It reports a consistent pattern of a maximum number of cases on Monday, declining steadily on every subsequent day before peaking seven days later.
Employers suspect that the weekly rise in cases of swine flu across the country is about employees using the pandemic as a new excuse for calling in sick on a Monday morning.
The pandemic flu line allows callers to get the antiviral drug Tamiflu and use the receipt to justify sick days and insurance claims.
Aaron Ross, chief executive of FirstCare, a consultancy that manages sickness-related absences from the workplace, said 43% of all his calls each week for all causes take place on Monday.
"The only natural conclusion is that people are using the service as a reason to take time off work," he said.
"A number of employers we have spoken to are suggesting employees are calling the hotline, requesting Tamiflu and then using it as justification to phone their manager to start an absence."
If a second wave of flu takes place later this year as predicted, employers should be braced for a new peak in absences, especially on Mondays.
The rate of infection in recent weeks has risen and fallen in line with health experts' predictions but they have found the weekly fluctuations more puzzling.
"We just don't know why," said Sir Liam Donaldson, chief medical officer, last week when asked to explain the Monday effect. There was no obvious medical reason, he said.