The probability of the three models’ in susceptibility to the development of moderate–severe COVID-19, taking into account the 455G > A polymorphism of the FGB gene, was determined as follows: codominant (p = 0.02), dominant (p = 0.01), superdominant (p = 0.01), and additive (p = 0.03), of which the dominant model with the lowest Akaike coefficient (AC = 16.03) is the most effective. The gene discussed is FGB; the disease is COVID-19.