South Australia will increase access to a vaccine for the Japanese encephalitis virus amid the growing risk of the mosquito-borne disease over summer.

The expanded vaccine rollout will target people who live or work in a postcode within five kilometres of the Murray River, Lake Alexandrina and Lake Albert.

To be eligible, residents must be aged two months or older and spend at least four hours outdoors on most days. The same four-hour outdoors requirement also applies to workers.

More than 900 people in selected high-risk groups, including piggery workers and those who work directly with mosquitoes, were vaccinated earlier this year through the first phase of the program.

Of the nine confirmed virus cases in SA this year, the majority had visited the Murray River region during their exposure period.

Health officials say a third consecutive La Nina weather event has resulted in conditions that increase the risk of mosquito-borne diseases.

Deputy Chief Public Health Officer Chris Lease said since Japanese encephalitis virus was declared a communicable disease incident of national significance in March this year, authorities had been working hard to put the best prevention measures in place.

"While these measures will go a long way in providing prevention, it is still important people remain vigilant in protecting themselves against mosquito bites," he said.

"This includes using repellent on exposed skin, wearing loose-fitting clothing outdoors, and ensuring that accommodation is fitted with mosquito nettings or screens."

Japanese encephalitis virus causes a rare but potentially serious infection that is transmitted through bites from infected Culex mosquitoes, commonly found in SA.

Most people don't experience any illness, however, a small number will develop encephalitis, which can be fatal or cause long-term neurological damage.

Symptoms include confusion, headaches, neck stiffness, tremors, drowsiness and seizures.