Flu vaccine

The ghost of influenza past and the hunt for a universal vaccine

Your first bout of flu may determine how you fare during the next pandemic. That’s why scientists are trying to understand immunologic imprinting. by Declan Butler

Police in Seattle, Washington, wear masks to protect themselves during the 1918 flu pandemic that killed nearly 50 million people. Credit: National Archives/Time Life/Getty

By the time she is about three years old, a child has usually endured her first influenza infection. If it’s a nasty bout, her temperature will rise and her muscles will ache. She’s probably young enough that she won’t recall the illness — but her immune system will.

When the virus enters her body, its presence prompts a pool of immature, unprogrammed immune cells to start competing to become the flu’s tracker and assassin. The winners — cells that bind most strongly to the virus — store a memory of the pathogen, ready to recognize and attack it the next time it strikes.