THE flu virus is a slippery customer, expert at escaping attack by our immune system. But there is a chink in its defences that could lead to a universal flu vaccine.

We can get flu repeatedly because the virus evolves: its surface proteins change, so the antibodies generated by one bout are not effective a second time. For the same reason, the vaccine for one year’s strain won’t work in later years. Now Wayne Marasco at Harvard University and his team may have found a way round this.

The flu virus’s main surface protein, haemagglutinin, is lollipop-shaped, and existing vaccines …