A spike of more than 1,500 coronavirus infections within days has dealt a sudden blow to Germany’s efforts to reopen the country, calling into question the durability of what had been widely considered a success story in managing the contagion in Europe.

The new clusters have been concentrated in slaughterhouses and crowded, low-income apartment blocks, which have been quarantined, but they are generating increasing concern that the infections could break out and spread among the broader public.

This week, those concerns spurred the authorities in the state of North Rhine-Westphalia to impose lockdowns in two counties — the first since the country’s broader reopening in May — after hundreds of workers at the Tönnies meatpacking plant in Gütersloh county tested positive for the virus. Neighboring Warendorf county, where many plant workers live, was the second county locked down.