Android users will also be alerted when exposure notifications are available in their state, then will be prompted to download a customized app developed by Google. This functionality will appear in Android 6.0 and higher later this month.

Meanwhile, iOS 13.7 will be released today with the new exposure notification system. The first states on the system will be Maryland, Nevada, Virginia, and Washington, D.C.

The unprecedented collaboration between Apple and Google was first announced in April. The interoperable system has been available for public health agencies to develop their own apps on — Virginia launched the first one in the U.S. in August and five more states have followed.

The more people who opt in, the better for public health. In a statement from Apple and Google, Christophe Fraser, from Oxford University’s department of health said: “We estimate that a well-staffed manual contact tracing workforce combined with 15% uptake could reduce infections by 15% and deaths by 11%.”

Contact tracing — the practice of identifying anyone an infected person has interacted with — is vital to a pandemic response. But the U.S. has struggled with it through a deadly mixture of overwhelming case loads, slow testing and uncooperative civilians. There is no federal contact tracing program, even as people can travel freely from state to state. The two dominant makers of smartphone operating systems can’t snap their fingers and create faster testing, responsible self-quarantining and widespread mask wearing, but the hope is that comprehensive exposure tracking can get people to assist in one fundamental of public health with as little friction as possible.