The vaccines are offered to children aged under five, people aged over 65 and a number of other groups including pregnant women, Aboriginal and Torres Strait Islanders and people with certain medical conditions.

"Flu is always circulating in the community, but we have seen the highest number of cases for this time of year in more than 10 years, so we have begun distributing vaccines as soon as they have become available," Health Minister Stephen Wade said.

Since January there have been 4485 confirmed cases of influenza in SA, compared to 1139 at the same time last year.

Mr Wade said under the free program, 90,000 children are eligible to receive the vaccine, which remains the best way to protect them from the disease and limit its spread.

SA Health's director of communicable disease control Louise Flood said it was not possible to predict the severity of a flu season.

"For many, the flu causes moderate symptoms that ease after a few days, but for others it can lead to severe illness and sometimes even death," Dr Flood said.

"I would urge everyone to contact their GP or immunisation provider to get a flu shot, especially those in the high-risk groups."

Australian Associated Press