The annual fight to keep the flu under control starts here.

Doctors are studying nose and throat swabs from flu sufferers sent from laboratories around the world, from Texas to the Solomon Islands. They are trying to predict which flu viruses will be most potent and common and should be included in the vaccine.

Scientists at the World Health Organization Collaborating Centre for Reference and Research on Influenza in Melbourne—one of six such centers globally—face an imprecise prediction process. This helps explain the vaccine’s varying effectiveness year to year. The U.S. Centers for Disease Control and Prevention said earlier this month that this year’s immunization may be less effective due to a mutation to a particular virus strain.

“We’re always playing catch-up with these viruses,” says Ian Barr, deputy director of the WHO’s Melbourne operation. “We have to crystal ball what’s happening and what’s going happen.”

In addition to studying flu viruses, the collaborating centers grow what are known as seed viruses for vaccine manufacturers, the first step in producing traditional flu vaccines in industrial-sized proportions. Most flu vaccines are inactivated forms of viruses that prompt the body to generate antibodies meant to fight an active flu bug in the body.

The WHO has spent more than six decades monitoring flu. Flu virus, which primarily infects the upper respiratory tract, is responsible for up to an estimated 500,000 deaths globally each year, the WHO says, including up to 50,000 people in the U.S., according to the Centers for Disease Control and Prevention.

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