In early 2020, the newly emerged coronavirus (SARS-CoV-2) began to spread around the world, and on 11th March 2020 the World Health Organization characterized the COVID-19 outbreak as a pandemic [1]. More than 3 years into the pandemic, there is plenty—and still increasing—international evidence to suggest that socioeconomically disadvantaged groups have a higher risk of infection with SARS-CoV-2 [2]. However, these socioeconomic inequalities in infections have changed during different phases of the pandemic [3]. In Germany, the virus emerged in spring 2020 predominantly among residents of more affluent regions. Soon afterwards, higher infection rates shifted to socioeconomically more deprived regions [4]. This pattern of changing socioeconomic inequalities in SARS-CoV-2 infection was also observable during the subsequent infection waves in Germany [5]. Considered cumulatively over the first two pandemic waves, people with a low socioeconomic position (SEP) had an up to twofold higher risk for SARS-CoV-2 infection than those with a high SEP [6]. Evidence from various countries also showed higher risks of severe disease progression such as COVID-19-related hospitalization [7, 8] and mortality [9, 10] in groups with a lower SEP. These inequalities might, to a certain degree, be attributable to unequal infection risks or greater vulnerability due to higher prevalences of comorbidities [11]. Contrarily, one might argue that higher infection rates during the early phases of the pandemic also mean that those who survived the infection were possibly better protected against severe COVID-19 after a subsequent SARS-CoV-2 infection due to the immunological response to their first antigen contact [12]. This may have been particularly relevant during the pre-vaccine phases of the pandemic, but changed after effective vaccines against COVID-19 became available.

COVID-19 vaccination aims to reduce severe courses of the disease and deaths and to prevent the transmission of SARS-CoV-2 [13]. In Germany, the COVID-19 vaccination campaign began in late December 2020 with BioNTechPfizer’s Comirnaty^® vaccine. Up until February 2023, six additional vaccines were approved in Germany [14]. However, because the vaccines’ availability was initially limited, some groups were prioritized at the beginning of the German vaccination campaign: In addition to individuals with an elevated risk of severe COVID-19, e.g., due to their age or other risk factors, priority was primarily given to people with a high occupational risk of exposure to SARS-CoV-2, such as healthcare workers. Exclusive access for prioritized groups was abolished in June 2021, 6 months after the start of the vaccination campaign. Since then, everyone aged 18 years and older has had the opportunity to be vaccinated against COVID-19. In December 2021, a booster dose was officially recommended in Germany by the National Immunization Technical Advisory Group (NITAG) for people above the age of 18 [13]. The vaccination against COVID-19 in Germany, and in most countries worldwide, was not mandatory but a free individual choice and free of charge. However, there were several administrative and legal restrictions for unvaccinated individuals, e.g., when travelling. After the data-collection period, mandatory vaccination for employees working in specific institutions such as nursing homes was temporarily introduced in Germany. Various studies, internationally and for Germany, indicate socioeconomic differences in willingness to be vaccinated and in COVID-19 vaccination uptake during the first months of vaccinations [15–17].

Research suggests that at least three SARS-CoV-2 antigen contacts through vaccination and/or infection may effectively protect against severe COVID-19, in particular through “hybrid immunity” [12, 18]. However, evidence on the association between SEP and the risk of infection with SARS-CoV-2, especially in later pandemic phases, COVID-19 vaccination and ‘hybrid immunity’ from both infection and vaccination is still scarce for Germany. This study aims to evaluate the socioeconomic patterning of different constellations of SARS-CoV-2 antigen contacts from infection and vaccination within the German adult population. It thus investigates how 1) SARS-CoV-2 infections, 2) COVID-19 vaccinations, and hence 3) possible protection against SARS-CoV-2 infection and a severe course of COVID-19 are socioeconomically distributed—2 years into the pandemic and after 1 year of the vaccination campaign in Germany.

Of the 10,448 participants, the majority (70.2%) participated in November and December 2021 (Table 1). The overall prevalence of SARS-CoV-2 infection was 9.8%. Most of the participants had been vaccinated at least once (94.0%) and had basic immunization (91.0%); one-third (33.2%) had had at least three SARS-CoV-2 antigen contacts.

After almost 2 years of COVID-19 pandemic and 1 year of the vaccination campaign, adults with lower education were more likely to have been infected with SARS-CoV-2. Also, low and medium SEP groups were less likely to have basic immunization or to have had at least three antigen contacts with SARS-CoV-2. Furthermore, adults with lower SEP were less likely to have been vaccinated and have received their first dose of COVID-19 vaccine later in the campaign. The results indicate that there is still prevention potential in low and medium socioeconomic groups, which together account for the majority of Germany’s population, in order to approach and maintain high protection against severe COVID-19 in the population as a whole.

The educational differences in infection risk with SARS-CoV-2 are in line with previous research from Germany for the first year of the pandemic [6]. A conceptual framework by Bambra [11] proposes unequal exposure (e.g., through opportunities to work from home), unequal transmission (e.g., through housing conditions) and unequal susceptibility (e.g., through comorbidities) as underlying pathways of health inequalities with emerging infectious diseases such as COVID-19. Empirically, individuals with lower SEP were less able to work from home [26], to reduce their contacts during the pandemic [27], and are generally more likely to live in crowded conditions [26]. Moreover, there are educational differences in factors such as risk perception or preventive behaviour regarding COVID-19 [28], which might contribute to a higher infection risk for low education groups. Our sensitivity analysis suggested that the household composition and employment status altered the education-infection association only marginally. This aspect could be investigated in more detail in further research in order to examine the explanatory extent of these and other potential confounding factors such as occupation [29], which was beyond the scope of our analyses.

Regarding COVID-19 vaccination, our results are consistent with previous research indicating socioeconomic differences in COVID-19 vaccination uptake [16]. In addition, several studies show that COVID-19 vaccine hesitancy is also associated with income and education [30, 31]. But there is a lack of studies that examine the reasons for the lower uptake and coverage of COVID-19 vaccination in socioeconomically disadvantaged groups. The internationally established “5C” model suggests several psychological reasons for the different uptake of vaccination in general [32, 33]. Research on this model shows that confidence (e.g., in the safety and effectiveness of the vaccine), constraints (structural and psychological barriers), complacency (perceived risk of the disease), calculation (extent of information searching, risk-benefit analysis) and collective responsibility (willingness to protect others) influence COVID-19 vaccination propensities [34–36]. Further research is needed to explore to what extent these five psychological antecedents of vaccination uptake contribute to explaining socioeconomic differences in COVID-19 vaccination prevalence. Misinformation about COVID-19, often from social media, reduces confidence in vaccine safety and is associated with SEP [37]. Other factors such as occupation are also evident to be associated with the COVID-19 vaccination prevalence [38], which was beyond the scope of our analyses but might have had an influence on our results.

The socioeconomic inequalities in SARS-CoV-2 infection and COVID-19 vaccination are also reflected in the antigen contacts. Our results show a socioeconomic gradient, particularly for the outcome of at least three SARS-CoV-2 antigen contacts. Research suggests that three antigen contacts provide a high protection against severe courses of COVID-19 [39], particularly the combination of SARS-CoV-2 infection and COVID-19 vaccination (“hybrid immunity”) [12]. Against this background, our results indicate that, despite their higher risk of infection during the early stages of the pandemic, socioeconomically disadvantaged groups might still be less protected from severe COVID-19 even after 1 year of the vaccination campaign. Furthermore, the results provide evidence that the socioeconomic gradient regarding the three antigen contacts is attributable less to socially unequal distribution of SARS-CoV-2 infection than to social inequalities in COVID-19 vaccination coverage.

This is the first study from Germany to analyse associations between SEP and SARS-CoV-2 infections, COVID-19 vaccination and antigen contacts after 1 year of the vaccination campaign. One of its major strengths is the seroepidemiological design, which enabled the identification not only of known but also of undetected infections via serological assays. Furthermore, combining the serological data with panel-survey data on socioeconomics enabled differentiated analysis by SEP groups. By assessing the dates of vaccination, we were also able to investigate, the average time to first vaccination by socioeconomic stratification.

Despite these strengths, several limitations should be noted. As our analyses regarding SARS-CoV-2 infections and vaccinations are partly based on self-reports of the participants, recall bias might have occurred, i.e., that the participants did not remember events accurately. This might have led to misreporting of self-reports, either overestimating or underestimating infections or even vaccinations. A further limitation is the fact that the antibody levels may have waned over time and hence contributed to an underestimation of the cumulative infection prevalence, especially among those individuals who had their antigen contacts long before their participation in the study. This concern might also have been relevant regarding vaccinations, where evidence reveals a rapid decline of vaccine effectiveness against SARS-CoV-2 infections but a remaining high protection against severe courses of COVID-19 [40]. Another limitation is that the booster vaccination rates increased strongly during the survey period, which might have limited the interpretation of our results regarding the three antigen contacts. Study participants who participated early in the observation period did not have the opportunity to get a booster vaccination compared to those who participated later, when the booster dose was already available. Considering the longer time to get vaccinated we found for participants with lower education or income, which might also have been the case for the booster vaccination, this might have led to an underestimation of three antigen contacts in these groups. Furthermore, we assume that high-education groups are more likely to participate in surveys than low-education groups, which might introduce self-selection bias and therefore an overrepresentation of higher educated groups in our sample. However, weighting procedures were applied to compensate for systematic non-response and to increase the generalizability of the results for the adult German population in private households [25].

Altogether, our findings suggest socioeconomic inequalities in SARS-CoV-2 infection, COVID-19 vaccination, basic immunization and at least three antigen contacts. These patterns indicate that socioeconomically disadvantaged groups are less protected from severe COVID-19 after almost 2 years of the pandemic and 1 year of the German vaccination campaign, but also that the medium socioeconomic groups still showed potential for prevention. Our results highlight the need for a stronger consideration of socioeconomic determinants in the management of pandemics with infectious diseases to prevent health inequalities. Health policies should consider socioeconomic factors that might influence the risk of infection, barriers to vaccination and vaccine hesitancy. Further research is needed, addressing the reasons for socioeconomic patterning of COVID-19 vaccination in order to design targeted interventions.

The data cannot be made publicly available because informed consent from participants did not cover the public deposition of data. However, the data underlying the analysis in this article is archived in the SOEP Research Data Centre (https://www.diw.de/en/diw_01.c.601584.en/data_access.html) in Berlin and can be accessed on site upon reasonable request.