As the sun went down, their partying got into full flow, with an unwanted guest: the coronavirus.

An outbreak among 18- to 25-year-olds at a seaside resort on the Brittany coast is crystallizing fears that the virus is flaring again in France, on the back of vacationers throwing COVID-19 caution to the summer winds.

With 72 infections by Wednesday — mostly among that age group — uncovered in a week of furious contact tracing, the cluster on the Quiberon peninsula was thought to have originated with a supermarket summer worker who partied with others at a nightspot.