(Bloomberg) -- The Covid-19 delta variant may account for a quarter of new cases in some parts of Italy, a startlingly rapid rise for the virus mutation, a new study shows.

The variant, first detected in India, has sent health care officials across Europe scrambling to respond, with some countries rethinking ambitious plans to reopen their economies.

Up to 25% of total cases in some Italian regions are linked to the delta variant, up from just 1% registered on May 18, daily Corriere della Sera reported Thursday, according to Pietro Forestieri, president of Ceinge-Biotecnologie Avanzate institute in Naples. “We need to be prepared for a progressive and constant increase in these numbers,” Forestieri said.

The U.K., where experts have cited the variant as a major driver behind new infections, on Wednesday reported its most new cases since the winter. Delta now accounts for at least 20% of new cases in Ireland and parts of Germany, while in localized hotspots such as Lisbon, the figure is above 60%.

The growing threat of the delta coronavirus variant in the European Union has prompted a fresh warning from the bloc’s disease prevention agency about the pace of vaccinations and not rushing re-openings.

In a threat assessment published Wednesday, the European Center for Disease Prevention and Control said it’s likely the variant will “circulate extensively during the summer, particularly among younger individuals that are not targeted for vaccination.”

Italy’s inoculation campaign got off to a rocky start, with complications linked to the AstraZeneca Plc vaccine slowing the initial roll-out. The country, which has hit its goal of 500,000 shots per day, now targets an 80% vaccination level by the end of September.

Still, the threat from the new variant makes the vaccination campaign all the more urgent. The Lazio region around Rome will move up its timetable for administering second vaccine doses, Governor Alessio D’Amato said Thursday. “We need to run faster than the variant, use all the doses available.”

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