One small Japanese study at the University of Shizuoka showed that nursing home residents who gargled with green tea three times a day for three months were less likely to get flu than those who did not.
Larger clinical trials are now under way, involving more than 600 teenagers. Young people were chosen because flu spreads quickly and easily among students.
Researchers have yet to discover exactly how catechins work, but it is believed they have a general anti-viral effect. Gargling is thought to be a more effective way to tackle viruses than drinking, because this way the bugs are exposed to the liquid for longer.