Revenue from CSL's flu vaccine-making business dropped sharply in the first half of the financial year amid a significant fall in immunisation rates, particularly in the US.
CSL Seqirus brought in revenue of $US1.66 billion ($2.6 billion) in the six months to December 31, down nine per cent from a year ago.
Sales of its Fluad egg-based flu shot dropped 17 per cent to $US829 million ($1.3 billion), while sales of its next-generation cell-based influenza vaccine, Flucelvax, fell 12 per cent to $US468 million ($745 million).
"CSL Seqirus was negatively impacted by significantly low influenza immunisation rates, particularly in the United States," said CSL chief executive and managing director Dr Paul McKenzie.
The US Centers for Disease Control and Prevention said in September that flu vaccine uptake in the US had been slowly increasing before the COVID-19 pandemic, but had since fallen to its lowest level in 12 flu seasons for children and six seasons for adults.
CSL Behring, which makes blood-derived therapy for people with rare diseases, fared much better, with revenue up 10 per cent to $US5.7 billion ($9 billion).
CSL credited strong demand for its immunoglobulin products and accelerated uptake for its new gene therapy product to treat the blood clotting disorder haemophilia B.
CSL Vifor, CSL's iron deficiency business, saw revenue rise six per cent to $1.08 billion.
Overall, CSL posted first-half revenue of $US8.48 billion ($13.5 billion), up five per cent from a year ago when adjusting for currency fluctuations.
It made $US2.07 billion ($3.3 billion) in net profit after tax, up five per cent on a constant currency basis.
"CSL delivered a solid result for the first half of the 2025 financial year led by CSL Behring," Dr McKenzie said.
At 10.22am, CSL shares were up 0.3 per cent to $271.03.
