Plymouth's coronavirus infection rate is the highest it has been in four months and public health experts say the Delta variant - originally from India - is the dominant strain.
According to the latest figures, 191 new cases of the virus have been recorded in the past seven days and the majority of those are among the under 25s.
“We are now seeing a steady rise in COVID-19 rates in Plymouth and although we remain below the average rates nationally, the gap between Plymouth and national rate has been getting smaller over the last few days," said Plymouth's Director of Public Health Ruth Harrell.
"The Delta variant is now dominant, and it spreads very quickly and this is showing in the number of cases we are seeing now.
“Around 60 per cent of cases are now seen in the under 25s – the age groups who don’t have the protection of vaccination yet – and this is reflected in the number of cases we're are seeing in schools."
Around 600 school pupils are currently self-isolating, according to the council.
While the number of coronavirus hospitalisations is much lower than during the first two waves of the virus, the number of people being treated in hospital is rising along with the number of deaths.
“While we are all weary of the impact the virus is having on daily life, we cannot afford to relax our guards now," continued Ms Harrell.
"Even with an excellent vaccination regime, as the legal restrictions are eased, we are still likely to have to make COVID safety part of our daily lives."
People in Plymouth are being urged to take two free lateral flow tests a week if they are regularly going out, socialising, shopping, eating or going into work or school.
"Some children and young people will now be leaving school after their exams and should continue to take regular tests as they will be out and about and meeting friends outside school," Ms Harrell added.
"Our experience so far shows how helpful they can be in reducing the spread of the virus by alerting those who have COVID but don’t have any symptoms.
“The tests are easily available through pharmacies, libraries, the test centre in the Place de Brest, at the mobile test site touring the city and can be ordered on line.
“The take up of vaccination has been fantastic, and I know that the opening up to all over 18’s created a surge of demand. Please, if for any reason you didn’t manage to get an appointment that suited you, do try again as more appointments are being opened up all of the time.
“Anyone who has any of the Covid symptoms (a cough, or a fever, or a change in taste or smell) should self-isolate and book a PCR test right away.”