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California’s smartphone-based COVID-19 exposure alert system had racked up 4 million signups by Friday afternoon, according to the state Department of Public Health, but needs more residents to opt in to be effective.

The system, dubbed CA Notify, officially launched Thursday and builds off of traditional contact tracing methods by sending smartphone alerts to people who’ve spent prolonged time with someone who later tests positive for the novel coronavirus.

Public health officials hope the tool will help slow the spread of the virus by providing people with timely information so they can self-isolate and get tested for the disease.

However, the system still needs a lot more of California’s 39.5 million residents to opt in, officials said. It launched as coronavirus cases are surging throughout the state.

Read the full story on LATimes.com.