Susceptibility to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and severity of coronavirus disease 2019 (COVID-19) are heterogeneous across different populations. This heterogeneity has been attributed to various individual risk factors (Williamson et al., 2020; Goel et al., 2021; Ng et al., 2021). The ABO blood group is a suspected candidate because of its known role in other infectious diseases (Cooling, 2015). Early in the pandemic, a report from China showed an association of blood group A with increased risk of SARS-CoV-2 infection and lower risk for individuals with group O (Li et al., 2020). A first large-scale genome-wide study reported similar results with respect to COVID-19 severity (Ellinghaus et al., 2020). Others demonstrated a protective role of blood group O compared to A regarding disease susceptibility or severity (Golinelli et al., 2020; Wu et al., 2020). However, current results are conflicting and several high-quality studies did not support ABO-dependent intrinsic susceptibility to SARS-CoV-2 infection or COVID-19 severity (Boudin and Dutasta, 2020; Zhang et al., 2020; Zietz et al., 2020; Anderson et al., 2021). One of the possible explanations could be an over-recruitment of individuals with blood group O in blood banks due to their universal donor status. Thus, studies using blood donors as a reference group would find that the frequency of blood group O is lower among their COVID-19 cases than in the blood bank reference population (Dzik et al., 2020; Pendu et al., 2021).

Several underlying mechanisms have been proposed to explain the association between ABO blood group and SARS-CoV-2 infection: First, genetic histo-blood group polymorphisms can lead to variable viral attachment. Second, it has been hypothesized that the SARS-CoV-2 receptor-binding domain (RBD) may act as a lectin and preferentially bind to blood group A glycoprotein expressed on respiratory epithelial cells of blood group A individuals, favoring SARS-CoV-2 infection, though this has been recently contradicted in vitro (Wu et al., 2021; Boukhari et al., 2022). Third, the interaction of the SARS-CoV-1 and -2 spike protein with the cellular angiotensin-converting enzyme 2 (ACE2) receptor is inhibited by anti-A natural antibodies (Guillon et al., 2008; Le Pendu and Ruvoën-Clouet, 2020).

Last, it has been suggested that natural anti-A and anti-B could neutralize the virus when transmission occurs in an ABO-incompatible manner and this may reduce virus transmissibility by at least 60% (Ellis, 2021). Natural anti-A and anti-B antibodies present in ABO-incompatible virus recipients could recognize and bind to A and/or B epitopes expressed on the viral envelope glycoproteins, prevent the interaction of the virus and ACE2 receptor, and consequently prevent entry into lung epithelial cells (Goel et al., 2021). This mechanism is supported by the finding that COVID-19 patients had significantly lower anti-A and anti-B antibody levels compared to controls (Deleers et al., 2021). Additionally, Boukhari et al. (2022) demonstrated only recently that ABO incompatibility between spouses strongly decreased the risk of SARS-CoV-2 transmission.

Although we and others have demonstrated lower SARS-CoV-2 susceptibility in children and adolescents compared to adults (Spielberger et al., 2021; Stich et al., 2021; Tönshoff et al., 2021), pediatric data on the impact of ABO blood group on SARS-CoV-2 infection susceptibility or COVID-19 severity are scarce (Bari et al., 2021). Moreover, the association between ABO/Rh blood group and SARS-CoV-2 infection within households has not been evaluated so far.

In this large multi-center family cohort study, we examined the prevalence of ABO/Rh blood groups in SARS-CoV-2 seropositive and seronegative children and adults. We evaluated the contribution of ABO/Rh blood group to SARS-CoV-2 susceptibility and symptomatic infection. Additionally, we investigated a role of ABO compatibility between infected and exposed individuals on the SARS-CoV-2 transmissibility.

This study found blood group A to be moderately overrepresented and blood group O to be moderately underrepresented in SARS-CoV-2 seropositive as compared to seronegative individuals and the reference population (Golinelli et al., 2020; Zietz et al., 2020; Ray et al., 2021). We observed this effect in adults, as previously reported by others (Pendu et al., 2021), but not in children. When considering the clustering of data in families, we found no evidence for blood groups A or AB as a substantial risk factor for SARS-CoV-2 infection in children or adults. This contrast to other studies might be explained by geographical, genetic, and methodological differences (Barnkob et al., 2020; Anderson et al., 2021). Here, we found a significantly lower frequency of Rh-positive blood groups among SARS-CoV-2 seropositive children and being Rh-negative doubled the risk for SARS-CoV-2 seropositivity. This contradicts previous reports in which Rh positivity constituted a risk factor for SARS-CoV-2 infection or severe COVID-19 (Latz et al., 2020; Zietz et al., 2020; Ray et al., 2021). Only one study from Sudan mentions a possible protective effect of presence of Rh-factor (Taha et al., 2020). A different ABO/Rh prevalence in the ethnically different Sudanese population prevents a comparison with our study. We can only hypothesize that the potential differences between adults and children may reflect a different underlying mechanism of ABO blood groups on SARS-CoV-2 infection.

Additionally, we investigated whether ABO incompatibility could potentially influence the risk for SARS-CoV-2 infection within the families. The frequency of transmission between index cases and exposed individuals was equally distributed between the ABO-compatible and ABO-incompatible group, and the risk for SARS-CoV-2 infection was not increased in ABO-compatible individuals in our cohort. This finding could be at least partially influenced by the natural phenomenon of a higher frequency of compatible blood groups within family cohorts due to inheritance of blood type, which makes ABO-compatible SARS-CoV-2 infections more likely. Therefore, to prove or reject the hypothesis of an effect of ABO compatibility on the risk of SARS-CoV-2 infection, transmission events need to be investigated in a cohort of unrelated individuals with known blood groups.

Since households represent the most important location for SARS-CoV-2 transmission, our data inform on the most common clinical scenario. In our cohort, the only parameters that positively correlated with the risk of intrafamilial transmission was the age of the exposed individual and the presence of symptoms in the index case, as described previously (Stich et al., 2021). Here, we confirm age group as a contributing factor, whereas ABO/Rh blood groups are not relevant for the risk of developing symptoms (Davies et al., 2020).

The strength of this multi-center study is its relatively large number of participants, and a separate analysis of infected and exposed children. Blood group determination within a household cohort of exposed SARS-CoV-2 seropositive and seronegative individuals allows controlling for genetic background and environmental influences. Nevertheless, the selection of participants and data collection may have been biased by voluntary participation and self-reporting of symptoms. Furthermore, we did not record data on ethnicity, and despite a relatively ethnic homogeneity in Southern Germany, this may have impacted the analysis. Finally, the findings apply to the SARS-CoV-2 variants circulating in Germany in spring 2020, and their validity in case of later virus variants (e.g., Delta and Omicron) is unclear.

In summary, we found that blood group A was moderately overrepresented in SARS-CoV-2-infected individuals, and the Rh-negative blood group was overrepresented in SARS-CoV-2-infected children. ABO compatibility did not increase the risk for SARS-CoV-2 infection. Individual ABO/Rh blood groups and ABO compatibility were not independent risk factors for SARS-CoV-2 acquisition, neither in children nor in adults.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by Ethics Committees of the University Hospitals Tübingen, Ulm, Freiburg and Heidelberg. Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin.

AJ, HR, AF, and CE conceived the study and designed the experiments. AJ, HR, RE, AF, PH, BT, MS, and K-MD procured the funding. AP, SH, TS, AD, H-JG, and TG performed and interpreted the serological assays. SE and CW performed the blood grouping experiments. HR, AJ, AF, CE, DF, MZ, E-MJ, SFNB, and JR collected the samples or organized the collection. AJ, HR, AF, CE, and K-MD curated the data. CE, AJ, and HR performed the data analysis and generated the tables. AJ and HR wrote the first draft of the manuscript. SS, JR, TG, BT, MS, DF, SNFB, ADMH, and CE revised the first draft of the manuscript critically for important intellectual content. All authors approved the final version of the manuscript and confirm that they had full access to all the data in the study and accept responsibility for the decision to submit for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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