A spike of 23,000 new cases was recorded at the end of August, and according to GROG estimates, about 71,800 people contracted the disease in metropolitan France from July 27 to August 31.
The Institute of Health Monitoring (InVS), a branch of the French health ministry, previously said there were 5,000 cases during the week of August 24-30.
GROG based its figures on a network of 5,000 doctors and pediatricians encountering acute respiratory infections in patients at the regional level, with an extrapolation made to come up with a total for the entire country.
"We are convinced that our figures are close to reality, with a margin of error of 20 percent," Cohen said.
In his opinion, he added, the health ministry had made its calculation "in a rush," using a "more restrictive notion of the definition of acute respiratory infection".
Setting the figure in context, however, he said that seasonal flu in France generally strikes 500,000 cases a week.
"We are currently facing a small wave ahead of a much bigger wave," the scale of which remains unknown, he said, adding that a major flu epidemic in France would strike two million to four million people.
Swine flu, or the A(H1N1) virus, the first pandemic to be declared by the World Health Organization in this century, has so far claimed 15 lives in France, out of at least 2,837 worldwide.
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