Remember, a long time has passed and a lot of people have died while Evidence-Based Medicine was facing the wrong way.
Selenium reduces COVID-19 mortality: A Bradford Hill analysis
1) On inspection of the Hubei data, it is notable that the cure rate in Enshi city, at 36.4%, was much higher than that of other Hubei cities, where the overall cure rate was 13.1% (Supplemental Table 1); indeed, the Enshi cure rate was significantly different from that in the rest of Hubei (P < 0.0001). Enshi is renowned for its high selenium intake and status [mean ± SD: hair selenium: 3.13 ± 1.91 mg/kg for females and 2.21 ± 1.14 mg/kg for males]—compare typical levels in Hubei of 0.55 mg/kg (10)—so much so that selenium toxicity was observed there in the 1960s. Selenium intake in Enshi was reported as 550 µg/d in 2013.
2) Serum samples (n = 166) from COVID-19 patients (n = 33) were collected consecutively and analyzed for total Se by X-ray fluorescence and selenoprotein P (SELENOP) by a validated ELISA. Both biomarkers showed the expected strong correlation (r = 0.7758, p < 0.001), pointing to an insufficient Se availability for optimal selenoprotein expression. In comparison with reference data from a European cross-sectional analysis (EPIC, n = 1915), the patients showed a pronounced deficit in total serum Se (mean ± SD, 50.8 ± 15.7 vs. 84.4 ± 23.4 µg/L) and SELENOP (3.0 ± 1.4 vs. 4.3 ± 1.0 mg/L) concentrations. A Se status below the 2.5th percentile of the reference population, i.e., [Se] < 45.7 µg/L and [SELENOP] < 2.56 mg/L, was present in 43.4% and 39.2% of COVID samples, respectively.
The Se status was significantly higher in samples from surviving COVID patients as compared with non-survivors (Se; 53.3 ± 16.2 vs. 40.8 ± 8.1 µg/L, SELENOP; 3.3 ± 1.3 vs. 2.1 ± 0.9 mg/L), recovering with time in survivors while remaining low or even declining in non-survivors.
Vitamins B1, B6, B12, D (25-hydroxyvitamin D), folate, selenium, and zinc levels were measured in 50 hospitalized patients with COVID-19. A total of 76% of the patients were vitamin D deficient and 42% were selenium deficient. No significant increase in the incidence of deficiency was found for vitamins B1, B6, and B12. folate, and zinc in patients with COVID-19. The COVID-19 group showed significantly lower vitamin D values than the healthy control group (150 people, age/sex matching). Severe vitamin D deficiency (based on 10 ng/dL) was found in 24% of the patients in the COVID-19 group and 7.3% of the control group. Among 12 patients with respiratory distress, 11 (91.7%) were deficient in at least one nutrient. However, patients without respiratory distress showed deficiency in 30/38 people (78.9%, P-value 0.425). These results suggest that a deficiency of vitamin D or selenium may decrease the immune defenses against COVID-19 and cause progression to severe disease; however, more precise and large-scale studies are needed.
100% of the patients in this study with severe outcomes, including death, were selenium deficient; 75% were vitamin D deficient; none were zinc deficient.
All epidemiological data about selenium and COVID-19 is consistent in direction and effect size. However, tests that could be done comparing COVID-19 risk in high and low selenium regions of Brazil, Scandinavia (selenium is supplemented in the food supply of Finland), and the USA would establish consistency further.
Selenium has much weaker or less consistent associations with other diseases, except those caused by other RNA viruses, e.g. when risk of hepatocellular cancer in viral hepatitis patients is compared with risk of osteoporosis.[4, 5]
4) Temporality - Strong
Prospective ecological comparisons are temporal by design. In the German study, the temporal association between low serum selenium levels and COVID-19 symptom severity was closely tracked.
5) Dose-response gradient - Very Strong
6) Plausibility - Very High
Reading references 2 and 3, as well as this review of the evidence written before reference 2 was published, should be persuasive. See also ref 17 for antiviral effects. This one goes up to 11.
7) Coherence - Very High
Selenium is well-studied and nothing in its story seems to contradict the idea that higher intakes will protect against COVID-19 mortality and reduce the severity of disease.
Dexamethasone, a drug which can reduce COVID-19 mortality in the ICU, enhances 1α,25-dihydroxyvitamin D3 effects by increasing vitamin D receptor transcription.
Selenium sufficiency is essential for the function of vitamin D in peripheral blood monocytes. Vitamin D status also correlates with COVID-19 survival.
This is an area of neglect, but overlaps with the next section as there are several trials of selenium supplementation in other viral diseases, and animal experiments in analogous conditions, and many mechanistic experiments that are non-specific. However the interaction between SARS-CoV-2 and selenoproteins has been confirmed by experiment.
9) Analogy - Strong
Selenium intake is protective, and selenium supplementation has been useful, in other viral illnesses.
However, the protective effect of high selenium intakes before infection in epidemiology appears stronger than the protective effect of selenium as a late intervention in disease.[6, 11]
We can add the most relevant of extra questions to any given set of criteria - "strength of the alternative hypothesis" would be a good one for any lipid hypothesis.
Bradford Hill stated that some interventions are easier to justify than others.
With nutrient intakes there is often an identifiable risk, with a J-shaped curve. With selenium the risk is selenosis, which is a condition that requires chronic high exposure (I have given myself mild selenosis with around 900mcg selenium a day and it was not a terrible condition to experience and was reversible). There could be other risks. Luckily we have an experiment that tells us where the limit is.
In a low selenium country, like New Zealand or Denmark, you don't want to take more than 200mcg of extra selenium long term. Pity the low dose arms here weren't retained in the intervention.
During 6871 person-years of follow-up, 158 deaths occurred. In an intention-to-treat analysis
[Edit: 1/09/20] There is also very good evidence that intravenous high dose selenite is safe in the ICU setting.
Totally 19 RCTs involving 3341 critically ill patients were carried out in which 1694 participates were in the selenium supplementation group, and 1647 in the control. The aggregated results suggested that compared with the control, intravenous selenium supplement as a single therapy could decrease the total mortality (RR = 0.86, 95% CI: 0.78–0.95, P = .002, TSA-adjusted 95% CI = 0.77–0.96, RIS = 4108, n = 3297) and may shorten the length of stay in hospital (MD −2.30, 95% CI −4.03 to −0.57, P = .009), but had no significant treatment effect on 28-days mortality (RR = 0.96, 95% CI: 0.85–1.09, P = .54) and could not shorten the length of ICU stay (MD −0.15, 95% CI −1.68 to 1.38, P = .84) in critically ill patients.
This, and an earlier analysis which found less benefit, did not single out viral illnesses as a subgroup - this is only evidence for safety - but the earlier analysis did find a) slightly lower mortality in trials without an initial bolus dose, b) no increased risk in patients with renal disease.
I will hypothesize briefly on selenium increasing mortality at 300 mcg/day in the Danish intervention study, a dose far too low to cause selenosis.
(The conventional signs of selenosis result from selenocysteine replacing cysteine in proteins, and the relative weakness of the Se-Se bond compared with the S-S bond.)
[Edit - hypothesis improved, 23/09/20]
Brazil nuts are a variable quantity, a sample of nuts sold in NZ in 2008 had an average of 19 mcg per nut and increased selenoprotein levels more than selenomethionine.
Plasma selenium increased by 64.2%, 61.0%, and 7.6%; plasma GPx by 8.3%, 3.4%, and -1.2%; and whole blood GPx by 13.2%, 5.3%, and 1.9% in the Brazil nut, selenomethionine, and placebo groups, respectively. Change over time at 12 wk in plasma selenium (P < 0.0001 for both groups) and plasma GPx activity in the Brazil nut (P < 0.001) and selenomethionine (P = 0.014) groups differed significantly from the placebo group but not from each other. The change in whole blood GPx activity was greater in the Brazil nut group than in the placebo (P = 0.002) and selenomethionine (P = 0.032) groups.
[Edit 02/09/20] - thanks to Mike Angell for this link; while all selenium sources are probably protective against death and ongoing harm from COVID-19, only selenite is likely to have an additional antiviral effect, and has low toxicity.
A rational protocol for using selenium in prevention and treatment of COVID-19, fully consistent with the evidence discussed here, is described out at the end of this paper:
 Yu MW, Horng IS, Hsu KH, Chiang YC, Liaw YF, Chen CJ. Plasma selenium levels and risk of hepatocellular carcinoma among men with chronic hepatitis virus infection. Am J Epidemiol. 1999;150(4):367-374. doi:10.1093/oxfordjournals.aje.a010016
 Wang, Y., Xie, D., Li, J. et al. Association between dietary selenium intake and the prevalence of osteoporosis: a cross-sectional study. BMC Musculoskelet Disord 20, 585 (2019). https://doi.org/10.1186/s12891-019-2958-5
 Bermano, G., Méplan, C., Mercer, D., & Hesketh, J. (2020). Selenium and viral infection: Are there lessons for COVID-19? British Journal of Nutrition, 1-37. doi:10.1017/S0007114520003128
 Hidalgo AA, Deeb KK, Pike JW, Johnson CS, Trump DL. Dexamethasone enhances 1alpha,25-dihydroxyvitamin D3 effects by increasing vitamin D receptor transcription. J Biol Chem. 2011;286(42):36228-36237. doi:10.1074/jbc.M111.244061
 Schütze N, Fritsche J, Ebert-Dümig R, et al. The selenoprotein thioredoxin reductase is expressed in peripheral blood monocytes and THP1 human myeloid leukemia cells--regulation by 1,25-dihydroxyvitamin D3 and selenite. Biofactors. 1999;10(4):329-338. doi:10.1002/biof.5520100403
 Martín Giménez, V.M., Inserra, F., Ferder, L. et al. Vitamin D deficiency in African Americans is associated with a high risk of severe disease and mortality by SARS-CoV-2. J Hum Hypertens (2020). https://doi.org/10.1038/s41371-020-00398-z
 Steinbrenner H, Al-Quraishy S, Dkhil MA, Wunderlich F, Sies H. Dietary selenium in adjuvant therapy of viral and bacterial infections. Adv Nutr. 2015;6(1):73-82. Published 2015 Jan 15. doi:10.3945/an.114.007575
 Rayman MP, Winther KH, Pastor-Barriuso R, et al. Effect of long-term selenium supplementation on mortality: Results from a multiple-dose, randomised controlled trial. Free Radic Biol Med. 2018;127:46-54. doi:10.1016/j.freeradbiomed.2018.02.015
 Zhao Y, Yang M, Mao Z, et al. The clinical outcomes of selenium supplementation on critically ill patients: A meta-analysis of randomized controlled trials. Medicine (Baltimore). 2019;98(20):e15473. doi:10.1097/MD.0000000000015473
 Manzanares W, Lemieux M, Elke G, Langlois PL, Bloos F, Heyland DK. High-dose intravenous selenium does not improve clinical outcomes in the critically ill: a systematic review and meta-analysis. Crit Care. 2016;20(1):356. Published 2016 Oct 28. doi:10.1186/s13054-016-1529-5
 Angelica Kuria, Hongdou Tian, Mei Li, Yinhe Wang, Jan Olav Aaseth, Jiajie Zang & Yang Cao (2020) Selenium status in the body and cardiovascular disease: a systematic review and meta-analysis, Critical Reviews in Food Science and Nutrition, DOI: 10.1080/10408398.2020.1803200 https://www.tandfonline.com/doi/full/10.1080/10408398.2020.1803200
 Thomson CD, Chisholm A, McLachlan SK, Campbell JM. Brazil nuts: an effective way to improve selenium status. Am J Clin Nutr. 2008;87(2):379-384. doi:10.1093/ajcn/87.2.379
 Kieliszek M, Lipinski B. Selenium supplementation in the prevention of coronavirus infections (COVID-19) [published online ahead of print, 2020 May 24]. Med Hypotheses. 2020;143:109878. doi:10.1016/j.mehy.2020.109878