Immunity Against SARS-CoV-2 in the German Population

Background: Early during the SARS-CoV-2 pandemic, national population-based seroprevalence surveys were conducted in some countries; however, this was not done in Germany. In particular, no seroprevalence surveys were planned for the summer of 2022. In the context of the IMMUNEBRIDGE project, the GUIDE study was carried out to estimate seroprevalence on the national and regional levels.

Methods: To obtain an overview of the population-wide immunity against SARS-CoV-2 among adults in Germany that would be as statistically robust as possible, serological tests were carried out using self-sampling dried blood spot cards in conjunction with surveys, one by telephone and one online. Blood samples were analyzed for the presence of antibodies to the S and N antigens of SARS-CoV-2.

Results: Among the 15 932 participants, antibodies to the S antigen were detected in 95.7%, and to the N antigen in 44.4%. In the higher-risk age groups of persons aged 65 and above and persons aged 80 and above, anti-S antibodies were found in 97,4% and 98.8%, respectively. Distinct regional differences in the distribution of anti-S and anti-N antibodies emerged. Immunity gaps were found both regionally and in particular subgroups of the population. High anti-N antibody levels were especially common in eastern German states, and high anti-S antibody levels in western German states.

Conclusion: These findings indicate that a large percentage of the adult German population has formed antibodies against the SARS-CoV-2 virus. This will markedly lower the probability of an overburdening of the health care system by hospitalization and high occupancy of intensive care units due to future SARS-CoV-2 waves, depending on the viral characteristics of then prevailing variants.

By November 2022, there were around 36 million officially registered SARS-CoV-2 infections in Germany (1). The proportion of undetected infections is estimated to be 1.5 to four times the number of recorded cases (2, 3). With the spread of the Omicron variant, it is likely that the proportion of unreported cases has increased even more.

According to information from the Robert Koch Institute (RKI), around 65 million people (77.9%) have received at least one dose of vaccine against SARS-CoV-2, 63.5 million people (76.3%) at least two doses, and 51.9 million people (62.4%) have had three doses (4). There are also uncertainties about the vaccination rate, since surveys indicate that there may be under-reporting of about 5% (5). In order to assess the pandemic risk situation, it is important to estimate the number of persons exposed to SARS-CoV-2 after vaccination or infection.

During the transition phase from a pandemic situation to recurrent seasonal waves, it is important to determine the level of underlying population immunity, not least to be able to estimate the need for population-based infection control measures. Seroprevalence studies conducted so far create a picture of the population’s antibody responses to SARS-CoV-2 up until February 2022, but include neither the Omicron waves nor the various stages of immune response, including hybrid immunity (6). Assessment of immunity in the population is a complex matter because many people were vaccinated at different times during the pandemic in a different order. They also suffered an infection – sometimes with different variants of the SARS-CoV-2 virus.

Furthermore, regional and demographic differences with regard to immunity in Germany and risk-specific differences in vaccination recommendations also need to be considered. The main focus here is on the elderly as well as on vulnerable patients who have a high risk of suffering severe disease due to their poor immune protection (7).

Seroprevalence of antibodies against the S and N antigens throughout Germany was therefore determined in a random sample of adults in order to gain a statistically robust overview as possible of the anti-SARS-CoV-2 immunity status of the adult population in Germany and to be able to assess regional and demographic differences. The presence of antibodies against the N antigen is an indication of previous infection, regardless of the person’s vaccination status, whereas presence of antibodies against the S antigen is a sign of either earlier infection or vaccination. The present results provide an overview of the humoral immunity status in the German-speaking adult population living in private households until September 2022, exposing immunity gaps both regionally and demographically.

Methods

Sample survey

The German-speaking population aged 18 years and over, living in private households in Germany, was defined as the study population.

Participants were identified using an online access panel, the PAYBACK Panel, and a telephone interview (CATI, computer assisted telephone interview). Based on predefined factors relating to gender, age groups, education, federal state, and regional distribution, a random selection was taken from more than 130 000 persons (PAYBACK), and 28 965 invitations were sent out (eFigure 1). In order to more fully cover the study population of the German-speaking community aged 65 years and over living in private households, the data of 1500 people aged 65 years and over were also gathered in addition by telephone (CATI). The survey results were verified using a random sample from the residents’ registration office of Berlin, taken as an example, in order to classify and validate the combined sample. All three samples were weighted using different methods. More detailed information on the sample survey and weighting may be found in the eMethods section and in the eTables 1, 2). The University Hospital of Bonn granted a positive ethics opinion to conduct the study (application 202/22). The study is registered under DRKS00029693.

eFigure 1

Presentation of the sample survey process

eTable 1

Utilization of the PAYBACK sample

eTable 2

Utilization of telephone questionnaire (CATI sample)

The questionnaire employed to survey the participants is attached in the online section of the present article (eQuestionnaire), where more detailed information on the serological laboratory analysis is also to be found.

Statistical data analysis

The characteristics of the study population and the number of declared vaccinations and infections were analyzed and estimated, stratified by age according to an appropriate weight variable. Stratification by age was performed for the groups 18–29, 30–34, 35–39, 40–49, 50–59, 60–64, 65–79, and over 80 years. Age-stratified estimates of the prevalence of antibodies to the S and N antigens were made using a second weight variable especially created for laboratory parameters, and the geographical distribution was presented according to NUTS2 and NUTS3 regions (NUTS, nomenclature of territorial units for statistics; administrative districts level as well as the rural and urban district level).

Antibody titers above 35.2 BAU/mL against the anti-S protein and an antibody ratio greater than, or equal to, 1.1 for the anti-N antigen were defined as positive. Another endpoint were those individuals without any immunity who stated in their questionnaire that they were neither vaccinated nor had been infected, and whose antibody tests were both negative. The proportion of persons in the sample with two booster vaccinations according to the current STIKO (German Standing Committee on Vaccination) recommendation was combined as one variable and presented by age groups (8).

All statistical analyses were performed using Stata 17.1. Estimates of total prevalences, mean values, stratified results, and corresponding 95% confidence intervals were implemented with the aid of the Survey Data [SVY] command and the two weight variables (9). Two-tailed t-tests were used to compare the mean values between groups in the sample. The geographic presentations (Figures 1, 2, eFigures 2–5) according to NUTS regions are based on map data from Eurostat (10) and were visualized with the Stata package SpMap. The detailed report on the methods of the sample survey and the code used for preparing and analyzing the data in the form of STATA do-files may be found at www.github.com/kaischulzewundling/guide.

Figure 1

Proportions of persons with proven positive S-antigen immunity in %, stratified by rural and urban districts (NUTS-3) and weighted by population data

Figure 2

Proportions of persons with proven positive N-antigen immunity in %, stratified by rural and urban districts (NUTS-3) and weighted by population data

eFigure 2

Proportions of persons without declared vaccination, infection, or proof of immunity in %, stratified by rural and urban districts (NUTS-3) and weighted by population variables

eFigure 3

Proportions of persons with proof of immunity to S antigen in %, stratified by administrative district (NUTS-2) and weighted by population data

eFigure 4

Proportions of persons with proof of immunity to N antigen in %, stratified by administrative district (NUTS-2) and weighted by population data

eFigure 5

Proportion of persons 60+ with two booster vaccinations according to the STIKO recommendation in %, stratified by rural and urban districts (NUTS-3)

Results

Included in the study were a total of n = 15 932 participants who completed the questionnaire between June and September 2022, either online or by telephone, and returned the dried blood spot test kits to the laboratory (study population characteristics in Table 1). It was not possible to evaluate 1203 dried blood spot cards as they had not been provided with enough blood. The participants were 52.0 years old on average (95% confidence interval: [51.7; 52.3]), of whom 52.3 % [51.5; 53.2] were female. 22.5% [21.9; 23.2] of the study population had a university degree and 78.1% [77.2; 78.9] were employed. 47.1% of participants were working full-time and 15.3% part-time, while 27.8% were retired. About 40% of the study participants were active or former smokers. The most common pre-existing conditions were hypertension (30.6%), lung diseases (12.1%), and diabetes (9.9%).

Table 1

Participant characteristics, weighted by population variables

A positive antibody test against the S antigen was found in 95.7% [95.3; 96.1] of the study participants and a positive antibody test against the N antigen in 44.4% [43.5; 45.3] (Table 2). Positive antibodies (to S or N antigen) were detected in a total of 14 398 study participants with evaluable dried blood spot samples. Prevalences were similar across age groups 18–64 for anti-S antigen, but significantly higher for the age groups of over 65-year-olds and above all for the over 80-year-olds with 98.8 % [97.2; 99.5]. For the N antigen, however, the estimated N antibody positivity decreased continuously with increasing age group (here, the age group 80+ had the lowest value of 28.5 % [24.4; 33.0]. The majority of participants were at least triple vaccinated. 57.4 % of the participants stated that they had not yet been infected [56.6; 58.3].

Table 2

Proportions of S and N antigen detections and declared vaccinations and infections, stratified according to age and weighted according to population variables (in % [95% confidence interval])

According to the STIKO recommendation for COVID vaccination, a second booster vaccination is recommended for all persons over 60 and all persons of the age group 18–59 with one or more risk factors. Altogether, 46.0% [45.2; 46.9] of the sample had one or more risk factors according to the recommendation or were over 60 years old (Table 3). In the age groups 18–29, 30–34, 35–39, and 40–49, between 16 and 32% of the persons had one or more risk factors for a more severe disease, depending on their group. The proportion of participants who had stated they had neither a positive antibody test against the S or N antigen nor declared in their questionnaire that they had been vaccinated or suffered infection was 1.2% of all study participants [1.1; 1.4] (eFigure 2). This proportion was the lowest at 0.3% [0.0; 2.0] in the age group of those over 80 years, while it was the highest in the age group 40–49 years at 1.67 % [1.2; 2.3]. Altogether, 27.0% [25.9; 28.1] of the participants fulfilled the vaccination recommendation for a second booster. The proportion of persons with a second booster vaccination according to the STIKO recommendation was the highest in the age group 80+ at 61.7% [56.9; 66.9] and the lowest in the age group of the 30 to 34-year-olds at 3.8 %.

Table 3

Proportions of persons with increased risk of severe disease*1, stratified by age and weighted by population variables (in % [95% CI])

The regional distribution of underlying population immunity at the rural or urban level is an important parameter (Figures 1, 2 and eFigure 2). The highest proportions of antibody detection against the S antigen was seen, amongst others, in the districts of Coburg, Fürth, Hof, Potsdam, Ravensburg, Rostock, and Tuttlingen at about 100%, whereas antibody detection against the S antigen was below 80% in the districts of Hohenlohe, Hildburghausen, Kempten, Memmingen, and Suhl.

There were also similar regional differences for antibodies to the N antigen. While antibody prevalence of more than 75% against the N antigen was found in districts such as Deggendorf, Hof, Memmingen, Rhein-Lahn, and Saale-Orla, the antibody prevalence in Ansbach, Baden-Baden, Bremerhaven, Emden, Osterholz, and Wolfsburg, for example, was comparatively low at below 20%. The regional differences become even more pronounced when presented by NUTS2 regions (eTable 3, eFigures 3, 4). There is a tendency to observe the highest proportions of positive anti-S antigen antibodies in the western federal states and the highest proportions of anti-N antigen antibodies in the eastern states.

eTable 3

Proportions of persons without vaccination, infection, or immunity, stratified by administrtive district (NUTS-2), weighted by population variables

The analysis of vaccination history conducted regionally at the level of rural or urban districts based on the STIKO recommendations also revealed large regional differences (presented in eFigure 5 for the age group 60+). There was a tendency for less vaccinations to be declared in the southeast and more vaccinations in the northwest. While less than 5% of the participants in the districts of Bautzen, Darmstadt, Frankfurt/Oder, and Stendal, amongst other places, reported having had a second booster vaccination at the time of the survey, this figure was more than 75% of the over 80-year-olds in Delmenhorst, Offenbach am Main, Rosenheim, Wilhelmshaven, and Worms.

Discussion

As part of the IMMUNEBRIDGE project, the GUIDE study was able, for the first time, to identify within in a very short time the exposure status to SARS-CoV-2 in the German speaking adult population using a self-sampling method. Anti-S seroprevalence in the general population was very high at 95.7 %. There were regional differences, however, indicating on the one hand a regional variation in the incidence of infection as well as different vaccination rates, which were particularly lower in the Eastern federal states. There was also an anti-N seropositivity of 44.4%, which is an indication of having been infected with SARS-CoV-2.

A series of seroprevalence studies of anti-SARS-CoV-2 antibodies has so far been conducted in Germany (6). To date, the majority of these, however, were restricted only to a certain region, so there has been no sample survey covering all areas of Germany. They were also conducted during the earlier times of the pandemic (and so do not take into consideration the largest proportion of the infections which have developed as the result of the appearance of the Omicron variant). Furthermore, these studies did not distinguish between antibodies to the S or N antigen, or there was no further reaching information on risk factors or number of vaccinations to be able to assess immunity in detail. The concept of subject acquisition and sampling by means of self-sampling and dried blood spot cards proved itself to be extraordinarily efficient and robust during the present study.

The GUIDE study, with its design, its sampling method, specimen collection modality, and its extensive questionnaire allowed a large group of participants to be mobilized within a time span of five months with a very high return rate. To the best of our knowledge, the GUIDE study is so far the only available study in Germany which has examined at this speed robust generalizations about the entire population with regard to seroprevalence of anti-SARS-CoV-2 antibodies. The combination of questionnaire with anti-S and anti-N antibody tests allowed a good assessment of the exposure status of the German speaking adult population and integration into current scenario modeling (11, 12).

Limitations

The study was limited to adults aged 18 years and over, while SARS-CoV-2-specific T-cell and memory-B-cell responses, neutralization assays, and other critical components of acquired immunity were not assessed. Even if antibody levels show that there is a certain underlying protection, they do not allow the degree and quality of the immunity to be fully assessed. Indeed, some people with detected antibodies can still suffer severe COVID-19 disease, especially if they belong to a high-risk group. Apart from antibody immunity, the number of vaccinations, in addition to laboratory tests for T and B cells, can most likely be regarded as a further surrogate of immunity and protection from severe disease. This way, complex endpoints can be formed which correlate well with markers of immunological protection, such as neutralizing antibodies (12, 13). Furthermore, it is also known from various systematic reviews and meta-analyses of epidemiological and vaccine effectiveness studies that immunity against (re-)infection decreases within a few months, yet is sustained against severe disease (14, 15). A recent meta-analysis established that a high level of lasting protection against severe disease exists from previous infection and hybrid immunity through vaccination and infection (but not against re-infection) (16).

Furthermore, high-risk groups in particular were underestimated due to the study design: Surveys on comorbidities are based on independent statements by the participants and allow only a rough classification of concomitant diseases since these data were not recorded in medical terms. Furthermore, unvaccinated individuals were possibly underrepresented in the study. According to data from the RKI, 86% of the adult population had received basic immunization, while this was 94% in the GUIDE study (4). Publication of the 7th report of the COVIMO study revealed that data from RKI’s digital vaccination rate monitoring possibly underestimates the actual vaccination rate by about 5%, while the COVIMO survey possibly over-estimates vaccination rates (5, 17).

Most recent estimates from the COVIMO study are very similar to those of the GUIDE study (COVIMO versus GUIDE 18–39 years: 86% versus ~90%; 40–59 years: 94% versus 95%; 60+ years: 96% versus ~97%) (17). Accordingly, a slight overestimation of the vaccination rate and underreporting of the unvaccinated may also be assumed in the GUIDE study. Qualitatively speaking, this underestimation does not change the conclusions, however, given the probably low absolute difference from actual values. Future studies would also benefit from data acquisition from the cohort of high-risk groups in the form of a prospective clinical trial, in addition to the population-based approaches of a representative analysis. Apart from strictly registering main and secondary diagnoses, serological examinations to gather data on humoral immunity, together with the necessary analysis of cellular immunity, could also be conducted.

It should also be noted that, since the end of the survey period, a further 3.5 million infections have been officially reported which are not reflected in the data and for which reason the anti-N-antibody prevalence is under-estimated.

Conclusion

Our findings show that a large proportion of the general population in Germany have humoral immunity to SARS-CoV-2. Depending on the characteristics of the particular SARS-CoV-2 variant, there will be a significant reduction in the likelihood of healthcare system overload scenarios due to hospitalizations and the need for intensive medical care for patients with COVID-19 in the next waves of the disease, compared with a situation without this immunity status in the population.

Protection against severe disease is due to a considerable level of cellular immunity, which is also further reinforced robustly against variant evolution (18, 19) and by repeated exposition. Although the GUIDE study does not measure cellular immunity directly, measurement of antibody levels and details about vaccinations and infections allow conclusions to be drawn about ongoing protection against severe disease (20). This is also suggested by the more moderate burden on intensive care units during the last two Omicron waves in the summer and fall of 2022 as compared with previous waves of infection. Predictions about the impact of future waves of infection alone, based on immunity status, are nevertheless not possible.

Funding

The study was funded via the Network of University Medicine (NUM) as part of the IMMUNEBRIDGE project by the Federal Ministry of Education and Research (BMBF) (Research number (FKZ 01KX2121).

Conflict of interest statement

NT is a member of the Board of Directors of the DGPI [German Society for Pediatric Infectious Diseases].

AP is spokesperson of the specialist and organ-specific working groups (FOSA) of Laboratory Medicine and the UAC use and access committee of the German National Pandemic Cohort Network “NAPKON” (both are committees of the Network of University Medicine).

MN is spokesperson of the FOSA of Laboratory Medicine, spokesperson of the advisory committee, and member of the control group of the NUM Research Information System (NUM-FIS). He is also responsible for the NUM-LIMS (all named institutions are committees of the Network of University Medicine).

AK received financial support from the German Research Foundation (DFG), the BMBF, the Federal Ministry of Health (BMG), the RKI, the Institute for Public Health North Rhine Westphalia, the Volkswagen Foundation, and the Innovation Fund of the Federal Joint Committee.

BL was supported by the Helmholtz Association, the Horizon 2020 Research and Innovation Program of the European Union, the IMMUNEBRIDGE project, and the Federal Ministry of Education and Research (BMBF) via the projects RESPINOW (Modeling Platform on Respiratory Infections) and OptimAgent, a standardized model-based framework to support public health decision-making processes. She is spokesperson of the Modeling Network for Serious Infectious Diseases, Deputy President of the German Society for Epidemiology, member of the Internal Advisory Board of the German Center for Infection Research, and member of the Steering Committee of TBNet (Tuberculosis Network).

HS received a fee for participation in the discussion round at a Johnson & Johnson Open House event. He was member of the Scientific Advisory Boards of AstraZeneca and Seqirus on an honorary basis. Johnson & Johnson also supports a study led by HS.

The other authors declare that no conflict of interest exists.

Manuscript received on 30 December 2022, revised version accepted on 13 March 2023.

Translated from the original German by Dr Grahame Larkin MD.

Corresponding author
Prof. Dr. med. Hendrik Streeck
Institute of Virology

University Hospital of Bonn

Venusberg Campus 1

Building 63

53127 Bonn

Cite this as:
Schulze-Wundling K, Ottensmeyer PF, Meyer-Schlinkmann KM, Deckena M, Krüger S, Schlinkert S, Budde A, Münstermann D, Töpfner N, Petersmann A, Nauck M, Karch A, Lange B, Blaschke S, Tiemann C, Streeck H: Immunity against SARS-CoV-2 in the German population. Dtsch Arztebl Int 2023; 120: 337–44. DOI: 10.3238/arztebl.m2023.0072

►Supplementary material

eMethods, eQuestionnaire, eTables, eFigures:
www.aerzteblatt-international.de/m2023.0072

References

1. Robert Koch-Institut: COVID-19-Dashboard. Robert Koch-Institut 2022 www.experience.arcgis.com/experience/478220a4c454480e823b17327b2bf1d4 (last accessed on 28 November 2022).

2. Robert Koch-Institut: Corona-Monitoring bundesweit (RKI-SOEP-Studie): Überblick zu ersten Ergebnissen. https://www.rki.de/DE/Content/Gesundheitsmonitoring/Studien/lid/Ergebnisse.pdf?__blob=publicationFile (last accessed on 10 March 2023.)

3. Dorn F, Fuest C, Gstrein D, Peichl A, Stöckli M: 2 2020 12. Oktober 2020 Corona-Infektionen und die Dunkelziffer: Vergleichen wir Äpfel mit Birnen? www.ifo.de (last accessed on 27 November 2022).

4. Bundesministerium für Gesundheit: COVID-19 Impfdashboard 2022 www.impfdashboard.de/ (last accessed on 27 December 2022).

5. Robert Koch-Institut: COVID-19 Impfquoten-Monitoring in Deutschland (COVIMO). Bericht 7. www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus/Projekte_RKI/COVIMO_Reports/covimo_studie_bericht_7.pdf?__blob=publicationFile (last accessed on 10 March 2022).

6. Robert Koch-Institut: Coronavirus SARS-CoV-2 – Ergebnisse zur SARS-CoV-2-Seroprävalenz in der Allgemeinbevölkerung – Aktualisierung September 2022. www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus/AK-Studien/Ergebnisse.html (last accessed on 10 March 2023).

7. Robert Koch-Institut: Epidemiologisches Bulletin STIKO: 22. Aktualisierung der COVID-19-Impfempfehlung. 2022. www.rki.de/epidbull (lastt accessed on 14 November 2022).

8. Robert Koch-Institut: Epidemiologisches Bulletin STIKO: 23. Aktualisierung der COVID-19-Impfempfehlung 2022. www.rki.de/epidbull (last accessed on 22 February 2023).

9. Newton HJ, Cox NJ, Garrett JM, Pagano M, Royston JP: Stata Technical Bulletin 45. 1998. www.stata.com/bookstore/stbj.html . (last accessed on 27 November 2022).

10. EUROSTAT. NUTS – GISCO: www.//ec.europa.eu/eurostat/web/gisco/geodata/reference-data/administrative-units-statistical-units/nuts (last accessed on 27 November 2022).

11. Lange B, Jäger V, Rücker V, et al.: Interimsanalyse des IMMUNEBRIDGE-Projektes zur Kommunikation von vorläufigen Ergebnissen an die Modellierungskonsortien der BMBF-geförderten Modellierungsplattform. 2022. www.//zenodo.org/record/6968574 (last accessed on 27 November 2022).

12. Lange B, Jäger VK, Rücker V, et al.: 2. Interimsanalyse des IMMUNEBRIDGE-Projektes zur Kommunikation von vorläufigen Ergebnissen an das Modellierungsnetz für schwere Infektionskrankheiten. 2022. www.//zenodo.org/record/7177592 (last accessed on 27 November 2022).

13. Gilbert PB, Donis RO, Koup RA, Fong Y, Plotkin SA, Follmann D: A Covid-19 milestone attained—correlate of protection for vaccines. N Engl J Med 2022; 387: 2203–6

14. Forecasting Team C-19, Lim S: Past SARS-CoV-2 infection protection against reinfection: a systematic review and meta-analysis. www.//papers.ssrn.com/abstract=4155225 (last accessed on 22 February 2023).

15. Shao W, Chen X, Zheng C, et al.: Effectiveness of COVID-19 vaccines against SARS-CoV-2 variants of concern in real-world: a literature review and meta-analysis. Emerg Microbes Infect 2022; 11: 2383–92

16. Bobrovitz N, Ware H, Ma X, et al.: Protective effectiveness of previous SARS-CoV-2 infection and hybrid immunity against the omicron variant and severe disease: a systematic review and meta-regression. Lancet Infect Dis 2023; S1473–3099(22)00801–5. Epub ahead of print

17. Robert Koch-Institut: COVID-19 Impfquoten-Monitoring in Deutschland (COVIMO). Bericht 10. 2022. https://www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus/Projekte_RKI/COVIMO_Reports/covimo_studie_bericht_10.pdf?__blob=publicationFile (last accessed on 22 February 2023).

18. Fujii SI, Yamasaki S, Iyoda T, Shimizu K: Association of cellular immunity with severity of COVID-19 from the perspective of antigen-specific memory T cell responses and cross-reactivity. Inflamm Regen 2022; 42: 50

19. Moss P: The T cell immune response against SARS-CoV-2. Nat Immunol 2022; 23: 186–93

20. Yan LN, Liu PP, Li XG, et al.: Neutralizing antibodies and cellular immune responses against SARS-CoV-2 sustained one and a half years after natural infection. Front Microbiol 2022; 12: 803031