More than 90 percent of the world's flu vaccine is being produced with the help of eggs, says Martin Friede, the chief vaccine researcher at the World Health Organization. File picture: Evan Vucci/AP

Geneva - While many hens lay eggs that are decorated and eaten during Easter season, others work in the name of global health and help produce hundreds of millions of flu vaccines all year long.

Every year, between 450 million and 500 million eggs are used for immunization shots, said Martin Friede, the chief vaccine researcher at the World Health Organization (WHO) in Geneva.

"More than 90 percent of the world's flu vaccine is being produced with the help of eggs," he recently told dpa in Geneva.

Scientists rely on products that cannot be bought in supermarkets.

The quality they need can only be found in special farms where hens are kept under strictly controlled conditions.

"It's more expensive than producing your omelette eggs," Friede said, adding that the chickens cannot be eaten.

"Too scrawny," he said.

Twice a year, WHO experts decide which influenza virus variants will dominate the next flu season.

Laboratories then produce the necessary viruses and send them to vaccine factories, where they are injected into eggs.

After around 10 days, billions of copies of the virus have matured in the egg white. They are then deactivated with heat or chemicals, turning them into harmless but effective vaccines.

Researchers have long been looking for alternative host cell cultures, including cells from a dog's kidney tumour and from insects.

The cell culture from the tumour can be reproduced infinitely, according to Friede.

But these approaches are not yet ready for mass production.

"Eggs will definitely continue to play a decisive role in the production of influenza vaccine for the next 20 years," Friede said.