Health agency Santé Publique France (SPF) issued the warning after the department crossed the alert threshold, with 50.1 new cases per 100,000 inhabitants in seven days. The threshold is 10 cases per 100,000 inhabitants, in seven days.
Additionally, the number of people hospitalised in the department doubled in the week of July 7-14, rising from eight to 16.
Mayenne is the first department in mainland France to be classed as “a high vulnerability situation”, following overseas territories French Guiana and Mayotte.
In a statement, SPF said: “The indicator of the level of vulnerability shows both the virus circulation and the impact on the health of the department population.”
For the week of June 28 to July 4, the number of cases throughout France was lower than the threshold of 10, except Mayenne, where it was at 45. This has now increased to 50.1.
The alert threshold is one of the major indicators being used to monitor the state of the epidemic in France, as deconfinement continues. It is partly calculated using the number of positive PCR tests, and the R number, representing how many other people one ill person will go on to infect.
When the R is less than one, the epidemic is retreating. When it rises to 2, the epidemic is spreading. Currently, the number in mainland France is 1.05 overall.
Recent SPF figures show that the mainland departments with the most people hospitalised - in relation to their population - were Hauts-de-Seine (38.5 per 100,000) and Indre (37.3).
Authorities in Mayenne are now calling for masks to become obligatory in enclosed public spaces as soon as possible.
France is set to make wearing a mask in enclosed public spaces compulsory nationwide, but only from next week. The government had initially said that the rule would only come into force from August 1, but this has now been brought forward.
Before the date change was announced, president of the Mayenne departmental council, Olivier Richefou, told the Agence France-Presse: “We are able to lockdown France within 48 hours, but we have to wait three weeks to be made to wear a mask? It’s risible.”
The call was similar to that made by many leading doctors, including geneticist Axel Kahn, who told news service FranceInfo: “I do not really see why we need to wait 15 days [to make masks compulsory].”
Mayenne has not yet imposed a decree across the entire department, but some mayors have introduced measures in specific towns - such as Yannick Borde, mayor of Saint-Berthevin, who said on Twitter that it would be mandatory to wear a mask “for people over the age of 11” in shops, and “all enclosed or confined spaces open to the public”.
The mayors of Château-Gontier, Changé and Cossé-le-Vivien have imposed similar rules.
Fears of a second wave
The rise in cases in Mayenne comes as a Seine-et-Marne hospital, Melun, warned that it was still seeing at least one hospital admission for Covid-19-related cases a day, and that the "virus is not finished...it is continuing".
Hospital head Mehrane Monchi told FranceInfo: "When we look at the averages, the trend is rising, little by little."
One patient is currently in intensive care - the first such admission at the hospital in three months.
A nurse at the hospital, Marion, said: "There is a little frustration and anger, because we know that the first defence against the coronavirus are the barrier methods and physical distancing, washing hands, and wearing a mask."
Her comments come after senior doctor and scientific advisor, Professor Jean-François Delfraissy warned that the public was "abandoning" barrier methods and safety recommendations.
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