Swine flu has been associated with the deaths of 20 people in WA and more than 4,000 people have been diagnosed with the disease.

More than 200,000 doses of the vaccination will be offered to people deemed to be the most at risk including pregnant woman, health care workers, people with chronic diseases and to Aboriginal communities.

WA's chief health officer, Dr Tarun Weeramanthri, says although it is near the end of winter, the vaccination will be effective.

"We are noticing a decline in cases at the moment, but we expect there to be continued transmission and we will be offering it to people in high-risk groups who may well choose to be vaccinated and reduce their risk," he said.