APHL, in collaboration with Google, Apple and Microsoft and with program funding and guidance from our CDC partners, provided support for the COVID-19 Exposure Notifications System (ENS) by hosting the National Key Server (NKS) and Multi-tenant Verification Server beginning in August of 2020. On May 11, 2023, in accordance with the expiration of the COVID-19 public health emergency declaration, APHL concluded operation of these servers.

​​​​​​​​​​​​This effort supported the novel coronavirus response by enabling state and territorial public health agencies to provide COVID-19 exposure notifications to residents who wished to receive them.

How ​Exposure Notification Works

An important way to combat the spread of an infectious disease like COVID-19 is through contact tracing. Public health officials contact, test, treat and advise people who may have been exposed to an affected person to break the chain of disease transmission. Exposure notifications augment that work by using privacy-preserving digital technology to tell someone they may have been exposed to the virus as quickly as possible. 

How APHL Supported Exposure Notificati​​on

APHL helped deliver this groundbreaking technology to public health agencies. An essential element of exposure notifications was a unified digital language for communication, known as exposure notification keys. Rather than each state and territorial public health agency bearing the burden of building and hosting its own key server, a NKS, hosted by APHL on the Microsoft Azure Cloud, securely hosted the keys of those affected users. This enabled exposure notifications across the US by assuring that users could find out when they may have been exposed by users from other states.

To reduce the effort needed by public health agencies to bring exposure notifications to their jurisdiction, APHL provided a multi-tenant verification server running on Google Cloud. As a part of ENS, a verification server is necessary to ensure a user has received a positive test result before uploading their temporary exposure keys to the NKS. Rather than each public health agency standing up its own verification server and deciding on a verification approach, providing one verification server reduced the time and complexity to deploy ENS.

Learn more about how exp​os​ure notification technology ​works and its privacy protections.​

Participating States 

The following states had an exposure notification app on the NKS:​​

  • Alabama
  • Alaska
  • Arizona
  • California
  • Colorado
  • Connecticut
  • Delaware
  • District of Columbia
  • Hawaii
  • Louisiana
  • Maryland
  • Massachusetts
  • Michigan
  • Minnesota
  • Missouri
  • Nevada
  • New Jersey
  • New Mexico
  • New York
  • North Carolina
  • North Dakota
  • Oregon*
  • Pennsylvania
  • Utah
  • Virginia
  • Washington
  • Wisconsin
  • Wyoming
​​Map of states with an exposure notification app

* Limited population pilot

Learn More About COVID-19 Exposure Notifications

Exposure Notifications in Use

Exposure Notification Frequently Asked Questions

Exposure Notification Privacy Policy

Exposure Notification Server Configurations

Partner Communications


Bringing COVID-19 Exposure Notification to the Public Health Community

COVID-19 Exposure Notifications Expand Among Public Health Community


Exposure Notification API Launches to Support Public H​ealth Agencies

Exposure Notifications: Using Technology to Help Public Health Authorities Fight COVID‑19