Health authorities say the flu has claimed 35 lives in South Australia this year, with more than ten times the number of cases than the same time in 2018.

The death toll from flu in South Australia has climbed to 35 this year as the total number of cases continues to skyrocket.

SA Health says 15,500 flu cases have now been confirmed in 2019 compared with less than 1400 at the same time last year.

The majority of the deaths have been among older people, including some in nursing homes. A teenager also is among the state's flu victims.

Communicable Disease Control Branch Director Louise Flood said the reason for the severity of this year's flu season could not be determined.

"That's not something that anyone will be able to answer," she told reporters on Monday.

Dr Flood said there had been about 70 reported outbreaks in residential care facilities.

Health authorities previously described SA's flu season as unprecedented.

They also said the number of reported cases was just the "tip of the iceberg" with many cases going unreported.