A major study suggests that coronavirus disease 2019 (Covid-19) infection rates in London have plunged by 80 percent in the past month as lockdown measures curb the spread of the virus.

Also, the United Kingdom was one of the first countries in the world to roll out a full-scale Covid-19 vaccination drive.

Imperial College London researchers tested 85,000 people across England between Feb. 4 and Feb. 13 as part of the monthly study. It found that about 1 in 200 people were infected, a fall of two thirds from the month before.

The decline varied across the country and was steepest in London, where a new and more contagious strain of the virus was identified late last year. In January, an estimated 1 in 30 people in London had the virus. That has now fallen to about 1 in 185.

Health Secretary Matt Hancock said the decline in cases was “encouraging … but we must not drop our guard.”

Britain has experienced Europe’s worst Covid-190 outbreak, with more than 118,000 deaths, and is in lockdown as a mass vaccination program pushes ahead at the continent’s fastest rate. So far some 16 million people have had a first dose, about a quarter of the population.