(Bloomberg) -- Coronavirus infections in the U.K. are leveling off after a sharp downturn in recent weeks -- and even rising slightly among teenagers this month, according to data released Friday.

The official estimate of the “R rate” -- the reproductive factor which measures how many people each infected person passes the virus on to -- increased to between 0.7 and 0.9 across the whole of the U.K, up from 0.6 to 0.9 last week.But the upper level reached 1.0 in all areas of England apart from London and the southwest, suggesting the virus is close to spreading exponentially again. The Department of Health said the figures did not yet fully reflect the reopening of schools in England this month -- signaling the rate is likely to go up in the weeks ahead.The data comes amid a surge in infections across much of Europe, forcing many governments on the continent to extend or impose national lockdowns.

Prime Minister Boris Johnson has warned the U.K. may need to tighten border regulations to prevent the arrival of new vaccine-resistant variants that threaten to undermine the country’s inoculation program, which has delivered doses to more than half of the adult population so far.

Separate data from the Office for National Statistics, based on tests carried out in the week ending March 20, shows that infections are rising slightly among 11- to 16-year-olds. This follows schools reopening in England on March 8, and is likely to be linked to a significant increase in testing among that age group.Overall, the proportion of people testing positive for the virus is plateauing in England, Wales and Northern Ireland, and rising in Scotland -- where schools re-opening earlier. One in 340 people in England had coronavirus in the week ending March 20, the ONS estimated.

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