Update: There is evidence this drug will be effective against SARS-CoV-2 based on "in vitro" studies - and in humans. In vitro means outside the body in "culture" - a laboratory experiment.
Here is the scientific reference to the study being used to fast track the use of this drug.
This link may not work. The title of the paper is:
In Vitro Antiviral Activity and Projection of Optimized Dosing Design of Hydroxychloroquine for the Treatment of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)
Here is the Presidential announcement on the use of hydroxychloroquine.
It's interesting that this drug is an immunosuppressant that also is well demonstrated to treat malaria. Apparently it's mode of action isn't the same as the biologic immune suppressant drugs that are more likely to make this infection worse. Here is a link for those who enjoy rigorous scientific speak.
Conclusion: I believe this drug has promise of effectiveness against the coronavirus because of its proven Antimicrobial Effects.
Hydroxychloroquine Antimicrobial Effects (taken from the article linked above)
In addition to its antimalarial effect, HCQ was found to be effective against bacterial and viral infections. Its antibacterial and antiviral effects are attributed to the alkalinization of intracellular acidic organelles infected by bacteria and to the inhibition of entry steps and viral proteins glycosylation. The most recognized antibacterial effect of HCQ is in Coxiella burnetii infections, where the treatment of choice for chronic Q-fever endocarditis consists of HCQ together with doxycycline for 18–36 months.
Comment from Dr. Lewis: Coxiella burnetii infection is commonly called Q-Fever. It is a virulent intracellular pathogen that we often test for. Why? Because it is at the root cause of many diseases - see list below. Importantly, the treatment for this bug is 12 - 36 month! This is not a 12 day dose of antibiotics - because these bugs live INSIDE cells and it takes a lot longer to get to and destroy them as a result.
Symptoms of Chronic Q Fever: Please read because many Americans have these symptoms but will never be tested for Q Fever.
Chronic Q fever may occur months to years after acute disease or may occur without a previous history of acute Q fever. Most cases of chronic Q fever occur in individuals with predisposing conditions such as existing heart valve or blood vessel (vascular) abnormalities or a compromised immune system.
The most common manifestation of chronic Q fever is inflammation the thin membrane lining the inside of the heart and heart valves (infective endocarditis), potentially damaging the heart valves or heart tissue. Affected individuals can develop congestive heart failure, a serious complication in which a limited ability to circulate blood to the lungs and the rest of the body results in fluid buildup in the heart, lungs and various body tissues.
Less commonly, chronic Q fever can present as infection of the bones and joints (osteoarticular infection) such as osteomyelitis or osteoarthritis, vascular infections, chronic hepatitis or chronic pulmonary disease. Osteoarticular infection can cause bone and joint pain. Chronic hepatitis can cause enlargement of the liver or jaundice. Chronic pulmonary disease can cause difficulty breathing (dyspnea) and other respiratory abnormalities.
Individuals with chronic Q fever may also experience a variety of nonspecific symptoms including prolonged fevers (although fevers are often absent), joint pain (arthralgia), muscle pain (myalgia), night sweats, chills, fatigue, and unintended weight loss.
Some individuals with Q fever develop long-term complications (long-term sequelae) such as chronic, persistent fatigue. Some researchers believe that infection with Q fever increases an individual’s risk of developing cardiovascular disease later in life.
More on hydroxychloroquine antimicrobial effects:
In the human immunodeficiency virus (HIV), HCQ was shown to inhibit virus replication in T cells, by inhibiting the surface envelope glycoprotein 120 [31–34]. This anti-HIV mechanism lacks cross-resistance with other antiretroviral drugs and was novel in the armamentarium of anti-HIV drugs. Paton et al.  showed that combined treatment with HCQ, hydroxyurea, and didanosine in antiretroviral naïve HIV patients with low viral load, decreased viral replication, and increased the CD4 count. However, in the current era of the highly active anti-retroviral therapy for HIV, the use of non anti-retroviral drugs such as HCQ is probably redundant.
Human corona virus (hCoV) caused a near pandemic of severe acute respiratory syndrome in 2002–2003. To date, no specific antiviral drugs for the prevention or treatment of hCoV infection are available. Recently, chloroquine was shown to inhibit the replication and spread of corona virus in vitro and to prevent infection with hCoV in newborn mice, showing promise as a potential therapy of this resistant virus.
What is hydroxychloroquine?
Hydroxychloroquine is a prescription drug. It comes as an oral tablet.
Hydroxychloroquine is available as the brand-name drug Plaquenil. It’s also available in a generic version. Generic drugs usually cost less than the brand-name version. In some cases, they may not be available in every strength or form as the brand-name drug.
Hydroxychloroquine may be used as part of a combination therapy. That means you may need to take it with other drugs.
Why it's used
How it works
Hydroxychloroquine is an antimalarial drug. It treats malaria by killing the parasites that cause the disease.
It isn’t fully understood how this drug works to treat lupus erythematosus or rheumatoid arthritis. However, it’s believed that this drug affects how your immune system works, which may be a benefit in lupus erythematosus and rheumatoid arthritis.
Hydroxychloroquine side effects
Hydroxychloroquine oral tablet doesn’t cause drowsiness, but it can cause other side effects.
More common side effects
The more common side effects that can occur with hydroxychloroquine include:
headache, dizziness, diarrhea, stomach cramps, vomiting
Mild side effects may go away within a few days or a couple of weeks. Talk to your doctor or pharmacist if they’re more severe or don’t go away.
Serious side effects
Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. Serious side effects and their symptoms can include the following:
blurred vision or other vision changes, ringing in your ears or hearing loss, angio edema, or rapid swelling of your skin hives mild or severe bronchospasm that causes trouble breathing, sore throat, unusual bleeding, or bruising blue-black skin color muscle, weakness hair loss or hair that becomes lighter, abnormal mood changes.
Consider taking this short risk assessment for the disease - COVID-19, which includes information on drugs that lower your ability to fight the coronavirus.
Thomas J. Lewis, Ph.D.