A team of researchers affiliated with a group of institutions in the United Kingdom and Brazil has partly solved the mystery of why some people are naturally less resistant to Covid-19 than others.       A new study shows that small differences in an individual’s genetic makeup may help explain why some people build a strong natural defense against Covid infection, while others become severely ill.   Researchers at the University of Glasgow’s Center for Virus Research have identified a specific protein called OAS1, which is believed to play a key role in shaping the early stages of an…