Allergies: Scientists have noted children with asthma and allergies surprisingly don't seem to be at high risk of developing serious cases of COVID-19. One theory is that those children have a reduced number of ACE2 receptors, the protein the virus latches onto before replicating inside the body. Without those receptors, the virus' chance of causing damage could decrease, meaning allergies may offer protection in this case.
Masks: Masks are discussed as a preventative measure, but they may contribute to more mild infections, as well. The most direct evidence of this theory is a comparison of two cruise ships. On the Diamond Princess, where masks weren't used, 47 percent of the positive cases were asymptomatic, whereas an Antarctic-bound Argentine cruise ship that had a similar outbreak, but provided masks to all passengers and crew, saw an 81 percent asymptomatic rate.