This New York Times article, by Carl Zimmer, describes remarkable research into the structure of covid19. The research showed covid19 is protected from the immune system, by sugar proteins attached to the surface of the virus.

Intracellular chlamydia consumes the ATP energy made by the cell; and when ATP is exhausted, consumes sugar. Chronic chlamydia also damages the ability of the cell to bring sugar into the cell, which leaves excess sugar outside of the cell, in the bloodstream and interstitial spaces.

Diabetes and obesity are high risk factors, for severe covid19; and it may be because the patients have a chronic underlying infection which leads to excess sugar in the blood stream. The excess sugar outside the cell and in the bloodstream is attaching to the virus and protecting the virus from the immune system.

https://www.nytimes.com/interactive/2020/health/coronavirus-unveiled.html