The rate of elderly people infected by flu in South Australia has more than doubled, the state government says.

Elderly South Australians are catching the flu at more than twice the rate of last year, the state government says.

People aged 65 or over account for more than 37 per cent of the state's influenza cases reported this year, up from 18 per cent in 2014.

SA Health chief medical officer Professor Paddy Phillips says the figures are concerning, particularly because the flu vaccine is available for free via a GP.

"As we age, the immune system gets weaker, making it easier for flu to take hold and cause serious complications such as pneumonia," he said in a statement on Monday.

"To help protect the elderly in our community we need to make sure that vaccination rates are as high as possible in this age group."

At least 15 aged-care facilities in SA have experienced flu outbreaks this year.

SOUTH AUSTRALIA'S FLU BLUES
- Outbreaks at more than 15 SA aged-care facilities
- Rate of infection in elderly has doubled
- About 370,000 doses of flu vaccine administered so far
- Influenza causes between 1500 and 2000 deaths in Australia every year