Effects of Iron Status on Adaptive Immunity and Vaccine Efficacy: A Review

Abstract

Vaccines can prevent infectious diseases, but their efficacy varies, and factors impacting vaccine effectiveness remain unclear. Iron deficiency is the most common nutrient deficiency, affecting >2 billion individuals. It is particularly common in areas with high infectious disease burden and in groups that are routinely vaccinated, such as infants, pregnant women, and the elderly. Recent evidence suggests that iron deficiency and low serum iron (hypoferremia) not only cause anemia but also may impair adaptive immunity and vaccine efficacy. A report of human immunodeficiency caused by defective iron transport underscored the necessity of iron for adaptive immune responses and spurred research in this area. Sufficient iron is essential for optimal production of plasmablasts and IgG responses by human B-cells in vitro and in vivo. The increased metabolism of activated lymphocytes depends on the high-iron acquisition, and hypoferremia, especially when occurring during lymphocyte expansion, adversely affects multiple facets of adaptive immunity, and may lead to prolonged inhibition of T-cell memory. In mice, hypoferremia suppresses the adaptive immune response to influenza infection, resulting in more severe pulmonary disease. In African infants, anemia and/or iron deficiency at the time of vaccination predict decreased response to diphtheria, pertussis, and pneumococcal vaccines, and response to measles vaccine may be increased by iron supplementation. In this review, we examine the emerging evidence that iron deficiency may limit adaptive immunity and vaccine responses. We discuss the molecular mechanisms and evidence from animal and human studies, highlight important unknowns, and propose a framework of key research questions to better understand iron–vaccine interactions.

Statement of Significance.

The significance of this review is that there has been substantial rigorous recent research on the topic of iron nutrition, adaptive immunity, and vaccine response, including animal, genetic, and human studies, and yet there is no comprehensive review in the nutrition literature on the topic.

Introduction

Vaccines play a crucial role in preventing deaths and illnesses caused by infectious diseases. Yet despite the high coverage of immunization programs, in 2019, 1 in 5 children globally remained unprotected, resulting in 1.5 million annual child deaths from preventable infectious diseases [1]. Vaccines often show lower efficacy in children in low- and middle-income countries (LMICs) [2,3], with the measles vaccine, for instance, demonstrating effectiveness generally <75% in African children [4]. Vaccines also tend to underperform in certain adult populations, such as the elderly and those with obesity and chronic kidney disease [[5], [6], [7]]. The specific factors contributing to reduced vaccine effectiveness in these populations remain unclear, but nutritional deficiencies may play a role [2,8,9]. Iron deficiency stands out as the most prevalent nutrient deficiency worldwide, affecting >2 billion individuals [10], and its prevalence may increase as climate change threatens food security [11].

In this review, we examine the increasing evidence indicating that iron deficiency may limit adaptive immunity and vaccine responses. In 2021, the European Hematology Association issued an expert opinion on vaccination in individuals with hematologic disorders and recommended correction of iron deficiency before the administration of COVID-19 vaccine [12]. This recommendation was based on recent genetic, preclinical, and clinical studies that collectively demonstrate that iron availability plays a critical role in regulating T- and B-cell responses to immunization. In this review, we discuss the molecular mechanisms of this effect (schematic diagram in Figure 1), examine the evidence from animal models, and review human studies linking iron deficiency to impaired adaptive immunity. We discuss ongoing intervention trials testing whether iron interventions used worldwide to combat anemia may improve vaccine performance. Finally, we highlight important unknowns and map out a framework of key research questions to explore and potentially benefit from iron–vaccine interactions.

FIGURE 1.

Schematic overview of the impact of iron deficiency on cellular and humoral adaptive immunity. Created with BioRender.com.

Iron Deficiency and Hypoferremia Are Common and Result from Both Iron-Poor Diets and Inflammation

The significance of iron in the context of immunity is underscored by the widespread prevalence of iron deficiency, especially in countries heavily burdened by infectious diseases. In 2016, an estimated 1.2 billion people worldwide suffered from anemia because of iron deficiency [13]. This condition is a leading cause of disability-adjusted life years in LMICs and the fourth leading cause globally, particularly impacting children and premenopausal women [13]. More than 40% of children aged <5 y worldwide are anemic, with most cases caused by iron deficiency [13,14]. African infants face particularly high rates of anemia, with >70% anemic at the time of routine vaccinations in their first year [15,16]. In the Gambia, infants have a high prevalence of anemia and extremely low serum iron concentrations (<5 μmol/L) for much of their first year of life [17,18]. Iron deficiency is not exclusive to LMICs; it is a concern in high-income countries as well, where breastfeeding infants, young children, women with heavy menstrual bleeding [19], pregnant women, and individuals with chronic inflammation are at higher risk [20,21]. Iron deficiency also contributes to the anemia of the elderly [22], and is associated with some malignancies including colorectal cancer [23].

Globally, poor nutrition is the primary cause of iron deficiency, due to iron-poor diets with low bioavailability [24] resulting in the depletion of body iron stores. However, “functional” iron deficiency can occur, alone or in parallel, because of chronically elevated hepcidin in the context of infection and inflammation [25]. This situation restricts duodenal iron absorption and sequesters body iron in the reticuloendothelial system and liver, irrespective of iron stores [26]. Functional iron deficiency is prevalent in areas with high infection rates [27]. In many common diseases, such as cancer, chronic kidney disease, obesity, or autoimmune disease [25], elevated hepcidin limits systemic iron availability and causes low serum iron (hypoferremia), and this often aggravates true iron deficiency because of depleted body iron stores. Common gastrointestinal diseases leading to malabsorption and increased iron losses also cause iron deficiency; for instance, in inflammatory bowel disease and celiac disease, characterized by local and systemic inflammation and disrupted gut epithelial integrity [28]. Thus, the high prevalence of iron deficiency worldwide arises in large part from a combination of low dietary iron and/or high hepcidin concentrations induced by inflammation and infection [24,27]. As discussed below, the resulting hypoferremia not only limits erythropoiesis but also may limit adaptive immune responses.

Iron and Immunometabolism

Several older reviews [8,29,30] proposed various mechanisms through which poor iron status might impact adaptive immunity. In certain animal and cell models, iron deficiency was associated with impaired T-cell activation and proliferation. In some studies, iron deficiency reduced B-cell numbers or function, but in others, it did not. Many of these older studies used now outdated methods to judge immune response and often used iron chelators to reduce iron availability, limiting their interpretation, as chelators do not equate to physiologically decreasing transferrin iron availability [8,[29], [30], [31]]. However, in the past decade, a growing number of well-designed studies have provided new insights into the effects of iron restriction on adaptive immunity. This review briefly summarizes older studies but focuses on these newer studies.

The first compelling genetic evidence supporting the involvement of iron in human adaptive immunity stems from the analysis of individuals within 2 families from Kuwait and Saudi Arabia characterized by severe immunodeficiency and heightened susceptibility to infections and early mortality [32]. These individuals were found to carry a hypomorphic mutation in transferrin receptor (TFRC), the gene encoding TFRC 1. This mutation diminishes the efficiency with which immune cells take up iron bound to transferrin from the bloodstream. Despite maintaining normal numbers of T-, B-, and NK-cells, these patients exhibited a deficiency in circulating IgG and a reduction in the count of circulating memory B-cells and neutrophils. Furthermore, their ex vivo T- and B-cell proliferation exhibited defects, which could be corrected by providing supraphysiologic concentrations of elemental iron, thus circumventing the impairment in TFRC function. Recently, mice harboring the analogous mutation in TFRC mirrored these lymphocyte activation defects in both ex vivo and in vivo settings, as discussed in more detail below [33]. These findings align with earlier studies conducted in animal models and in vitro, demonstrating the significance of transferrin-bound iron uptake for optimal lymphocyte activation [34,35]. Complete inhibition of transferrin-bound iron uptake is known to impede lymphocyte development [36]. The seminal study conducted by Jabara et al. [32] is the first report of human immunodeficiency caused by defective iron transport and underscores the necessity of iron for adaptive immune responses.

A recent Chinese study [37] highlighted the significance of iron in the activation of cyclin E and the proliferation of B-cells. Compared with control mice, in mice with iron deficiency because of iron-deficient diets, there was a significant decrease in circulating mature B-cells and mature B-cells in the bone marrow. However, the population of immature B-cells in the bone marrow remained unaffected. There was no significant difference in the proportion of splenic follicular B-cells and marginal zone B-cells between iron-deficient mice and control mice. Nevertheless, when these mice were immunized with 2 types of T-independent (TI) antigens, the secretion of antigen-specific IgG3 and IgM was significantly diminished in iron-deficient mice compared with control mice. Similarly, in mice with iron deficiency, immunization with a T-dependent antigen, 2,4-dinitrophenyl – keyhole limpet hemocyanin, resulted in a drastic reduction in antigen-specific IgG1 and IgM production compared with control mice. Ten days postimmunization, iron-deficient mice exhibited a significant reduction in splenic germinal center (GC) B-cells, and fewer GC regions were present compared with control mice. Together these results indicate that proliferation of activated B-cells and antibody production during an immune response is more sensitive to iron deficiency than the steady-state development of B-cells. Moreover, similar inhibitory effects on mature B-cell populations in the spleen and circulating mature B-cells in the bone marrow were observed in mice with a knockout of 6-transmembrane epithelial antigen of the prostate 3, a ferrireductase crucial for iron uptake. Iron was demonstrated to play an essential role in B-cell proliferation, as both iron deficiency and inhibition of α-ketoglutarate suppressed cyclin E1 induction and the entry of B-cells into the S phase upon activation. Three iron-dependent demethylases were identified as responsible for histone 3 lysine 9 demethylation at the cyclin E1 promoter, cyclin E1 induction, and B-cell proliferation. These important findings underscore the critical role of iron availability to support the proliferation of activated antigen-specific B-cells [37].

Frost et al. [33] in a series of studies using cell cultures and various genetic and animal models, demonstrated that hypoferremia induced by injecting minihepcidin, a hepcidin mimetic, can hinder the response to vaccines and influenza virus infections. The authors also demonstrated that iron regulatory proteins (IRP 1/2) controlling TFRC-mediated T-cell iron acquisition are crucial for T-cell proliferation after activation. The significant increase in efforts to acquire extracellular iron by activated T-cells was highlighted by the massive increase in TFRC expression observed in activated T-cells, reaching ∼1 million copies of protein per cell within 24 h [38], coupled with increased concentrations of transmembrane iron importers. Following activation, CD8 T-cells altered their ability to acquire iron, which is essential for T-cell mitochondrial and effector functions, optimal metabolic activity, and cell-cycle progression. Even a temporary decrease in serum iron during primary immune responses negatively affected the quality and quantity of subsequent T-cell memory and recall capacity in mice. Low-iron availability inhibited iron-dependent processes such as oxidative metabolism and DNA synthesis in T-cells. This finding is similar to those shown in B-cells, where DNA and histone demethylases are iron-dependent, and iron chelation reduces their activity and alters cell-cycle behavior [37,39]. It should be noted that beyond these processes, other essential cellular pathways integral to T-cell biology, including epigenetic reprogramming and hypoxia sensing, also depend on iron and these merit further investigation. The data of Frost et al. [33] also suggested that reduced CD25 expression and impaired IL-2 sensing may contribute to the diminished quality of effector/memory CD8 T-cells observed during iron deficiency [40]. Yarosz et al. [41] also reported that iron may modulate lymphocyte responses to IL-2R signaling. Within CD4 T-cells, iron plays a pivotal role in regulating the expression of proinflammatory cytokines, specifically IL-2 and granulocyte-macrophage colony-stimulating factor (GM-CSF) (but not IFNγ and TNFα); this regulation occurs through interactions with the RNA binding protein known as PCBP1 [42].

Frost et al. [33] also observed impaired antibody responses in iron-deficient piglets. Growing piglets rapidly become iron deficient and anemic unless supplemented with iron. When iron-supplemented and nonsupplemented piglets were vaccinated against Mycoplasma hyopneumoniae, the iron-supplemented piglets had normal hemoglobin and serum iron and exhibited a 2-fold increase in vaccine-specific antibody response 2 wk postvaccination compared with the iron-deficient animals (Figure 2). These animal data convincingly indicate that, in the context of iron deficiency anemia, iron supplementation not only maintains hemoglobin but also enhances responses to vaccination. Frost et al. [43] then studied mice made hypoferremic by minihepcidin and exposed to influenza A virus respiratory infection. Hypoferremia attenuated T-cell, B-cell, and neutralizing antibody responses to the infection. Hypoferremic mice displayed fewer virus-specific CD8 T-cells in the spleen and lungs, reduced granzyme B-expressing splenic CD8 T-cells, and fewer T-follicular helper cells (Tfh), antigen-experienced CD44+ CD4 cells, and GC B-cells in the mediastinal lymph nodes during infection. Lung mRNA expression of proinflammatory cytokines IL-6 and TNF-α was higher. On day 10 postinfection, cellular immune responses in the spleen were reduced, circulating influenza neutralizing antibodies were undetectable, and lung viral load was significantly higher. The mice exhibited more severe inflammation and lung pathology and failed to recover from infection-induced weight loss (Figure 3A and B). These data demonstrate that in mice with hypoferremia, iron limitation suppressed adaptive immune response, and reduced virus clearance ability, resulting in more severe pulmonary disease with prolonged morbidity. It should be noted that the more severe disease in the iron-deficient mice may have also been because of defects in innate immunity, specifically in neutrophil numbers and function, induced by iron restriction. In a similar model of hepcidin-induced hypoferremia in mice, the same researchers reported reduced granulocyte numbers, reduced inflammation-induced neutrophil accumulation in the spleen and peritoneal cavity, and impaired neutrophil phagocytosis and killing of bacteria [43]. A parallel study confirmed the particular sensitivity of neutrophil production to lack of iron availability (via IRP1/2 deletion) [44], and in humans, the TFRC mutation mentioned earlier was also clearly associated with neutropenia [45]. In summary, the studies of Frost et al. [33,43] clearly show that the heightened metabolism of activated lymphocytes relies on increased iron acquisition, and that hypoferremia, especially when occurring during lymphocyte expansion, can adversely impact multiple facets of immunity.

FIGURE 2.

Correcting iron deficiency improves vaccine response in piglets. Iron supplementation restored normal iron concentrations in iron-deficient piglets. The antibody response to Mycoplasma hyopneumoniae vaccination was assessed by measuring the improvement in percentage block at postnatal day 28 compared with the preimmunization percentage block at postnatal day 14. Two-tailed unpaired Student’s t-test. Adapted with permission from Frost et al. [33].

FIGURE 3.

Iron deficiency suppresses adaptive immune responses to influenza infection in mice. Respiratory influenza A virus infection in mice treated with control or treated with mHep to induce prolonged hypoferremia. Focal inflammation of the airway was observed in all infected mice. Mice treated with mHep exhibited suppressed lymphocyte responses and more pronounced perivascular/peribronchiolar lung inflammation, extending into the alveolar parenchyma. (A) Leder staining of representative lung sections from mice on day 10 postinfluenza infection from each group: 200×, scale bar 20 μm. Polymorphonuclear cells stained with pink cytoplasm indicated by arrows. (B) Lung inflammation and neutrophil infiltration scoring. Means ± SD. Kruskal–Wallis test with correction for multiple comparisons. mHep, minihepcidin. Adapted with permission from Frost et al. [33].

Iron Deficiency and Vaccine Responses in Humans

Most older studies on iron status and vaccines have been small and cross-sectional, and have produced equivocal results [8,30]. In South Africa, the response to a diphtheria toxoid vaccine was measured in children with iron deficiency anemia, milder iron deficiency, and nonanemic controls; the numbers with a positive response were 12 of 14 in the controls, 2 of 11 in the anemic children and 0 of 7 in those with iron deficiency [46]. An observational study in 8 LMICs assessing links between enteropathogenic infection, undernutrition, and child health (n = 1449) found borderline significant associations between soluble transferrin receptor (sTfR) and oral polio vaccine response at ages 7 and 15 mo [47]. In a cross-sectional study of Ecuadorian children (n = 1162) who had previously received diphtheria and tetanus vaccines, children with anemia aged > 1 y had lower antibody concentrations than controls and more anemic children were seronegative for diphtheria compared with controls (12.5 compared with 18.1%, respectively) [48]. Vaccine trials for diseases such as diphtheria, tetanus [49], and typhoid [50] did not demonstrate a clear association between iron deficiency and vaccine efficacy, although these studies were likely underpowered. Notably, lower vaccine efficacy has been reported for specific vaccines, including measles and live attenuated influenza vaccine, among infants in LMICs where the prevalence of iron deficiency is high [2,4].

In a recent cross-sectional study conducted in China [37], the correlation between iron metabolism and antibody responses to the measles vaccine was investigated. The study involved a heterogeneous population of 118 individuals aged ≥10 y, and it measured measles-vaccine-specific IgG antibody titers, serum iron concentrations, and transferrin saturation. Individuals with iron deficiency (defined as serum iron <50 μg/dL) exhibited markedly lower measles vaccine-specific IgG antibody titers compared with those with normal iron concentrations (serum iron ≥50 μg/dL) (Figure 4). Similar trends were observed for transferrin saturation, where individuals with iron deficiency (defined as transferrin saturation <16%) showed lower antibody titers. Furthermore, individuals with a low-antibody response (measles vaccine antibody titer <200 mIU/mL) had significantly lower concentrations of serum iron compared with those with a normal (measles vaccine antibody titer between 200 and 800 mIU/mL) or high antibody response (measles vaccine antibody titer >800 mIU/mL). The same pattern was observed for transferrin saturation, highlighting a consistent association between iron status and the strength of the antibody response to the measles vaccine. It should be noted that many potential confounders of these associations were not reported [37].

FIGURE 4.

Response to the measles vaccine is positively correlated with serum iron in humans. (A) Serum iron concentration in iron deficient vs. normal subjects. (B) Measles vaccine-specific IgG antibody titers in patients with iron deficiency (defined as serum iron <50 μg/dL) vs. normal subjects. ∗∗P < 0.01, Student’s t-test. Adapted with permission from Jiang et al. [37].

A recent prospective observational study revealed a connection between anemia in young children and alterations in their immune system composition [51]. The study focused on 15 children aged <5 y from Tanzania and Mozambique, using detailed immunophenotyping of longitudinal blood samples collected during a malaria vaccine phase III trial. The comparison involved gene expression and immune profiles in blood samples from age-matched anemic (<8.5 g/dL hemoglobin, n = 6) and nonanemic (>10.5 g/dL hemoglobin, n = 9) children. The analysis indicated that anemic children exhibited lower frequencies of recent thymic emigrant T-cells, isotype-switched memory B-cells, and plasmablasts, crucial components for protective immunity following vaccination. To delve into the mechanisms behind this detrimental effect, the authors conducted studies in cell cultures [51]. Cellular iron concentrations were manipulated by supplementing media with apo-transferrin to facilitate iron uptake by lymphocytes and by limiting intracellular iron using an iron chelator. Increased cellular iron enhanced B-cell proliferation, and conversely, proliferation was impaired when iron availability was reduced. Increased bioavailability of iron also enhanced plasmablast differentiation, accompanied by an increase in class-switched antibody production in the cell cultures. This demonstration that sufficient iron is essential for optimal production of plasmablasts and IgG responses by human B-cells in vitro is consistent with in vivo work in mice [37,52,53], including recent studies in the context of immune responses to murine malaria infection [54]. Taken together, these findings indicate a direct impact of iron supply on B-cell biology, suggesting that dietary iron deficiency or infection may impair the development of adaptive immunity in children.

Convincing human data linking iron deficiency to impaired vaccine response comes from studies in young children in Kenya [55]. In Kenya, most infants have iron deficiency anemia around the time they receive their routine pediatric vaccinations [15,16]. Two complementary studies with differing designs were done to determine if anemia and/or iron deficiency during infancy affect vaccine response [55]. In a birth cohort study, 303 infants were followed from birth to age 18 mo, and all received 3-valent oral polio, diphtheria-tetanus-whole cell pertussis-Haemophilus influenzae type b (Hib) vaccine, 10-valent pneumococcal-conjugate vaccine, and measles vaccine. Primary outcomes were antivaccine-IgG and seroconversion at age 24 wk and 18 mo. Already at age 10 and 14 wk, just over half of infants were anemic. At age 24 wk and 12 mo, >90% of infants were anemic, nearly all the anemia was moderate or severe, and low serum ferritin and high sTfR suggested iron depletion in most infants. Controlling for sex, birthweight, anthropometric indices, and maternal transfer of antibodies, hemoglobin at the time of vaccination was the strongest positive predictor of the following: 1) antidiphtheria- and antipertussis-IgG at 24 wk and 18 mo, 2) antipertussis filamentous hemagglutinin-IgG at 24 wk, and 3) antipneumococcus 19-IgG at 18 mo. Anemia and serum sTfR at the time of vaccination were the strongest predictors of seroconversion against diphtheria and pneumococcus 19 at 18 mo.

In the second study, a randomized trial cohort follow-up [55], children received a micronutrient powder with 5 mg iron daily or without iron for 4 mo starting at age 7.5 mo and received measles vaccine at 9 and 18 mo; primary outcomes were antimeasles-IgG, seroconversion and avidity at age 11.5 mo and 4.5 y. In the trial, 155 infants were recruited, and 127 and 88 were assessed at age 11.5 mo and 4.5 y. Significant benefits of iron were seen in the primary measles vaccine response (at 11.5 mo) but not at 4.5 y. Compared with infants that did not receive iron, those who received iron at the time of vaccination had higher antimeasles vaccine-IgG (P = 0.0415), lower seronegativity rate (15.3 compared with 23.8%, P = 0.0531) and greater IgG avidity (29.1 compared with 26.8%, P = 0.0425) at 11.5 mo (Figure 5). Most measles-related deaths worldwide occur in young children in LMICs [56] and the measles vaccine has the lowest effectiveness in the WHO Africa region where its median effectiveness is only 73%, with a range of 26%–95% [56,57]. In summary, these Kenyan studies suggest that anemia and iron deficiency at the time of vaccination predict decreased response to diphtheria, pertussis, and pneumococcal vaccines and that primary response to the measles vaccine may be increased by iron supplementation at time of vaccination.

FIGURE 5.

In a randomized controlled trial of iron fortification in Kenyan infants, antimeasles serum IgG concentrations, seroconversion, and IgG avidity were greater in the iron group compared with the control group. (A) Antimeasles serum IgG concentrations at age 7.5 and 11.5 mo (at baseline and end of intervention), (B) seroconversion at age 11.5 mo (at end of intervention), and (C) IgG avidity at age 11.5 mo (at end of intervention). Adapted with permission from Stoffel et al. [55].

There are few data from high-income countries examining links between iron and vaccine response. One small study found that among elderly hospitalized patients receiving influenza vaccines, nonresponse was associated with low serum iron concentrations (mean, 8.34 μmol/L compared with 16.00 μmol/L in responders) [58]. Certain high-income populations that are at risk for iron deficiency, such as individuals with celiac disease [59], obesity [5], and chronic kidney disease [6], are reported to exhibit poor responses to the hepatitis B virus vaccine. Vaccines generally underperform in patients with inflammatory bowel disease, who are often iron-deficient [60]. The inactivated flu vaccine has been reported to perform inadequately in chronic kidney disease patients [61] and the elderly [7], 2 populations at higher risk for iron deficiency. Other mechanisms that may affect vaccine response in these patients include the use of immunosuppressive therapy, immune dysregulation from uremia, immunosenescence, and the presence of comorbidities, which can further impair immune function.

A rare type of iron deficiency anemia in humans, known as iron-refractory iron deficiency anemia (IRIDA), is caused by elevated hepcidin concentrations in individuals with mutations in a protease responsible for suppressing hepcidin [62]. In a small cross-sectional study, IRIDA patients (n = 12) with iron deficiency anemia and hypoferremia were compared with iron-sufficient controls of similar age [33]). The IRIDA group displayed lower serum concentrations of IgG against Hib (P = 0.066) and anti-Streptococcus pneumoniae serotype 1 (PS1) (P = 0.005) (Figure 6), the latter a major cause of invasive pneumococcal disease globally [63]. Specifically for antiPS1, none of the IRIDA patients reached an antibody concentration exceeding the WHO-recommended protective threshold against S. pneumoniae serotypes. Although these findings indicate IRIDA is associated with reduced antibody concentrations against several significant pathogens, many potential confounders were not reported.

FIGURE 6.

Iron-refractory anemic humans have lower concentrations of antibodies to Streptococcus pneumoniae. Comparison of the frequency of IRIDA patients with TMPRSS6 mutations and non-anemic healthy controls with antibody concentrations against Streptococcus pneumoniae serotype 1 exceeding the protective threshold of 0.35 mg/mL. Fisher exact test. IRIDA, iron-refractory iron deficiency anemia; TMPRSS6, transmembrane serine protease 6. Adapted with permission from Frost et al. [33].

As mentioned above, the European Hematology Association in 2021 recommended correction of iron deficiency before administration of COVID-19 vaccine [12]. The direct evidence to support this recommendation is lacking. Several small retrospective studies suggested that patients with iron deficiency may be at increased risk of severe COVID-19 disease [[64], [65], [66]]. However, in these studies, low serum iron may have been a marker of disease severity because of underlying inflammation-induced hypoferremia. In a study of patients with chronic kidney disease, higher serum ferritin concentrations were associated with an improved response to COVID-19 vaccination [67]. A large retrospective, longitudinal cohort study in Israel [68] examined the effectiveness of the BNT162b2 mRNA COVID-19 vaccine in preventing COVID–19-related severe respiratory morbidity and death in individuals with or without a history of iron deficiency in the previous 2 y. The COVID-19 vaccine was >90% effective in preventing severe acute respiratory syndrom coronavirus 2 (SARS-CoV-2) infection in the 3 wk after the second vaccination, irrespective of iron status. In a recent randomized controlled trial of intravenous iron compared with control in iron-deficient Dutch kidney transplant recipients (n = 48) given mRNA-1273 or mRNA-BNT162b2 SARS-CoV-2 vaccines, the co-primary endpoints were SARS-CoV-2-specific antireceptor binding domain-IgG and T-lymphocyte reactivity against SARS-CoV-2 at 4 wk after the second vaccination. Iron supplementation corrected iron deficiency but did not improve the humoral or cellular immune response against SARS-CoV-2 after 3 vaccinations [69]. Another controlled intervention trial that gave iron-deficient anemic Kenyan women intravenous iron at the time of vaccination with the ChAdOx1 SARS-CoV-2 vaccine reported significantly higher antispike protein- and antireceptor binding domain-IgG after the first vaccination but not after the second (unpublished data).

Of note, because of the distinct immune responses elicited by different vaccine types [70], the impact of iron deficiency may vary across vaccines. Live vaccines like measles and protein antigens from diphtheria and pertussis typically induce robust T-dependent responses, involving the activation of antigen-specific Tfh cells and the formation of GCs that produce memory B-cells. These processes are highly dependent on iron [33,37]. On the other hand, bacterial polysaccharide antigens, such as those in Hib and pneumococcal polysaccharide vaccines (PnPs), typically induce TI responses and do not lead to the generation of Tfh cells or GCs, especially if not effectively conjugated with an adjuvant protein [71]. Consequently, the immunogenicity of polysaccharide antigens, even when conjugated, might be less influenced by iron availability. This might contribute to the observation in the Kenyan birth cohort study [55], where anemia was predictive of the response to diphtheria, pertussis, and measles vaccines, but not to Hib and most serotypes of PnPs.

The Differing Role of Iron Availability in the Innate Compared with the Adaptive Immune Response

An apparent paradox raised by the above data suggesting iron deficiency and hypoferremia impair adaptive immunity is the well-recognized protective role that hepcidin-induced hypoferremia plays in the innate immune response [72]. Hepcidin plays a dual role in regulating iron concentrations and responding to acute inflammation. It's expression is triggered by inflammation, primarily through the cytokine IL-6 [73]. In times of infection or inflammation, hepcidin concentrations typically increase, leading to a reduction in macrophagic iron recycling and an accumulation of iron in reticuloendothelial cells. This limits the availability of systemic iron, restricting its ability to support pathogen growth [72]. Thus, hepcidin-induced hypoferremia is a crucial aspect of the innate immune response to specific infections [[74], [75], [76]]. This form of nutritional immunity is proposed to be essential for controlling extracellular siderophilic bacterial pathogens. In mice, hepcidin-mediated hypoferremia protects against certain bacterial infections [74] and the liver stage of Plasmodium infection [76]. In humans, elevated hepcidin and hypoferremia are observed after experimental norovirus, typhoid, and malaria infections [[77], [78], [79]], and during initial viremia in HIV-1-infection [80]. However, although inflammatory hypoferremia can be beneficial in infections with single-cell organisms that rely on acquiring iron from their host, there is a lack of evidence supporting the protective role of low serum iron in viral infections. A potential explanation for this apparent paradox is that denying iron to certain siderophilic pathogens benefits the host, but if the iron requirements of the pathogen are relatively low (such as for viral replication), inflammatory hypoferremia instead risks inhibiting a protective adaptive immune response, allowing viral infection to persist and potentially exacerbating disease [81]. This trade-off needs to be better understood and should be a focus of future research.

Recently Completed or Ongoing Intervention Trials of Iron and Vaccine Response

A search of clinical trials registries in January 2024 (clinicaltrials.gov, PACTR) identified 6 recently completed or ongoing trials examining the effects of iron interventions or iron status on vaccine response. These include interventions with iron supplements and iron-fortified foods, and studies in infants, women of reproductive age, and pregnant women using a variety of different vaccines, in Africa and Asia. As these trials begin to report results, the link between iron status and vaccine response should become clearer.

Future Research in the Field of Iron and Adaptive Immunity

Mechanistic effects of low iron on lymphocytes

Metabolic reorganization occurs after lymphocyte activation and influences the proliferation, polarization, and fate of T- and B-cells. But how does iron deficiency influence these processes, and does low iron differentially affect the generation and longevity of particular T-cell subsets and/or specific aspects of B-cell physiology?

Effect on lymphoid tissues and barrier-site immunity

Low iron availability inhibits the development of immune responses and has cell-intrinsic effects on activated lymphocytes. However, does systemic iron deficiency also influence the structure and function of lymphoid organs and affect immunity in specific tissues, for example, barrier tissues like the gut and skin?

Chronic effects on immune development across populations

Persistent iron deficiency is common in many LMIC populations, particularly in infants and children. Does chronic iron deficiency have long-term effects on the development of the immune system during childhood, either through altering hematopoietic production and function of immune cell subsets, or on the generation and maintenance of immunologic memory?

Interactions of iron deficiency with other nutrients

Iron deficiency frequently is accompanied by other nutritional deficits that may affect iron absorption and/or metabolism, e.g., riboflavin and vitamin A deficiency, and other deficiencies that impact immunity, e.g., zinc. Do coexisting nutritional deficiencies exacerbate the impairment of immune responses by low iron availability? Conversely, are there any nutritional supplements besides, or in addition to iron, that can counteract the effects of iron deficiency on immunity, and if so, what is the mechanism?

Which vaccine types are more susceptible to the effects of iron deficiency?

As mentioned above, it appears that vaccines that induce TI and T-dependent responses may have different sensitivities to iron deficiency. However, it will be important to discover how other important vaccine types (e.g., mRNA platforms, oral vaccines, and live attenuated vaccines) perform under conditions of iron deficiency, both in terms of the acute response and induction of memory. It will also be of value to investigate the effects of hypoferremia on other immunotherapies, for example, checkpoint inhibition and cellular therapies.

Iron-oriented therapeutic interventions to improve immunity

If data indicates that increasing iron availability improves immunity, then determining the best and safest mechanism to supply iron (route, dose, and timing regimens) to the responding immune system will need to be determined. Determining the immune benefits of correcting differing severities of iron deficiency, i.e., depleted stores, iron-deficient erythropoiesis, and iron deficiency anemia, would also be important. Various modes of intervention, e.g., oral iron [82,83], intravenous iron [84], and hepcidin inhibition [[85], [86], [87], [88]], could be investigated.

Discussion

As our knowledge of systemic iron homeostasis and immunity advances, iron is gaining recognition as a crucial factor in adaptive immunity. The availability of iron seems to play a central role in shaping the adaptive immune response to infections and vaccinations. If iron deficiency limits adaptive immunity, then normalizing iron status could be an appealing intervention to enhance vaccine responses. However, the evidence supporting the therapeutic targeting of iron to support adaptive immunity is currently limited. It is critical to conduct prospective randomized controlled trials to rigorously test the hypothesis that iron could enhance immune responses to vaccines in individuals with iron deficiency. Ongoing research in this area is actively exploring these possibilities. These trials will be essential to determine which vaccines are particularly affected by iron deficiency and could benefit from iron supplementation, identify the optimal iron delivery regimen for maximizing a sustained vaccine response, and pinpoint the specific populations that should be targeted for iron interventions. Given the global prevalence of anemia and the substantial burden of vaccine-preventable diseases, even if iron deficiency only moderately diminishes the effectiveness of a subset of vaccines, counteracting this could yield significant benefits.

Author contributions

The authors’ responsibilities were as follows – both authors: conceived the idea for the manuscript; NUS: wrote the first draft; HD: edited the manuscript; and both authors: approved the final version of the manuscript.

Conflict of interest

The authors have no competing interests to declare.

Funding

NUS received support from the Fondation Jean-Jacques et Felicia Lopez-Loreta pour l’Excellence Académique. HD received support from the UK Medical Research Council, MRC Human Immunology Unit grant (MCU_12010/10).