Results of the nationwide antibody study

From July 2020 to August 2021, a research team from the HZI, with external support, studied the antibody status of more than 26,000 people in seven county districts and thus collected important data on the pandemic event.

One of the pitfalls of the SARS-CoV-2 coronavirus is its ability to cause only mild cold symptoms in many infected people or even to go completely unnoticed. This is precisely its great success: The virus causes the majority of its hosts to fall ill only mildly or not at all and is passed on unchecked. Nevertheless, a minority becomes seriously ill with Covid-19, mainly people with a weakened immune system due to their age or a pre-existing condition. The very high total number of infected people – there are now over six million registered cases in Germany – entails a large amount of severe courses of the disease despite the low percentage, which have cost the lives of more than 100,000 people. In order to be able to derive appropriate protective measures, it is important to know approximately how high the number of unreported infections is, in which areas of life most infections occur and to how many people an infected person passes on the virus on average.

In July 2020, the Department of Epidemiology of the Helmholtz Centre for Infection Research (HZI) launched the nationwide study "MuSPAD" to gain insights into these aspects. MuSPAD stands for "Multilocal and Serial Prevalence Study on Antibodies to SARS-CoV-2 Coronavirus in Germany" and ran until August 2021. As part of the study, the research team used blood samples to determine the antibody status in the population in different counties that were affected to varying degrees by the pandemic.

"We set up a study centre at each study site, where nine milliliters of blood were taken from each participant," says HZI scientist Manuela Harries. The blood samples were then tested for antibodies against SARS-CoV-2 to find out whether the test person had already had contact with the virus. "In addition, the participants answered a questionnaire about their living conditions. From these data we can, for example, deduce risk factors for an infection," says Harries. The selected study sites were Reutlingen, Freiburg, Aachen, Osnabrück, Magdeburg, Chemnitz and Vorpommern-Greifswald. In the first round, invitations for voluntary participation went out to more than 70,000 people, for the second round again to just under 50,000. The addresses for the invitations were drawn as random samples by the residents' registration offices in order to reach a representative cross-section of the population.

"For a scientific study, the willingness to participate was extremely high," says HZI scientist Daniela Gornyk. Many people had a great interest in finding out whether they had perhaps already been through an infection without realising it. In the study centres, the service providers BOS 112 and Johanniter in particular supported the HZI team, while the cities and districts called on their populations to participate via the media. Despite the help, the study remained an organisational challenge: It sometimes took a weekend to print and cut out thousands of barcodes. In addition, there were also negative reactions to the invitations, which in individual cases resulted in complaints and even insults. Much more often, however, the opposite occurred: Numerous people without invitations were eager to take part in the study. "We were pleased about that, but unfortunately we couldn't take on any additional volunteers so as not to distort the representative cross-section," says Gornyk.

In the end, almost 17,000 people aged 18 to 99 took part in the first round. In the second round, almost 40 per cent of them participated again, while more than 9,000 unvaccinated people were newly added at six study sites. In order to obtain representative results for each of the county districts, the research team calculated a factor in each case to compensate for the difference in the age and gender distribution of the participants compared to the distribution in the entire county district population. The seroprevalence thus weighted by age and gender, i.e. the proportion of people with antibodies in their blood, was between 1.3 and 2.6 per cent in Freiburg, Reutlingen, Aachen and Osnabrück during the first sampling until October 2020.
"The majority of reported cases at the first study sites in July to October resulted from infections during the first wave, which indicates a high under-reporting by a factor of three to five due to the not yet so intensive testing and many asymptomatic infections in the summer," says Manuela Harries. In the study sites that only had their turn after the second or third wave until August 2021, another 2.4 to 19.9 percent of the population that had not yet been vaccinated had experienced an infection. The last study site was Chemnitz in July 2021, where the weighted seroprevalence among participants who had not yet been vaccinated was 32.4 per cent. This may come from both a higher proportion of people with an already known SARS-CoV-2 infection agreeing to participate upon invitation, or a larger proportion of the unvaccinated compared to the vaccinated having already undergone infection.

MuSPAD found that for every reported SARS-CoV-2 infection, there were two to five people actually infected, with infections detected more frequently in people over 80 years of age. The under-reporting – i.e. the number of unreported cases – was lower in the second and third waves than in the first. During the first survey, 2.7 percent of the participants stated that they had already been in quarantine. During the second survey, the figure was 3.2 per cent. In addition, 7.6 and 4.6 per cent had voluntarily entered quarantine. Antibodies against SARS-CoV-2 were present in 14 percent of all people who had been in quarantine. Rounded up, this means: On average, seven people had to go into quarantine so that one infected person could be secured among them. Incidentally, among the participants who had never been in quarantine, 2.1 percent had antibodies against SARS-CoV-2 in their blood.

"The study gives us reliable figures on regional and age-specific undercoverage and on the incidence of infection in different phases of the pandemic."
Manuela Harries, HZI Scientist

The survey on living conditions also yielded interesting results: For example, the proportion of people who smoke daily was 10.1 percent in Freiburg, but 23.8 percent in Greifswald. They fell ill less frequently, but often more severely. In Freiburg, 70.2 percent had a university degree, in Chemnitz 38 percent. In this context, it is known that people with a higher socio-economic status fall ill less often, which may be due, for example, to the fact that they more often have the opportunity to transfer their work to their home office and thus reduce their daily contacts.

"The study gives us reliable figures on regional and age-specific undercoverage and on the incidence of infection in different phases of the pandemic," says Manuela Harries. "It thus helps to better understand pandemic events and the effectiveness of protective measures in different regions and population groups in Germany."

Harries hopes to gain further insights from an additional study round that took place in Hanover: There, almost 3000 people who have already participated in Germany's largest population study – the NAKO Health Study – accepted the MuSPAD team's invitation. "By comparing our findings with the NAKO data already collected on the health status, lifestyle or hereditary factors of the participants, we may be able to derive statements about the long-term effects of a coronavirus infection." However, the first results are not expected until 2022.

Author: Andreas Fischer

Published: December 2021

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