To keep up with antigenic drift, scientists are constantly tweaking the flu vaccine, which is designed to respond to a surface protein called hemagglutinin, targeting what Fauci calls the “head” of the protein. “When you make a good response, the good news is you get protected. The problem is, the head is that part of the protein that has a propensity to mutate a lot.”

The other end of the protein—the “stem”—is much more resistant to mutations. A vaccine that targets the hemagglutinin stem has the potential to provide protection against all subtypes of influenza and work regardless of antigenic drift, offering an essentially universal defense against the flu. NIAID, part of the National Institutes of Health (NIH), is currently working to develop a candidate for a universal flu vaccine in a Phase 1 clinical trial, the first time the vaccine candidate has been given to people. Results on the safety and efficacy of the vaccine are due in early 2020.

Along with protecting against the seasonal influenza, a universal vaccine would also arm humanity with a weapon against the next pandemic strain of the flu. Flu pandemics come along occasionally and unpredictably, usually when a subtype of influenza jumps from animals to humans. This phenomenon, called antigenic shift, introduces a flu so novel to humans that our immune systems are caught entirely off guard.