead of having a national app like the U.K. But as of December, 18 states, including California, participate in a national key server program that allows the system to work across state lines even though they are distinct apps.Power of the push notificationAdoption remains an issue for these apps, especially in the United States. They generally work better when there are more people participating because there are more devices tracking potential close contacts. A study from earlier this year suggested that 60% of a population would need to use one of these systems to stop Covid-19 transmission, but subsequent research suggests lower uptake can still slow the spread of the virus.California’s early adoption rate has been more robust than some large earlier states, and that’s largely due to push notifications that started going out to Californians on Thursday.To boost adoption, Apple and Google introduced an update to its system in September called called exposure notifications express. California is using Exposure Notifications Express. Instead of building a full app, governments can instead submit a configuration file to Apple and Google and get the system working quickly. iPhone users don’t even need to download an app under Exposure Notifications Express.But perhaps most importantly, it also allows public health departments to send push notifications to residents when the system is available, boosting adoption.Not all Californians have received push notifications so far, but they will continue to go out until they’re sent to all smartphones in the region, CDPH said in an email. Notifications are being sent by Apple to iOS users and Google to Android users who are physically located in California. When people receive the alert is dependent on the location of the phone, CDPH said. Someone who stays home for a number of days may not receive an alert until the next time they go to a new location.Exposure Notifications Express has boosted adoption in the regions that are using it. Google said on Friday that in Colorado, 20% of people have opted in to that state’s system. In Washington DC, 367,000 people have joined the system, which would amount to 53% of the district’s population.Both of those public health departments are using Exposure Notifications Express. EN Express systems have also been deployed in Connecticut, Maryland, and Nevada, which used the system to send a push notification to residents despite having an existing app.Measuring the efficacy of these systems is also difficult because of the way they are designed which does not allow public health departments to collect personal information such as name, phone number, or identity of contacts, which would enable them to measure how many potential infections that the apps caught that wouldn’t have been discovered another way.In a trial period where CA Notify was tested at several California universities, roughly 28,000 people participated in the system, and 80 codes were issued that would allow a user to alert the people they may have been nearby, CDPH said.Google says that there are “other anecdotal signs” that the exposure system is helping, citing an episode where Virginia’s governor tested positive and his staff got notifications.