The coronavirus is a serious, often-deadly pathogen, yet the Centers for Disease Control and Prevention estimates 40 percent of all cases are asymptomatic. In some isolated outbreaks in prisons and food processing plants where thousands of people contracted COVID-19, as many as 94 percent of infected individuals presented no symptoms. The Washington Post spoke to experts and suggested four possible reasons as to why, though it's important to note the research in all cases is in early stages.
T-Cells: T-cells, a type of white blood cell that generally provides longer-lasting immunity than antibodies, may be the key to understanding resistance. One research group found that, among uninfected blood samples donated to a blood bank between 2015 and 2018, a "remarkable" 40 to 60 percent recognized the coronavirus, suggesting some people may have an immune response based on memory of other, less potent coronaviruses.
Vaccines: The Mayo Clinic is studying whether vaccines for other pathogens can protect against the virus, as has been proven in other situations. Seven types of vaccines given one, two, or five years in the past were found to be associated with a lower rate of coronavirus infection, particularly pneumonia and polio vaccines.
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. Read more at The Washington Post.