Public health authorities in the United States and Europe warned on Sunday that Australia's severe influenza season foreshadows a difficult Northern Hemisphere winter. They urged early autumn vaccination as cases surge and children's vaccine stocks deplete.

Influenza cases in the Illawarra region of New South Wales tripled from one month to the next, according to New South Wales Health. Free vaccines allocated for children in the area have run out, and Australian clinicians have described emergency departments filling with respiratory patients earlier than usual.

Epidemiologists routinely treat the Southern Hemisphere winter, which runs from June to September, as a preview of the strains and severity likely to hit Europe and North America months later. The US Centers for Disease Control and Prevention and the UK Health Security Agency track Australian surveillance data to guide vaccine composition and messaging.

The warnings coincided with the launch of autumn vaccination drives in the Northern Hemisphere. The NHS opened its combined COVID and flu campaign on September 1, and pharmacies across the United States began offering seasonal shots.

Health officials stressed that early uptake among children and older adults would be critical if the Australian pattern held. A UK Health Security Agency spokesperson said monitoring of Australian influenza activity was informing preparations for the coming winter and encouraged eligible groups to accept vaccination as soon as appointments became available.