A genetic variant raises one's risk of developing Alzheimer's may also make those same people more susceptible to contracting COVID-19. People that have two copies of the APOE gene (called the APOE4 gene) are 14 times as likely to develop Alzheimer's disease. Those same people are twice as likely to test positive for the corona virus as well. The APOE gene is essentially involved in handling cholesterol in the body. The protein also plays a role in certain immune system functions. Scientists are not sure why people with APOE genes are more vulnerable to the corona virus.
I think because the gene plays such a role in the immune system, it may be making it more vulnerable to exposure, thus giving people are more increased chance of contracting it compared to people who do not express the gene.
Second link: https://www.alz.org/