For instance, researchers noticed during the pandemic that people who received the BCG vaccine against tuberculosis—which is not recommended in the U.S. but is commonly used in countries where the disease is endemic—had extra protection against COVID-19. That meshed with decades of observations that the shot prevents a smattering of other diseases. While the vaccine itself has a mixed success rate, the way it works has piqued scientists' interest: BCG revs up the innate immune system, it turns out, which is not specific to a given pathogen, and provides broad, albeit low-level, protection against many different infections.