Previous tracking of variants relied on flash surveys that screened a small proportion of positive Covid tests and extrapolated the results to wider areas.
However French testing laboratories are now rolling out a 'test multiplex' – which tests for traditional Covid and new variants at the same time.
Currently 50 percent of tests are screened using this new technique and the aim is to get to 100 percent as quickly as possible.
This has enabled public health body Santé Publique France to release its first detailed, département-by-département breakdown of variants, based on the new testing technique.
Nationally the UK variant of the virus accounts for 36 percent of all cases, with the South African and Brazilian variants accounting for five percent of cases.
However this hides wide regional variations.
There are 10 départements showing more than 50 percent of cases as the UK variant, among them the Nord département which contains Dunkerque and several départements popular with British people including Vienne in west France and parts of Brittany.
Vienne's MP Sacha Houlié told Le Parisien: “It's not necessarily a surprise, we have a large immigrant community from Great Britain in the south of the départment. On the other hand, there is currently no major outbreak of infections.”
Younger people accounted for the highest percentage of cases, with 44.6 percent of cases in the 0-9 age group being the UK variant, 39.7 percent in the 10-19, 37.1 in the 20-29 and 40.8 in the 30-39 age group, falling to 20.3 percent in the over 90s.
Several flash surveys had indicated high levels of variants in the greater Paris Île-de-France region, but the more detailed survey shows only average levels of the UK variant in Paris and its suburbs.
The areas with the high levels of variants do not necessarily correspond with places that have a lot of cases – the southern city of Nice which has seen worryingly high case numbers and great pressure on local hospitals has only average levels of the UK variant and low levels of the South African and Brazilian variants.
The départements of Lozère and Corse-de-Sud are not included in Santé Publique France's data set, apparently due to a low number of tests meaning there is not enough meaningful data.
South African and Brazilian variants
These represent a much lower percentage and many cases – such as a cluster of cases in the south west département of Dordogne, have been traced to travel or a specific cluster.
— Le Parisien Infog (@LeParisienInfog) February 20, 2021
However the eastern département of Moselle has reported a high level of cases – 300 in a single week – which local officials say they cannot trace to a cluster.
In general the north east départements of France running along the Belgian and German borders have seen high levels of the South African and Brazilian variants.