Covid cases in Oxford have increased fivefold in one week, almost entirely among 15 to 24-year-olds, according to the latest official figures.
The number of positive cases rose from 175 to more than 899 in the seven days to 30 June - which saw the city's fourth-highest number of daily cases.
The rate (cases per 100,000 people) went up from 115 to 590 - an increase of 414% - over the same period.
It now has the fifth-highest rate of coronavirus cases in England.
The national rate stands at 223 per 100,000 people - the highest since the start of February.
Director for public health at Oxfordshire County Council, Ansaf Azhar, urged people aged under 30 to get tested at additional testing facilities brought in to Oxford at the weekend.
He added: "We continue to urge young people to come forward and get the vaccine and we ask people to get back to basics and remember to keep a distance from others, wash their hands regularly, avoid indoor socialising and if you are indoors, make sure the room is ventilated."