Recently an unprecedented study was conducted in the UK; a Covid-19 “Human Challenge Trial.” In it, 36 unvaccinated young adults were recruited to be injected with the coronavirus SARS-CoV-2 through the nose. While the ethics of this study are undoubtedly questionable, the results were even more surprising. None of these people got seriously sick, and, more importantly, only half of these people exposed to the virus were infected. The study raised an important question: Why didn’t everyone get sick? Even though the virus was injected directly into their noses, some weren’t infected. For answers, we turned to Dr. Yuhong Dong, an expert in infectious diseases and antiviral drug development. “We may think it’s incredible, but in the real world, we’re often exposed to an environment with the same virus concentration where some people get infected and some don’t,” Dr. Dong said. “What’s the reason? The study was designed to answer …