A major threat to the next pandemic: Vaccine hesitancy

In April, more than 1,000 vaccine experts gathered in Washington, D.C. for the first time since the pandemic began. Over four days, scientists, doctors and drugmakers pored over cutting-edge research and tackled some of the most pressing questions in the world of vaccines. 

Talk of Covid-19 vaccines was, of course, unavoidable. But high on the agenda at the World Vaccine Congress was a vaccine for another mysterious illness that could strike at any moment: Disease X.

Disease X is not any particular virus, bacteria or other germ, but a term used...