It is certain that cigarette-smoking and heavy drinking result in serious negative effects on immune function.  These behaviors should be discouraged for the benefit of both individual and society, especially in the time of coronavirus. 

We also need to address the touchy subject of how smoked and ingested marijuana could influence the course of the pandemic.  Obviously, pot is prevalent as never before.  Attitudes have relaxed, and many states have approved recreational and medicinal marijuana.  A good number of medical personnel consider marijuana benign, at least in adults.  Apart from writing prescriptions, they may use it themselves in retirement.  Seniors use marijuana, and some nursing homes and assisted care facilities allow it. 

I searched the literature to determine whether or not marijuana affects resistance to coronavirus.  What I found — or actually, didn't find — was an unpleasant surprise.  The question is unanswered, not only for coronavirus, but for all viruses.  From a strict scientific perspective, there just aren't sufficient data on human subjects to conclude whether marijuana has a negative, positive, or neutral effect on viral infections.

But there are some fairly strong leads. 

Effects are predictable, because the immune cells that protect humans from viral infection exhibit cannabinoid receptors.  Animal studies demonstrate that those effects are negative in at least some types of bacterial and viral infection.  For instance, marijuana smoke significantly aggravates staphylococcal infection by blocking immune function, and THC decreases immune function in lungs and increases mortality in experimental Legionnaires.  In the case of viral infection, animals given THC after exposure to influenza exhibit decreased immune cell function and substantially increased viral loads — again, specifically in lungs (Hernandez-Cervantes R, 2017. PubMed Identifier 29151103).

The first fifty-one coronavirus deaths in the United States were in the four states that topped legal marijuana sales last year.  There may be big international differences in spread and mortality not entirely explicable on the basis of hygiene and quality of medical care.  These add a layer of suspicion to the animal data.  In my opinion, there is sufficient knowledge to recommend that individuals refrain from marijuana in these times.  And there could be no better time for the CDC to seek the truth.