These recommendations from the Eastern Virginia Medical School (EVMS) have recently been updated on November 30th, 2020. It is extensive info (39 pages) on many stages of treatment for Covid-19, including prevention. This is their disclaimer:
Disclaimer: The information provided in this protocol is to provide guidance to physicians on the prevention of infection with SARS -CoV-2 as well as the early treatment and management of the hyper-inflammatory cytokine “storm” of COVID-19. Our guidance should only be used by medical professionals in formulating their approach to COVID-19. Patients should always consult with their physician before starting any medical treatment.
The prophylaxis (preventative) and early treatment of symptoms recommendations are summarized in this statement:
For prophylaxis and treatment of the early symptomatic phase we suggest the combination of Quercetin (a plant polyphenol), Vitamin C and Zinc. This is based on intriguing basic science data which indicates that: a. Zinc is essential for innate and adaptive immunity. In addition, Zinc inhibits RNA dependent RNA polymerase in vitro against SARS-CoV-2 virus. b. Quercetin has direct viricidal properties against a range of viruses, including SARS-CoV2, and is a potent anti-oxidant and anti-inflammatory agent. [24,29,34,34,319-326] In addition, quercetin acts as a zinc ionophore.  c. Vitamin C improves the potency of Quercetin and has antiviral and anti-inflammatory activity. 24. It should also be noted that Vitamin D may be a very powerful prophylactic and treatment strategy against COVID-19. Vitamin D deficiency explains, in part, the enormous geographic variation in mortality of this disease. [11,328]
This is a screenshot from my years old flyer on flu prevention and early treatment:
The Vitamin D3 recommendation is for three days only as it is a high dose but safe for short periods (400 mcg = 40,000 Iu's) The Ester-C contains the important ingredient Quercetin. And of course a highly absorbable zinc is equally important. This has a lot of similarity to the EVMS recommendations as mentioned above.
There are 338 references in the 39 page report by the EVMS. Here is the link for those who want or need more info: https://www.evms.edu/media/evms_public/departments/internal_medicine/EVMS_Critical_Care_COVID-19_Protocol.pdf
The following outlines the more extensive recommendations for early treatment:
Symptomatic patients at home (for the duration of acute symptoms)
• Vitamin C 500 mg BID* and Quercetin 250-500 mg BID*
• Zinc 75-100 mg/day (elemental zinc)
• Melatonin 10 mg at night (the optimal dose is unknown) 
• Vitamin D3 2000-4000 iu/day. Calcifediol 200 μg is an alternative. 
• Highly recommended: Ivermectin 150-200 ug/kg orally (repeat on day 3). [1-5,62,64-74] See Table 1, Figure 5 and ClinTrials.gov NCT04523831. See drug-drug interactions above.
• ASA** 81 -325 mg/day (unless contraindicated). ASA** has anti-inflammatory,
antithrombotic, and antiviral effects.[75,76] Platelet activation may play a major role in
propagating the prothrombotic state associated with COVID-19. 
• B complex vitamins
My additions: *BID=twice a day, **ASA=aspirin
Now they elaborate extensively on this in the report by adding other "Highly Recommended" "Optional" and "Not Recommended" over the counter drugs, supplements and prescription medication.
I also want to note that included in their conclusion is the statement: "Vitamin D deficiency explains, in part, the enormous geographic variation in mortality of this disease. [11,328] ". It should also be noted that vitamin D deficiency may also play a part in the increase in disease severity for certain ethnicities and races. This graphic shows that this could be a possible factor in these differences.
Thank you Dr. John Campbell for your enlightening summaries on youtube. Here is a link: https://www.youtube.com/watch?v=ZqZLMoLvhgk
For a more in-depth and scholarly post on vitamin d-high dose and covid-19: https://www.youtube.com/watch?v=3mth0LqC9z4 from Dr Mobeen Syed. Excellent.
Knowing your vitamin D status is easy. If you want to get your vitamin D levels measured, call my office at 479-845-1445 or email me at firstname.lastname@example.org and we will get that done for you. It is best to get to optimal levels before a virus strikes, and not after.
Thanks for reading!