Editor’s note: Two new strains of the coronavirus that causes COVID-19 called B.1.1.7 and B.1.351 have been found in the U.K. and South Africa and are thought to be more transmissible. In this interview, David Kennedy, a biologist who studies the evolution of infectious diseases at Penn State, explains how these new strains are different, what “more transmissible” means, what that means for the public and whether the vaccines will be effective against them.