Helminth Seropositivity Inversely Correlated with Th1 and Th17 Cytokines and Severe COVID-19

1. Introduction

The COVID-19 pandemic, caused by SARS-CoV-2, has significantly impacted global health and economies since its emergence in Wuhan, China, in late 2019 [1]. As the virus started to spread globally, healthcare professionals faced a plethora of clinical presentations and outcomes, ranging from asymptomatic infections to severe acute respiratory distress and death [2]. Governments have implemented various public health measures, including lockdowns, social distancing, and vaccination campaigns to mitigate the spread of the virus. Despite these efforts, the pandemic has resulted in 775 million confirmed cases and more than 7 million deaths as of July 7, 2024 [3]. Older age and comorbidities, such as hypertension, diabetes, cardiovascular diseases, chronic respiratory conditions, and obesity, were found to be associated with severe clinical outcomes [4].

A striking aspect of the pandemic is the variation in cases and fatality rates across regions. In contrast to high-income countries, such as the United States and several European nations, the trajectory of the pandemic in Africa has been unexpectedly mild, with lower case numbers and fatality rates [5,6]. While it is widely accepted that younger demographics on the continent have contributed to the milder impact of the pandemic, emerging hypotheses suggest that the high prevalence of helminth infections in many African populations may also have played a role [7,8,9].

Helminths are parasitic worms that are widespread in many regions of the world, particularly in sub-Saharan Africa [10]. It is estimated that at least 1.5 billion people, predominantly in sub-Saharan Africa, are currently infected with helminths, including soil-transmitted helminths, schistosomes, and lymphatic filariae, leading to significant health issues such as malnutrition, anemia, chronic illness, and perpetuating cycles of poverty in these regions​ [10]​. In addition, helminths are renowned for their ability to dampen pro-inflammatory responses and promote anti-inflammatory pathways, skewing their host’s immune system towards Th2 and regulatory responses, including cytokines such as IL-4, IL-5, IL-13, and TGF-β [11,12,13,14]. Through these unique properties, they can potentially affect the course of viral co-infections, such as SARS-CoV-2, altering the severity and progression of diseases.

The impact of helminth co-infection on the clinical outcomes of viral infections has not yet been fully elucidated [15]. Emerging hypotheses suggest that helminth co-infections can attenuate the severity of COVID-19 through their immunomodulatory properties [16]. Previous studies have demonstrated that helminths can significantly affect host susceptibility and the clinical outcomes of viral and bacterial infections [17,18,19,20,21]. A recent in vitro study investigating the impact of antigen extracts from three major helminth parasites, Onchocerca volvulus, Brugia malayi, and Ascaris lumbricoides, on immune reactivity to SARS-CoV-2 peptides demonstrated that all tested helminth antigens significantly reduced the activation of SARS-CoV-2-reactive CD4+ T helper cells [22,23]. However, the flip side has potential disadvantages, as helminth infections have been associated with diminished vaccine efficacy, increased susceptibility to concurrent infections, and potential interference with tumor immunosurveillance [18,24,25,26,27,28,29,30,31,32,33,34].

One potential explanation is that helminth infections may trigger a Th2-skewed and regulatory immune response, which in turn inhibits or regulates the Th1 immune response. Suppression of the Th1 immune response can also dampen the hyperinflammation linked to severe COVID-19, resulting in improved health outcomes [15,26,35,36,37].

Building on these findings, the present study aimed to elucidate the immunomodulatory effects of helminths on SARS-CoV-2 specific Th1 and Th17 immune responses and their associated clinical outcomes in an African population. This study also examined the proportion of asymptomatic COVID-19 cases among individuals concurrently exposed to helminth antigens. By examining the relationship between helminth seropositivity, cytokine profiles, and disease severity, we sought to shed light on the complex interplay between helminth co-infection and COVID-19 severity. This investigation promises to enhance our understanding of the dynamic interplay between parasitic infections and viral immunity, offering new insights into immune regulation determinants of disease outcomes and shaping public health policies in helminth endemic areas.

2. Materials and Methods

2.1. Participants, Sample Collection, and Ethics

A retrospective cohort cross-sectional study was conducted using 104 archived plasma samples from the Ashanti region collected during the third wave of the pandemic from June 2022 to December 2022. To avoid bias and ensure representativeness, all participants, including a diverse patient population across the region and reflecting the region's demographics, were included in the study. Although the initial recruitment goal was to recruit 200 participants, the final sample size was 104 participants because of the low incidence of active COVID-19 infections in the region. Recruitment was closed at the end of the wave in Ghana to maintain consistency.

The recruitment was conducted by teams from the Kumasi Center for Collaborative Research in Tropical Medicine (KCCR) in Ghana. All personnel involved in sample collection were thoroughly trained and provided with standardized protocols. The demographic and clinical data of all participants were collected using predefined questionnaires. Blood samples were collected in EDTA tubes under aseptic conditions and subsequently processed for plasma by density gradient centrifugation as previously described [38]. The plasma was then aliquoted into tubes and stored at -20°C before being shipped to the Bonn University Hospital, Institute of Medical Microbiology, Immunology, and Parasitology (IMMIP) for further analysis. Samples were collected and prepared according to the guidelines of the Committee on Human Research and Publication and Ethics at the School of Medical Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana (reference CHRPE/AP/072/21). Ethical approval for the investigations performed in Germany was obtained from the Ethics Committee of the University Hospital Bonn (Lfd. Nr. 439/20).

2.2. Quantification of SARS-CoV-2 Specific IgA and IgG and Severity Classification

Enzyme-linked immunosorbent assays (ELISAs) were performed at the IMMIP, University Hospital Bonn, Germany. The Euroimmun SARS-CoV-2 IgG/IgA ELISA kit (Euroimmun, Lübeck, Germany) was used to quantify SARS-CoV-2 specific IgG and IgA with the automated Euroimmun analyzer I, adhering to the manufacturer’s instructions and following the protocol previously described [39]. Optical density (OD) was measured at 450 nm. The expressions of SARS-CoV-2 specific IgG and IgA were calculated, and the results were interpreted according to the manufacturer’s protocol. Initial testing by PCR, as previously described [40] was used to grade COVID-19 severity according to the National Institutes of Health (NIH) guidelines as follows: Asymptomatic or presymptomatic infection are Individuals who tested positive for SARS-CoV-2 by PCR but had no symptoms consistent with COVID-19. Mild illness refers to individuals exhibiting any of the various signs and symptoms of COVID-19 (e.g., fever, cough, sore throat, malaise, headache, muscle pain, nausea, vomiting, diarrhoea, loss of taste and smell) but did not have shortness of breath or dyspnoea. Moderate illness refers to individuals who showed evidence of lower respiratory disease during clinical assessment and had an oxygen saturation measured by pulse oximetry (SpO₂) ≥94% on room air at sea level. Severe illness refers to individuals who had SpO₂ <94% on room air at sea level, a ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO₂/FiO₂) <300 mm Hg, a respiratory rate >30 breaths/min, or lung infiltrates >50%[41].

2.3. Quantification of Helminth-Specific IgG

Ascaris lumbricoides, Strongyloides ratti, and Acanthocheilonema viteae IgG ELISA Kits (DRG Instruments GmbH, Marburg, Germany and Bordier Affinity Products, Chantanerie, Switzerland) were used to detect specific IgG in the plasma of recruited individuals. All samples were examined for Strongyloides ratti, Acanthocheilonema viteae, and Ascaris lumbricoides. Strongyloides ratti was used as a homologous antigen for human Strongyloides species, and Acanthocheilonema viteae was used as a homologous antigen for human filarial nematode species. The procedure was executed manually according to the manufacturer’s protocol as previously reported [17]. Optical density (OD) was measured at 450/620 nm.

2.4. Quantification of Neutralizing Antibody Levels

To assess the levels of neutralizing antibodies specific to SARS-CoV-2 variants, we employed the SARS-CoV-2 Variants Neutralizing Antibody 6-Plex ProcartaPlex Panel (Thermo Fisher Scientific) following the manufacturer's protocol as previously outlined [17]. This assay facilitates the quantification of SARS-CoV-2 neutralizing antibodies in serum and plasma, enabling direct comparison of neutralizing antibody levels against five SARS-CoV-2 variants, namely the wild-type and variants B.1.1.7 (α), B.1.351 (β), P1 (γ), B.1.617.2 (δ), and B.1.1.529 (ο). The samples were analyzed using a MagPix Luminex instrument. Data analysis was conducted using the ProcartaPlex Analyst application (Thermo Fisher Scientific) and the provided neutralization (%) equation.

2.5. Quantification of Systemic Cytokine and Chemokine Expression

To assess systemic cytokine expression, the Cytokine Storm 21-Plex Human ProcartaPlex Panel (Thermo Fisher Scientific) was used according to the manufacturer's instructions, as previously described [17]. This panel includes 21 cytokines and markers associated with cytokine release syndrome (CRS), including G-CSF (CSF-3), GM-CSF, IFN alpha, IFN gamma, IL-1 beta, IL-2, IL-4, IL-5, IL-6, IL-8 (CXCL8), IL-10, IL-12p70, IL-13, IL-17A (CTLA-8), IL-18, IP-10 (CXCL10), MCP-1 (CCL2), MIP-1 alpha (CCL3), MIP-1 beta (CCL4), TNF alpha, and TNF beta. Cytokine quantification was performed using the MagPix Luminex instrument. Data were analyzed using the ProcartaPlex™ online analysis application (ThermoFisher Scientific).

2.6. Statistical Analysis

The primary objective of this study was to examine the proportion of asymptomatic COVID-19 cases among individuals concurrently exposed to helminth antigens. The baseline characteristics were reported as the median with interquartile range for continuous variables such as age, Ascaris lumbricoides, Strongyloides ratti, and Acanthocheilonema viteae specific IgG, and as proportions for categorical variables such as sex and infection status (mono- and co-exposure). The χ² test was used to compare the proportions across different categories of COVID-19 severity, providing additional insights into the distribution of categorical variables. The differences in helminth-specific IgG and cytokine expression among the severity groups were analyzed using the Kruskal-Wallis test. Due to the small number of severe cases in our study, moderate and severe cases were combined for the analysis. The Mann-Whitney U test was used to compare cytokine expression and SARS-CoV-2 specific IgA/IgG between the mono- and co-exposed groups. The data analysis was performed using SPSS V. 26. All graphs were generated using GraphPad Prism version 10.2.0. (La Jolla, CA, USA).

3. Results

3.1. Demographic and Clinical Characteristics

The demographic and clinical characteristics of the study population are summarized in Error! Reference source not found.. The study included 52.9% (95% CI: 42.8% - 62.8%) males and 47.1% (95% CI: 37.2% - 57.2%) females aged 13–90 years. The median age of the participants was 33 years (IQR: 20 - 48). The most participative age group consisted of individuals aged between 11 and 22 years, accounting for 28.8% (95% CI: 20.4% - 38.6%), while the least were individuals aged between 44 and 54 years [10.6% (95% CI: 5.4% - 18.1%)]. Of the 104 participants, 29.8% (95% CI: 21.2% - 39.6%) were healthy, 25.0% (95% CI: 17.0% - 34.4%) were asymptomatic, 15.4% (95% CI: 9.1% - 23.8%) were mild, 27.9% (95% CI: 19.5% - 37.5%) were moderate, while only 1.9% (95% CI: 0.2% - 6.8%) exhibited severe SARS-CoV-2 related symptoms. Moderate/severe cases were predominantly observed in older age groups, suggesting a potentially higher risk of severe infection among older individuals (≥55) compared to younger age groups (11-22).

Table 1. Demographic and clinical data of the study population.

		Clinical outcome n (%)
	Number Examined n (%)	Uninfected n = 31	Asymptomatic n = 26	Mild n = 16	Moderate n =29	Severe n = 2
Gender
Male	55 (52.9)	17 (54.8)	18 (69.2)	6 (37.5)	14 (48.3)	0 (0.0)
Female	49 (47.1)	14 (45.2)	8 (30.8)	10 (62.5)	15 (51.7)	2 (100.0)
Age group
11-22	30 (28.8)	14 (45.2)	11 (42.2)	0 (0.0)	5 (17.2)	0 (0.0)
22-32	22 (21.2)	7 (22.6)	4 (15.4)	6 (37.5)	4 (13.8)	1 (50.0)
33-43	20 (19.2)	4 (12.9)	7 (26.9)	3 (18.8)	6 (20.7)	0 (0.0)
44-54	11 (10.6)	5 (16.1)	3 (11.5)	1 (6.5)	2 (6.9)	0 (0.0)
≥55	21 (20.2)	1 (3.2)	1 (3.8)	6 (37.5)	12 (41.4)	1 (50.0)

This table shows demographic and clinical characteristics across different COVID-19 severity levels: asymptomatic, mild, and severe. Each category displays the total number of individuals, age category, and the percentage of individuals who are female and male for each category; the number (n) of individuals included in the study is provided, age group and gender with their percentages in bracket. The number examined column provides the corresponding data for the overall sample.

Table 1. Demographic and clinical data of the study population.

		Clinical outcome n (%)
	Number Examined n (%)	Uninfected n = 31	Asymptomatic n = 26	Mild n = 16	Moderate n =29	Severe n = 2
Gender
Male	55 (52.9)	17 (54.8)	18 (69.2)	6 (37.5)	14 (48.3)	0 (0.0)
Female	49 (47.1)	14 (45.2)	8 (30.8)	10 (62.5)	15 (51.7)	2 (100.0)
Age group
11-22	30 (28.8)	14 (45.2)	11 (42.2)	0 (0.0)	5 (17.2)	0 (0.0)
22-32	22 (21.2)	7 (22.6)	4 (15.4)	6 (37.5)	4 (13.8)	1 (50.0)
33-43	20 (19.2)	4 (12.9)	7 (26.9)	3 (18.8)	6 (20.7)	0 (0.0)
44-54	11 (10.6)	5 (16.1)	3 (11.5)	1 (6.5)	2 (6.9)	0 (0.0)
≥55	21 (20.2)	1 (3.2)	1 (3.8)	6 (37.5)	12 (41.4)	1 (50.0)

This table shows demographic and clinical characteristics across different COVID-19 severity levels: asymptomatic, mild, and severe. Each category displays the total number of individuals, age category, and the percentage of individuals who are female and male for each category; the number (n) of individuals included in the study is provided, age group and gender with their percentages in bracket. The number examined column provides the corresponding data for the overall sample.

3.2. Assessment of SARS-CoV-2 and Helminth Co-exposure Rates

A SARS-CoV-2 seroprevalence rate of 52.9% (95% CI: 42.8% – 62.8%) (Error! Reference source not found.1A) and helminth seroprevalence rate of 46.2% (95% CI: 36.3% - 56.2%) were observed. Ascaris lumbricoides emerged as the predominant helminth exposure at 40.4% (95% CI: 30.9% - 50.5%), whereas the least frequent helminth seropositivity was against Strongyloides ratti [2.9%, (95% CI: 0.6% - 8.2%)]. Cases of multiple helminth seroreactivity were observed only between A. limbricoides and S. ratti at 0.96% (95% CI: 0.0% - 5.2%) (Figure 1B). 50.0% (95% CI: 35.2% - 64.8%) of helminth seropositive individuals were also SARS-CoV-2 seropositive, whereas 43.6% (95% CI: 30.3% - 57.7%) of SARS-CoV-2 seropositive individuals were also helminths seropositive (Figure 1C)

3.3. Helminth-exposed individuals exhibit asymptomatic COVID-19 Clinical outcomes

To determine the influence of helminth exposure on COVID-19 severity, a multivariate regression model was used. Age was found to have a significant impact on disease severity, with older individuals having higher odds of severe COVID-19 outcomes (OR = 1.046, p = 0.003). After adjusting for age and sex, exposure to helminths was significantly associated with a strong protective effect against severe COVID-19 outcomes, with non-exposed individuals having a higher risk of developing more severe COVID-19 outcomes than exposed individuals (OR= 4.115, p=0.018) (Error! Reference source not found.). Specifically, A. lumbricoides exposed individuals had a lower risk of developing severe COVID-19 than non-exposed individuals (OR= 4.038, p=0.028). The other helminths were not statistically significant.

Table 2. Multivariate regression to explore the relationship between infectious status and COVID-19 clinical outcome.

Predictors	Estimate	aOR	Std. Error	Wald	df	p-value	95% CI
							L B	U B
Age	0.046	1.047	0.016	8.530	1	0.003	1.015	1.080
Gender (Female)	-0.946	0.388	0.571	2.751	1	0.097	0.127	1.188
A. lumbricoides Seroreactive (no)	1.396	4.038	0.635	4.830	1	0.028	1.163	14.020
A. viteae seroreactive (no)	0.451	1.570	1.251	0.130	1	0.718	0.135	18.252
Any Helminth (no)	1.415	4.115	0.596	5.628	1	0.018	1.279	13.242

This table presents the results of a multivariable regression analysis investigating factors associated with the severity of COVID-19. Variables include age, gender, Ascaris lumbricoides seroreactivity, Acanthocheilonema viteae seroreactivity, Strongyloides ratti seroreactivity, and any (pooled) helminth seroreactivity with their corresponding estimates, odds ratios (OR), standard errors, p-values, and 95% confidence intervals.

Table 2. Multivariate regression to explore the relationship between infectious status and COVID-19 clinical outcome.

Predictors	Estimate	aOR	Std. Error	Wald	df	p-value	95% CI
							L B	U B
Age	0.046	1.047	0.016	8.530	1	0.003	1.015	1.080
Gender (Female)	-0.946	0.388	0.571	2.751	1	0.097	0.127	1.188
A. lumbricoides Seroreactive (no)	1.396	4.038	0.635	4.830	1	0.028	1.163	14.020
A. viteae seroreactive (no)	0.451	1.570	1.251	0.130	1	0.718	0.135	18.252
Any Helminth (no)	1.415	4.115	0.596	5.628	1	0.018	1.279	13.242

This table presents the results of a multivariable regression analysis investigating factors associated with the severity of COVID-19. Variables include age, gender, Ascaris lumbricoides seroreactivity, Acanthocheilonema viteae seroreactivity, Strongyloides ratti seroreactivity, and any (pooled) helminth seroreactivity with their corresponding estimates, odds ratios (OR), standard errors, p-values, and 95% confidence intervals.

3.4. Helminth Seroprevalence Inversely Correlates with COVID-19 Severity

To further analyze the relationship between helminth seropositivity and COVID-19 severity, we investigated the prevalence of helminth seropositivity across different levels of COVID-19 severity. The seroprevalence of A. lumbricoides was significantly higher than those of S. ratti and A. viteae (homologous of multiple human filarial species). The results further indicated that the highest cumulative seroprevalence of helminths (61.5%) was observed in the asymptomatic infected SARS-CoV-2 group. In contrast, mild and moderate COVID-19 patients exhibited 1.4 (43.8%) and 1.8 (34.5%) times lower helminth seropositivity than the asymptomatic group. Notably, the 2 severe COVID-19 cases investigated in this study were seronegative for all the three helminths tested (Figure 2).

3.5. Elevated Helminth-Specific IgG Levels Correlate with Asymptomatic SARS-CoV-2 Infection

Next, we compared the relative expression levels of helminth-specific IgG in asymptomatic, mild, and moderate/severe cases. The data show that helminth-specific IgG indices were significantly higher in asymptomatic SARS-CoV-2-infected individuals (Asy) than in moderate and severe COVID-19 patients (Mod/Sev). The results for all the tested helminths followed this trend, peaking in asymptomatic individuals and diminishing with increasing disease severity. Asymptomatic individuals exhibited 1.5 times the levels of helminth-specific IgG compared to those with moderate/severe symptoms (Figure 3).

3.6. Low Expression of SARS-CoV-2- specific IgA/ IgG and Neutralization Potency in Helminth-co-exposed Individuals

Furthermore, we aimed to identify the influence of helminth exposure on the development of humoral immunity against SARS-CoV-2. The results indicated that individuals exposed to helminths showed a significantly reduced expression of SARS-CoV-2 specific IgA /IgG (p = 0.0099, p= 0.0260) (Figure 4A,B) and neutralization potency against the wild type and variants (Figure 5). Similarly, helminth-specific IgG was negatively correlated with SARS-COV-2 neutralizing antibody expression (Figure 4C).

3.7. Lower Expression of Th1 and Th17 and Elevated Expression of Th2 Cytokines in Helminth Co-Exposed Individuals

To determine the immunological basis underlying the association between helminths and SARS-CoV-2 co-exposure, we used the Mann-Whitney U test to compare systemic Th1, Th2, and Th17 cytokines between SARS-CoV-2 mono-exposure (n = 31) and helminth co-exposure (n = 24). The results are shown in Figure 5. Co-exposed individuals exhibited elevated IL-10, IL-13, and IL-5 expression compared with mono-exposed individuals (p <0.0001, p <0.0001, and p = 0.0002, respectively). In contrast, mono-exposed participants displayed significantly higher expression of Type 1 and Th17 pro-inflammatory cytokines, including G-CSF, GM-CSF, IFN alpha, IFN gamma, IL-1 β, IL-2, IL-6, IL-8, IL-17A, IL-18, and IP-10 (p= 0.0132,0.0464, <0.0001, 0.0009, 0.0263, 0.0522, 0.0299, 0.0231, 0.0040, and 0.0278, respectively) (Figure 6).

3.8. Reduced Expression of Th1 and Th17 Cytokines in Asymptomatic SARS-CoV-2 Exposed Individuals.

To better understand the immunological differences across the spectrum of COVID-19 clinical presentations, we analyzed the systemic expression of different cytokines in the COVID-19 clinical groups. The data revealed a distinct cytokine profile associated with disease severity. Asymptomatic individuals exhibited a significantly higher expression of Th2 cytokines compared to patients with moderate and severe COVID-19, specifically IL-5 (p = 0.0444) and IL-13 (p = 0.0389). Conversely, they displayed significantly lower levels of Th1- and Th17-related pro-inflammatory cytokines, including IFN-γ (p = 0.0158), IFN-α (p = 0.0004), IL-6 (p = 0.0008), IL-8 (p = 0.0046), IL-17 (p = 0.0036), IL-12 (p = 0.0393), IL-18 (p = 0.0006), and IP-10 (p = 0.0004). In contrast, moderate to severe COVID-19 cases showed reduced expression of Th2 cytokines and notably higher levels of Th1 and Th17 cytokines. Specifically, these patients exhibited significantly higher levels of IFN-γ, IFN-α, IL-6, IL-8, IL-17, IL-12, IL-18, and IP-10 compared to asymptomatic and mild cases. However, levels of other cytokines, including G-CSF (CSF-3), GM-CSF, IL-1β, IL-4, IL-2, MCP-1 (CCL2), MIP-1α (CCL3), TNF-α, MIP-1β (CCL4), and TNF-β, were not significantly different across the groups. These findings highlight the critical role of cytokine regulation in determining clinical outcomes of COVID-19 (Figure 7).

4. Discussion

The analysis of the demographic data in our study confirms the initial findings suggesting that factors such as older age and male sex play a role in the severity of COVID-19 [17,42,43,44,45,46,47]. After adjusting for age and sex, our main findings support the hypothesis that co-exposure to helminths significantly leads to asymptomatic SARS-CoV-2 clinical outcomes. It also demonstrated that the presence of helminth antigens modulates the immune responses to SARS-CoV-2 in such a way that pro-inflammatory cytokine responses, antibody responses (IgA and IgG), and SARS-CoV-2 CD4+ reactive T cells are reduced, leading to better health outcomes despite the reduction in humoral immunity. This suggests that previous and current helminth exposure may reduce the risk of developing severe COVID-19 [8,17,48].

Helminths induce gut microbiome changes, which can affect the host's immune response. Therefore, they can likely influence SARS-CoV-2 pathogenesis in two different ways: either by directly modulating the immune system or by influencing the microbiome balance [49,50,51]. Interactions between enteric helminths and microbiota have been shown to protect against pulmonary viral infections in animal models [50], and this protection has been proven effective. Understanding the effects of environmental pathogen exposure and lifestyle on the immune system can provide a better understanding of the effects of helminth exposure on COVID-19 clinical outcomes as well as the differential course of the pandemic worldwide [20]. These factors have been proven to affect the immune system and inflammatory response, particularly in low- and medium-income countries where helminths are endemic [52,53,54,55,56]. These parasites can affect immune system functionality, modulating the immune response to SARS-CoV-2, causing COVID-19 to spread differently in different regions of the world. While it is important to acknowledge the potential influence of other pathogens, environmental factors, and demographic factors, such as age and sex, on COVID-19 clinical outcomes, our study suggests that helminths are key contributing factors. This assertion is corroborated by global epidemiological data, as regions with high helminth endemicity frequently demonstrated lower rates of severe COVID-19 cases and mortality compared to areas with low helminth endemicity [35]. Moreover, experimental studies have demonstrated that helminth infections can modulate the host immune response in a way that may mitigate the severity of other infectious diseases, including respiratory infections [12,13,14,38,57]. Simultaneously, we found that helminth-exposed individuals exhibited lower expression of SARS-CoV-2 specific immunoglobulins (IgA and IgG) and a lower neutralizing potential. This result indicates that co-exposure to helminths leads to a diminished humoral response to SARS-CoV-2 and a reduced efficacy of antibodies in binding to the virus's receptor binding domain (RBD). This result suggests that co-exposure to helminths leads to a diminished humoral response to SARS-CoV-2 and reduced efficacy of antibodies in inhibiting the receptor-binding domain (RBD) of the virus. At first glance, this may seem contradictory, as antibody-mediated immune responses are typically recognized for their crucial role in viral clearance and in enhancing overall health outcomes. However, in the context of SARS-CoV-2 infection, disease severity is more closely linked to an excessive inflammatory response, also known as a cytokine storm, rather than a weak humoral response [58,59,60,61,62]. If co-exposure to helminth antigens modulates the cytokine storm, it is plausible that the overall health outcome might be better despite low immunoglobulin levels [61]. Additionally, moderate levels of neutralizing antibodies may still be adequate to clear the infection. On the other hand, it also indicates that helminth antigen exposure may diminish vaccine efficacy against certain pathogens [15,26,27,28,31,63,64]. Our findings, while preliminary, illustrate the vital role of considering exposure to multiple pathogens during clinical trials and how essential it is to balance the advantages and disadvantages introduced by helminth co-endemicity when considering health outcomes. This is particularly important in populations already burdened by neglected tropical diseases (NTDs), such as those in low-income and middle-income countries.

Co-exposed individuals showed lower expression of Th1 and Th17 cytokines, including G-CSF, IFN-α, IFN-γ, IL-1 β, IL-17A, IL-18, IP-10, IL-6, IL-2, and GM-CSF, while upregulating the expression of Th2 cytokines such as IL-5, IL-10, and IL-13. The higher expression of Th2 cytokines and IL-10 might be the reason for the low expression of Th1 and Th17 cytokines, which may lead to asymptomatic outcomes. IL-10 is widely recognized as a powerful immune-regulatory cytokine [65]. While our data did not specifically identify the source of IL-10, it is likely that antigen-presenting cells, particularly dendritic cells and monocytes, are the primary sources [66]. Previous studies have highlighted monocytes as the primary contributor of IL-10 in peripheral blood cells [67]. This shows that helminths are capable of modulating the immune system in such a way that the regulatory responses are enhanced [11,68,69,70,71,72,73]. The Th2-skewed immune response induced by helminth exposure has also been associated with tissue repair and remodelling, which could contribute to mitigating the damage to lung tissues caused by COVID-19 pneumonia [15,26,35,36,37]. Moreso, some helminths have been associated with reduced risk or severity of certain comorbidities such as inflammatory bowel disease (IBD), autoimmune disorders, and allergic diseases [74]. If helminth exposure indirectly mitigates the severity of these conditions, individuals may have better overall health outcomes if they contract COVID-19 [75,76,77,78].

Our study's strength lies in its capacity to address gaps in the current understanding of the interaction between helminth infections and COVID-19 clinical outcomes in African samples. It provides compelling evidence that exposure to helminth antigens confers protection against severe COVID-19. However, the main limitation of our work is that the samples were collected from a single African country. This geographic limitation may affect the generalizability of the results to other helminth-endemic areas or to the entire African continent. Additionally, while correlations between helminth seropositivity and reduced severity of COVID-19 were observed in this cross-sectional cohort study, further investigations are needed to determine the causal relationships.

Although our findings are informative, our sample size of 104 participants is relatively small for drawing definitive conclusions. Future studies with larger sample sizes would provide more statistical power and reliable estimates of the observed effects, thus enhancing the robustness and applicability of the findings to broader populations.

Our study used serological assays to detect helminth infections. Although serology is a useful tool for detecting past infections and exposure, it has limitations in discriminating between past and current infections [79]. Identification of patent helminth infection by serology is challenging in endemic areas because of long-lasting antibody responses and repeated exposure [80]. Despite these limitations, serological assays remain crucial in epidemiological studies and public health surveillance. Even though serology does not discriminate between past and current infections, it reveals the immune system's reaction status to pathogens. For SARS-CoV-2 response mitigation, the presence of helminths, particularly those in the gut, is less important than the immune system's reaction, which spills over to influence the response against SARS-CoV-2. This broadens the scope of our investigation, providing a valuable tool for analyzing the correlations between exposure to various helminths, current and past helminth infections, and COVID-19 severity.

The interaction between helminth infections and vaccine efficacy, particularly in the context of SARS-CoV-2, is a critical area of research that has implications for vaccination strategies in helminth-endemic regions. Our findings indicate a notable reduction in SARS-CoV-2-specific IgA and IgG levels, as well as diminished neutralizing antibody responses in individuals exposed to helminths. This immune modulation is primarily due to a Th2-skewed immune response, characterized by elevated levels of cytokines such as IL-4 and IL-10, which are known to suppress Th1 responses and antibody production essential for effective vaccination against viral pathogens like SARS-CoV-2 [15,33,81]. The presence of helminth infections can significantly hinder the efficacy of vaccines that rely on robust Th1 and antibody-mediated immune responses. Studies have shown that chronic helminth infections can lead to decreased vaccine efficacy, as evidenced by findings that indicate established helminth infections at the time of vaccination adversely affect vaccine-specific immune responses [33,82]. This is particularly concerning in populations where helminth infections are prevalent, as the immune response necessary for effective vaccination may be compromised [81]. To mitigate these challenges, integrating adjunctive anti-helminthic treatments into vaccination campaigns could enhance vaccine responses. Evidence from previous studies suggests that deworming programs can improve vaccine efficacy, as demonstrated with BCG and influenza vaccines, where the removal of helminth infections resulted in enhanced immunogenicity and protective efficacy [33,83]. Furthermore, modified vaccine formulations that include adjuvants designed to enhance Th1 responses or counteract the effects of regulatory cytokines like IL-10 may also improve vaccine effectiveness in populations exposed to helminths [15,81]. Revised vaccination schedules could also play a role in ensuring adequate immunity in these populations. This might involve administering additional booster doses or utilizing higher antigen loads to overcome the immune suppression caused by helminth infections [33,81]. Our study underscores the necessity for vaccine trials that take into account co-endemic helminth infections, as this consideration is crucial for ensuring equitable vaccine efficacy in regions burdened by both infectious and parasitic diseases. Future research should aim to elucidate the specific mechanisms by which helminth exposure alters vaccine-induced immune responses, facilitating the development of tailored public health interventions that effectively address these challenges. In summary, the interplay between helminth infections and vaccine responses necessitates a multifaceted approach to vaccination strategies in endemic regions. By integrating anti-helminthic treatments, modifying vaccine formulations, and adjusting vaccination schedules, public health initiatives can enhance vaccine efficacy and improve health outcomes in populations affected by both viral and parasitic infections.

5. Conclusions

In conclusion, our study provides preliminary evidence that current or past helminth infections may modulate the immune response to SARS-CoV-2. This modulation appears to reduce Th1 and Th17 pro-inflammatory cytokine responses and the antibody response (IgA and IgG), resulting in diminished humoral immunity to SARS-CoV-2 and leading to better health outcomes (Figure 8). Our findings underscore the importance of considering helminth-induced immune modulation in disease control but also in vaccination strategies, as understanding these interactions could help optimize vaccine design and efficacy in populations with a high prevalence of helminth infections.

Graphical Summary

Figure 8. Schematic illustration of modulatory effects of helminth in a COVID-19 co-exposure scenario. This figure illustrates the immune response in severe COVID-19, characterized by the activation of pro-inflammatory cytokines through TH1 and Th17 pathways, leading to a cytokine storm. It also shows the immunomodulatory effects of helminth co-exposure in COVID-19 patients. In such co-exposure scenarios, COVID-19 patients exhibit a suppressed TH1 and Th17 response due to the Th2-mediated immune response from helminths. Additionally, the figure highlights the impact of helminths on the development of humoral immunity against SARS-CoV-2, including the suppression of SARS-CoV-2 specific IgA/IgG and neutralizing antibody expression. This figure was created using https://app.biorender.com.

Figure 8. Schematic illustration of modulatory effects of helminth in a COVID-19 co-exposure scenario. This figure illustrates the immune response in severe COVID-19, characterized by the activation of pro-inflammatory cytokines through TH1 and Th17 pathways, leading to a cytokine storm. It also shows the immunomodulatory effects of helminth co-exposure in COVID-19 patients. In such co-exposure scenarios, COVID-19 patients exhibit a suppressed TH1 and Th17 response due to the Th2-mediated immune response from helminths. Additionally, the figure highlights the impact of helminths on the development of humoral immunity against SARS-CoV-2, including the suppression of SARS-CoV-2 specific IgA/IgG and neutralizing antibody expression. This figure was created using https://app.biorender.com.