Governments across Europe and in the U.S. say they have driven down a key variable to levels that suggest that the lockdowns that plunged their economies into the deep freeze are succeeding in containing the spread of the deadly new coronavirus.

The variable measures the average number of other people each carrier of the new coronavirus infects. This infection ratio will be avidly watched as policy makers calibrate the task of rolling back measures that have kept hundreds of millions of people at home.

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