The Efficacy of Multivitamin, Vitamin A, Vitamin B, Vitamin C, and Vitamin D Supplements in the Prevention and Management of COVID-19 and Long-COVID: An Updated Systematic Review and Meta-Analysis of Randomized Clinical Trials

This review aims to evaluate the efficacy of any vitamin administration(s) in preventing and managing COVID-19 and/or long-COVID. Databases were searched up to May 2023 to identify randomized clinical trials comparing data on the effects of vitamin supplementation(s) versus placebo or standard of care on the two conditions of interest. Inverse-variance random-effects meta-analyses were conducted to estimate pooled risk ratios (RRs) and 95% confidence intervals (CIs) for all-cause mortality between supplemented and non-supplemented individuals. Overall, 37 articles were included: two regarded COVID-19 and long-COVID prevention and 35 records the COVID-19 management. The effects of vitamin D in preventing COVID-19 and long-COVID were contrasting. Similarly, no conclusion could be drawn on the efficacy of multivitamins, vitamin A, and vitamin B in COVID-19 management. A few positive findings were reported in some vitamin C trials but results were inconsistent in most outcomes, excluding all-cause mortality (RR = 0.84; 95% CI: 0.72–0.97). Vitamin D results were mixed in most aspects, including mortality, in which benefits were observed in regular administrations only (RR = 0.67; 95% CI: 0.49–0.91). Despite some benefits, results were mostly contradictory. Variety in recruitment and treatment protocols might explain this heterogeneity. Better-designed studies are needed to clarify these vitamins’ potential effects against SARS-CoV-2.


Introduction
Viral infections are a significant global health concern [1].They can lead to a wide range of illnesses, from the common cold to more severe and lethal diseases like influenza but also acquired immunodeficiency syndrome or COVID-19 [1,2].Public health practices [3], including handwashing, social distancing, and other non-pharmacological treatments, are a cornerstone in reducing the spread of viral infections and minimizing their impact on individuals and communities [4].Combined with vaccinations, they have mitigated the effects of the COVID-19 pandemic, during which searching for any effective supplemental treatment or preventive measure rekindled the interest in the role of micronutrients [5,6].
It is well-known that vitamins play a crucial role in infections [7] as they are essential for the proper functioning of the immune system.Clinical studies show that vitamin D deficiency is associated with a higher risk of respiratory infections, including COVID- 19 [8]; some authors have reported the positive effect of vitamin A supplementation on the risk of severe diseases via an immunomodulatory and anti-inflammatory effect, replicable for COVID-19 [9,10], while a review has concluded that some reduction in duration and severity of common cold symptoms can be observed in regular supplementation trials [11].characteristics of the intervention (i.e., target population, sample size, vitamin administered, vitamin status at baseline, vitamin dose, vitamin route, vitamin frequency of administration, and follow-up time); area of evaluation (i.e., prevention or management of COVID-19 and/or long-COVID); main findings; and side effects or adverse events.Effects were classified into four categories: (i) immunological, hematological, and laboratory outcomes; (ii) clinical outcomes; (iii) length of hospitalization; and (iv) all-cause mortality.Articles providing data on different outcomes but from people enrolled in the same trial were grouped.
Two independent authors performed the quality assessment of the articles included in the systematic review using the revised Cochrane Risk-of-Bias tool version 2 for randomized studies [22].Any discrepancy was resolved by consensus.
Then, given the high heterogeneity of the intervention protocols applied and the outcomes investigated, for each vitamin/multivitamin complex, a narrative synthesis of the main results was performed, distinguishing the two areas of evaluation (i.e., prevention or management of COVID-19 and/or long-COVID).In addition, for vitamins C and D, separate inverse-variance random-effects meta-analyses were performed to estimate pooled risk ratios (RRs) and 95% confidence intervals (CIs) for all-cause mortality, comparing the cumulative incidence of deceased patients between the intervention and control group.The Cochrane I 2 metric was used to quantify heterogeneity.It was considered statistically significant at p-value < 0.05 and substantial heterogeneity was defined as I 2 > 50% [23].In the main analysis, we stratified by time-interval (i.e., mortality ≤ 14 days vs. mortality > 14 days from enrollment) considering the longest follow-up available, while in sensitivity analyses, whenever possible, we stratified by hospitalization setting (i.e., intensive care unit (ICU)-hospitalized patients vs. hospitalized patients vs. non-hospitalized patients), vitamin status at baseline (i.e., 100% vitamin deficient patients vs. other), route of administration (i.e., oral vs. intravenous administration), and frequency of administration (i.e., single vs. multiple administrations).Lastly, the small-study effect, potentially caused by publication bias, was investigated by visual inspection through funnel plot asymmetry.

Results
Overall, 13,523 records were identified by database search (Figure 1).After duplicate removal and screening by title and abstract, 67 articles were selected as eligible for full-text analysis, from which 30 were excluded with reasons, for a total of 37 articles ultimately included in the systematic review and 20 articles meta-analyzed.

Characteristics of the Included Studies in the Prevention of COVID-19 and/or Long-COVID
Two studies [25,26] published in 2022 and conducted in the United Kingdom [25] and Mexico [26], respectively, investigated vitamin D supplementation in the prevention of COVID-19, one of which extended the analysis to include long-COVID data [25] (Table 1).While one study [25] recruited a large number of non-hospitalized subjects (over 5000 people), the other RCT [26] enrolled a smaller sample of healthcare workers (less than 350 participants).In both cases [25,26], approximately three-quarters of the sample were females, while the proportion of individuals with vitamin D deficiency ranged from 67% in one trial [26] to 100% in the other RCT [25].Intervention protocols included the oral supplementation of cholecalciferol/vitamin D3 with different dosages, from 800 international units (IU) [25] to 4000 IU [26], administered once daily for six months [25] or 30 days [26], with a follow-up of 6 months [25] and 45 days [26], respectively.The risk of bias was judged as high in both studies [25,26] (Table S2).

Characteristics of the Included Studies in the Prevention of COVID-19 and/or Long-COVID
Two studies [25,26] published in 2022 and conducted in the United Kingdom [25] and Mexico [26], respectively, investigated vitamin D supplementation in the prevention of COVID-19, one of which extended the analysis to include long-COVID data [25] (Table 1).While one study [25] recruited a large number of non-hospitalized subjects (over 5000 people), the other RCT [26] enrolled a smaller sample of healthcare workers (less than 350 participants).In both cases [25,26], approximately three-quarters of the sample were females, while the proportion of individuals with vitamin D deficiency ranged from 67% in one trial [26] to 100% in the other RCT [25].Intervention protocols included the oral supplementation of cholecalciferol/vitamin D3 with different dosages, from 800 international units (IU) [25] to 4000 IU [26], administered once daily for six months [25] or 30 days [26], with a follow-up of 6 months [25] and 45 days [26], respectively.The risk of bias was judged as high in both studies [25,26] (Table S2).

Vitamin Co-Administration
Three studies investigated multivitamin supplementation in managing adult patients with COVID-19, two of which were conducted in Iran [9,27] and one in Mexico [28] (Table 2).The number of individuals enrolled was small in all trials (less than 100 patients), with the proportion of females ranging from 35% [28] to 49% [9].The baseline levels of vitamins were measured in one case only [9].Protocol interventions were heterogeneous: in one study [9], vitamin A, vitamin D, vitamin E, vitamin C, and vitamin B-complex were administrated intravenously with a varying dosage and frequency (from four times a day to only once in 7 days); in the second trial [27], vitamins C and E were administered orally once a day until discharge; in the last study [28], vitamins C and D were administered orally two times a day for 21 days.Follow-up ranged from 7 [9] to 40 days [28] or until hospital discharge [27].The overall risk of bias was considered with some concerns [9], high [27] and low [28] (Table S2).

Vitamin A
Two studies conducted in 2022 in Iran examined vitamin A supplementation in the management of COVID-19 in hospitalized [30] and non-hospitalized patients [29], respectively (Table 2).In both cases, the sample was small (less than 200 participants) with a female proportion of around 40%.Vitamin A levels were not tested at baseline but it was supplemented intramuscularly at a dosage of 50,000 IU in hospitalized patients [30] and orally at half of the dosage in outpatients [29].In both studies, patients took vitamin A daily, with a similar duration of treatment (two weeks in hospitalized patients and 10 days in non-hospitalized patients).Participants were followed until hospital discharge [30] or for 10 days [29].The overall risk of bias was low [30] and with some concerns [29] (Table S2).

Vitamin B
Vitamin B supplementation in COVID-19 management was investigated in two studies conducted in 2022 in Iran [31] and in China [32] (Table 2).Participants were enrolled among hospitalized patients with COVID- 19 [32] and in one case they were recruited from the ICU setting [31].The population was small, ranging from 24 [32] to 85 [31] individuals, of whom about 50% were females.The levels of vitamin B at baseline were not assessed in any study and the route of administration was specified only in one case [31], in which vitamin B was supplemented intramuscularly.Vitamin form and dosage differed between the two studies as well as the duration of treatment and follow-up.Specifically, in one case, vitamin B complex was supplemented daily for two weeks with an additional two weeks of follow-up [31], while only nicotinamide was administered five times daily for a shorter time and follow-up (two days for both) [32].The overall risk of bias was considered with some concerns [31] and high [32] (Table S2).

Main Findings of Vitamin Administration in the Prevention of COVID-19 and/or Long-COVID
One of the two articles [25] that analyzed the effects of administering vitamin D in COVID-19 and long-COVID prevention compared a high dose of supplementation, a low dose of supplementation, and no supplementation at all, while the other RCT [26] compared a high dose of supplementation to a placebo (Table 3).While vitamin D administration seemed to not influence the prevention of long-COVID symptoms at month six in the only study that analyzed it [25], data about the prevention of COVID-19 risk were contrasting, with one study [25] reporting a non-significant difference between treated and untreated in the prevention of SARS-CoV-2 infection, risk of hospitalization, and mortality at six months, whereas the Mexican RCT [26] found a significantly lower proportion of infections among vitamin D-supplemented individuals.Side effects did not differ between the intervention and control groups in both studies [25,26].In the three trials investigating multivitamin supplementation, the combination of vitamins was compared with no supplementation in two cases [9,28] or with the standard of care [27] (Table 4).Results were contrasting: a reduction in some inflammatory parameters in supplemented patients was registered in one study [9] but not in the other [27], whereas Leal-Martínez et al. [28] reported a significantly higher hydric balance and lower gastrointestinal distension among treated individuals but a non-significant difference in post-COVID syndrome, weight decrease, and gastrointestinal symptoms at day 40 from enrollment.Similarly, in the intervention group, a lower gravity score and length of hospitalization were found in the trial by Beigmohammadi et al. [9]; a lower mortality and higher saturation were registered by Leal-Martínez et al. [28], while Hakamifard et al. [27] did not report any difference between the two arms in clinical outcomes such as ICU-admission rate, temperature, and pulse rate, apart from a lower respiratory rate that was reported in patients who had received vitamins C and E. Side effects were not reported in the only study in which they were assessed [9].

Vitamin A
The effects of vitamin A were evaluated by comparing its administration with the standard of care with [29] or without a placebo [30] (Table 4).No significant difference was found in the trial of Somi et al. between the two groups, neither in clinical outcomes such as ICU-admission rate, need for invasive or non-invasive mechanical ventilation, time for clinical response and treatment strategies, nor in mortality and in length of hospitalization.By contrast, a significant improvement in some clinical symptoms such as fever, body aches, weakness, and fatigue and also significant differences in immunological response were detected in the other trial [29].Only one study [30] investigated side effects and found no events.

Vitamin B
The effects of vitamin B were investigated by comparing the administration of vitamin B complex with nutritional complex without vitamin B in one trial [31] and vitamin B with the standard of care in the other one [32] (Table 4).No immunological difference was detected in both trials between the two arms, as well as no difference in hematological parameters or in organ function indexes [31,32].Similarly, clinical outcomes such as oxygenation parameters, clinical aggravation, and mortality showed no difference between groups in the two studies [31,32].The effects of vitamin B on the duration of hospitalization were not evaluated in the studies, in which the side effects were also not investigated [31,32].

Vitamin C
The effects of vitamin C were mainly evaluated by comparing its administration with the standard of care, with [36,38,43] or without a placebo [33,35,37,41,42] (Table 4).No difference was observed in the four trials [38,39,41,43] investigating immunological response and other hematological and laboratory parameters, except for lower levels of Interleukin-6 detected after one week among patients treated with vitamin C in the study of Zhang et al. [43].All studies assessed the clinical outcomes: of these, five trials [34,36,38,39,43] did not find any difference in severity scores, including SOFA (sequential organ failure assessment) or GCS (Glasgow Coma Scale), while five trials [33,[35][36][37]43] did not report any difference in clinical improvement, oxygenation parameters, need for invasive or noninvasive mechanical ventilation, intubation, or admission to the ICU.By contrast, three authors [35,41,43] registered better respiratory parameters during the first day of hospitalization among supplemented individuals, even though the overall clinical improvement was not found to differ in two trials [42,43], was faster in one case [37], or was greater in another study [40].
As for length of hospitalization, four trials found no difference between the intervention and control group [33,38,41,43], one study [37] found it was shorter among treated individuals, whereas another study [35] found that it was longer, but with no difference after restriction to the ICU-admitted patient subgroup.Out of the six studies that analyzed side effects [33,35,39,40,42,43], only one found a higher proportion of events in the intervention group [42].

Vitamin D
Vitamin D administration was compared to placebo in five trials [44,45,47,54,55], to standard of care in two studies [46,59], and no supplementation in two RCTs [48,52], whereas it was added to standard of care (with or without placebo) in the remaining six trials [49][50][51]53,58,60] (Table 5).Eight authors [45,46,48,51,55,[57][58][59] did not report a difference between the intervention and control group in any of the immunological, hematological, and laboratory parameters investigated, whereas three trials [47, 52,53] registered better outcomes during hospital stay but only in a few factors, including neutrophil cell count, neutrophil-to-lymphocyte ratio, and natural killer cell count.A significantly longer duration of viral RNA conversion was also reported in the supplemented group by Abroug et al. [44].As for the clinical effects, severity scores such as SOFA were not found to differ [54], whereas some respiratory parameters were better among treated individuals in two cases [49,50] but not in another trial [52].A need for intubation, respiratory support, and ICU admission rate were non significantly different between the two groups in most cases [46][47][48][49][50]52,[54][55][56], even though two trials reported positive results favoring supplemented individuals [48,60].Signs or symptoms of greater clinical improvement during hospital stay or at discharge were mentioned in two [49,59] out of the six RCTs that investigated such outcomes [44][45][46]48,49,59].Furthermore, no difference was found in the duration of infection or the proportion of patients with re-infection at one year between the two groups [44,59].Length of hospitalization was found to be higher among supplemented individuals in one trial [47], it was shorter in another trial [49], and did not differ in the other RCTs, considering both the overall patients [46,49,50,53,55,56], only those admitted to the ICU [46,49,53,54], or only those with vitamin D deficiency at baseline [52,55,56].Side effects were not detected in five trials [46,47,49,50,53] or were not found to differ in the other four RCTs that were investigated [51,54,55,60].

Discussion
Despite growing evidence [61,62] that proves the role of supplement compounds in supporting the immune system, the debate about the use of natural agents in the prevention and management of viral infections is still far from solved [63][64][65].Indeed, although it is well-known that vitamins are critical to making the immune system work properly [66,67], the results coming from clinical trials on the role of these substances in COVID-19 disease are inconclusive [13,68].It is not surprising that many reviews have already been published on the topic [13,15,69,70] but they mostly focused on one vitamin only [14,15,68,71] or a specific outcome in COVID-19 patients [13,69,70].Alternatively, or in addition, they had different inclusion criteria [12,14,68], such as the investigation of vitamin administration in combination with other substances or they were not updated [14,69,[71][72][73].Therefore, we systematically reviewed all evidence on the role of any vitamin in the prevention and management of COVID-19 but also long-COVID, with the aim to provide healthcare professionals with a wider overview of the topic, including the latest available evidence, and considering simultaneously several aspects, such as different populations, settings, vitamin dosages, and route of administrations.We found that the prevention of COVID-19 was little investigated and in relation to vitamin D only.Moreover, given the contrasting findings, no clear conclusion could be drawn on the potential role of such vitamins in reducing the SARS-CoV-2 infection risk [25,26].Similarly, even though vitamin D seems to provide some benefits in the long-term complications of COVID-19 [74], its effects in the prevention of long-COVID symptoms were poorly studied, together with the effects of vitamin co-administration in the management of COVID-19 patients, both of which did not show any robust result.However, considering the low quality of most trials included and the relatively safe profile of these substances, additional data could be produced in the near future, allowing a more conclusive judgment on the potential benefits of vitamin D as a prevention therapy and vitamin co-administration as a supplemental treatment for COVID-19 patients [75,76].
As for the efficacy of individual vitamin supplementation in managing SARS-CoV-2 infections, we found limited evidence on vitamins A and B, with most outcomes showing no additional benefit [29][30][31][32].As for vitamin C, even though its role as an antioxidant agent [77,78] and necessary micronutrient for leukocyte function is well recognized [79], a few positive findings were reported in some trials but results were largely inconsistent in relation to most outcomes.Among these, all-cause mortality was the only one in which COVID-19 vitamin-supplemented individuals seemed to have significant benefits, in line with a recent meta-analysis that found reduced mortality rates after vitamin C administration in many diseases [80].These results also align with other similar reviews conducted on COVID-19 subjects [13,15,71] and might be explained by the fact that patients with pneumonia, sepsis, and/or multiple organ failure, as happens in critically ill COVID-19 patients, have usually high oxidative stress [13,81].Furthermore, vitamin C may have a role in various immunity and inflammation pathways against COVID-19 by regulating the growth and function of innate and adaptive immune cells, the production of antibodies, and the suppression of pro-inflammatory cytokines, potentially helping to balance the immune response and mitigate excessive inflammation [82,83].Nevertheless, most randomized trials that investigated all-cause mortality incidence had some concerns or were at high risk of bias [37,39,41,42].In addition, the treatment protocols were largely heterogeneous as for dose, route, and frequency of administration.For this reason, further studies are necessary to confirm our results, especially well-conducted RCTs that would contribute to populating the funnel plot and probably mitigate any potential presence of publication bias [84].
As for vitamin D, it is widely recognized for its role in the regulation of the immune system and how its deficiency is linked to inflammation.On the one hand, SARS-CoV-2 infection can cause an inflammation status leading to vitamin D deficiency [85], while, on the other hand, lower serum vitamin D levels may contribute to a dysregulation of the renin-angiotensin system and thus may increase the risk of developing a cytokine storm in COVID-19 [86,87].For these reasons, it has been the object of many investigations in the last years [14,73], but its efficacy in COVID-19 management is still contradictory [68], as also found in this systematic review, in which the effects of vitamin D supplementation were mostly inconsistent.In this regard, it is worth mentioning the small sample sizes and low quality of the retrieved RCTs, together with the considerable heterogeneity among the included studies in terms of drug dosing, population characteristics, COVID-19 severity, and treatment strategies that may have affected these findings [68].Interestingly, despite the hypothesis that individuals with low baseline 25-OH vitamin D levels may benefit to a different extent from the supplementation [68,88,89], we did not detect any increased effect, as already reported in the literature [68].By contrast, this review found a reduced mortality rate in individuals supplemented regularly with vitamin D, a finding that aligns with a large population study in which an observed inverse association between habitual use of vitamin D supplements and COVID-19 infection was observed [90] and that suggests the presence of a positive effect of this vitamin on several inflammatory mechanisms induced by SARS-CoV-2 [90][91][92][93].
Additionally, it is known that treating SARS-CoV-2 infections is more effective in the early stages of the disease and therefore, by the time COVID-19 patients are hospitalized due to their critical conditions, the treatment is likely to be less effective [94].This may be the case for vitamin administration as well, as both vitamin C and D help counterbalance the cytokine excess in the early phases of SARS-CoV-2 infection [83,87].Moreover, the choice between different vitamin D forms may be relevant in this context due to their different pharmacokinetic profiles [95].However, since most studies included patients already hospitalized, further research is needed to explore how the timing of vitamin supplementation affects the outcomes in COVID-19 patients.
However, as several trials are still ongoing or have not been published yet [96], new data could soon become available that may allow us to alleviate some uncertainty about the findings and may contribute to making the funnel plot more balanced.Alternatively, or in addition, given the high tolerability consistently mentioned across the studies, large RCTs with standardized recruitment and treatment protocols should be designed to further explore the clinical benefits of vitamin D supplementation in COVID-19 patients and better clarify the mechanisms by which it acts [68].
This study has some strengths and limitations.The main strength is the updated and comprehensive collection of data on the topic.Indeed, to the best of our knowledge, this is the first review that synthesizes available evidence on the role of any vitamin in both the prevention and management of COVID-19 and long-COVID, providing results for multivitamin complex and vitamins A, B, C, and D.Moreover, we studied all effects related to vitamin supplementation in patients with confirmed SARS-CoV-2 infection, including any immunological, hematological, laboratory, and clinical outcomes.By contrast, the limitations of the current review are mostly related to the primary studies included.Since most of them were conducted in low-middle income countries, enrolled a small sample size, and/or great proportions of individuals with vitamin deficiency, the generalizability of our findings may be limited.In addition, none of the studies provided information on the patients' SARS-CoV-2 vaccination status or reported outcome data based on age groups and/or type or number of comorbidities, factors that could have been considered in the stratified analyses.Moreover, the included studies were conducted across different countries and at different times during the COVID-19 pandemic and they did not specify the SARS-CoV-2 variant responsible for the infections [97], nor did they assess the effect of supplements on the management or prevention of infections caused by different SARS-CoV-2 variants, limiting the opportunity to consider these aspects in the analyses.Furthermore, large heterogeneity in the recruitment and treatment protocols, as well as in the outcome definitions, was found, limiting the opportunity to provide a quantitative synthesis for most aspects.Lastly, the concerning quality of the trials, coupled with some uncertainties in the findings due to the potential presence of publication bias, made the interpretation of the results particularly challenging.This was not unexpected, given that, as highlighted by Honarmand et al. [98], several biases affected the largest part of the RTCs on COVID-19 management and prevention, making it imperative to prioritize rigorous trial design to address the demand for effective and safe treatment and prevention options during healthcare crises.For all these reasons, despite some positive findings, further and better-designed studies are needed to clarify the potential benefits of vitamin administration in relation to both the prevention and management of COVID-19 and long-COVID, using common pre-established daily dosages and standardized recruitment and intervention protocols.

Figure 1 .
Figure 1.Flow diagram of the review process.

Figure 1 .
Figure 1.Flow diagram of the review process.

Table 1 .
Characteristics of the studies retrieved from the literature search and included in the systematic review that analyzed vitamin supplementation and prevention of COVID-19 and/or long-COVID.

Table 2 .
Characteristics of the studies included in the systematic review that analyzed the effects of vitamin supplementation in the management of COVID-19 by vitamin type.

Table 3 .
Main effects of vitamin administration in the prevention of COVID-19 and/or long-COVID.
COVID-19: Coronavirus disease 2019.FACIT: functional assessment of chronic illness therapy.MRC: United Kingdom Medical Research Council.NA: not assessed.# this trial evaluated two different outcomes.

Table 4 .
Main effects of supplementation with multivitamins and vitamin A, vitamin B, and vitamin C in the management of COVID-19.

Table 5 .
Main effects of vitamin D administration in the management of COVID-19.