It’s important to note that these figures are a rough estimate, calculated by dividing an app’s total downloads by the number of people in a given country 14 or older. The dataset doesn’t cover about half of the countries with national contact tracing apps. It also doesn’t include China, which has no standalone contact tracing app: Instead, a contact tracing feature was automatically added to AliPay and WeChat, which are used by hundreds of millions of people within the country.

To come up with the 15% threshold, the researchers assumed that app usage would be distributed randomly across the country. But in reality, it will vary locally: Big cities, for example, will probably have the highest adoption rates. The researchers stress that local rates will have the biggest impact on whether an app will protect a particular community.

Within Sensor Tower’s available dataset, Qatar’s Ehteraz app is a clear leader in adoption rate at 91.8%—towering over its closest peer, Israel’s HaMagen, which has seen a 22.3% adoption rate. India’s Aarogya Setu app has by far the greatest raw number of downloads, at 127.6 million—more than double the total of every other country in the dataset combined.

Privacy advocates have singled out both Qatar and India for security flaws in their apps. India’s government has mandated that public and private sector workers, as well as residents of hard-hit containment zones, download Aarogya Setu, despite concerns that it could expose infected users’ location data to private companies and hackers. The Qatari app, which citizens must download or face up to three years in prison and a $55,000 fine, reportedly left over 1 million users’ national ID numbers, health status, and location data vulnerable to hackers.

Contact tracing apps continue to roll out unevenly around the world. In Europe, debates over how to design apps to balance privacy and efficacy have slowed deployment, and even forced some countries to scrap their offerings. The US has adopted a piecemeal approach, allowing states to build their own apps one by one. Regardless of its strategy, each county will need higher adoption figures for digital contact tracing to make a meaningful difference in public health.

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