But the new exposure notification system released by Google and Apple on Wednesday has turned these roles on their head, albeit in ways that at least some public health authorities say will make their job more difficult.

In Europe, countries like France and the United Kingdom are pushing for aggressive digital contact tracing efforts that involve government collection of broad new caches of location data.

As Matthew Green described in Future Tense, they have designed a system that will work only with contract tracing apps that employ a decentralized model for data storage—meaning that data is held on individual phones, rather than in a centralized database.

Your phone will know who you crossed paths with in the park, but Google and Apple won’t.

This system won’t work with apps that log location information or reveal the identity of those who tested positive.

Apple and Google have made assurances that compatible apps will delete data after it’s no longer needed, and they’ve issued technical white papers so that experts can review the design specifications.

The Apple and Google initiative responds to a widespread push to use digital contact tracing to support the fight against the virus.

With respect to digital contract tracing, success depends on at least four factors—four factors that suggest the Apple and Google system is not likely to be particularly effective in meeting the stated public health goals.

Apple and Google have repeatedly emphasized that the use of their contract tracing systems will be opt-in only, meaning that they won’t be used unless people download compatible apps and them on.

That is also true for a range of separate apps that don’t depend on the Google and Apple system.

But in Singapore, only about 20 percent of people are using the country’s Trace Together app, which means that there’s only a 4 percent chance that two people who are exposed to each other will both have the app.

In the U.S., surveys suggest that  most, people won’t opt-in.

And if people ignore notifications, the apps will have a limited effect.

In other words, the apps only work as one part in a larger containment strategy.

Meanwhile, we shouldn’t place all our eggs in the digital contract tracing basket, which has proven to be far less effective than one would hope.

We should couple any digital effort with increased funding and support for human contact tracing