Shortages of flu shots this year are not related to drugmakers ramping up production of potential COVID-19 vaccines, the head of vaccines at GlaxoSmithKline (GSK) said on Wednesday.

Speaking at the Financial Times' virtual healthcare conference, Roger Conner said GSK, the world's biggest vaccine maker by sales, had supplied doses for seasonal influenza, commonly known as flu, at its maximum possible levels this year.

A surge in demand for flu shots has led to shortages in some European cities, raising fears of a potentially lethal "twindemic" as COVID-19 cases also spike.

GSK has supplied more than 85 million flu vaccine doses worldwide, with 50 million doses for the United states alone.

"The reason why there's constraint on flu this year is just an unprecedented demand against the normal capacity. I don't think it's actually driven by the fact that coronavirus (vaccine) manufacturing is eating that capacity in any way," Conner said.

He said manufacturing of flu shots at GSK and its vaccine booster for potential COVID-19 vaccines were part of separate production processes.