ARCHIV - 26.04.2021, Berlin: Menschen warten im Corona-Impfzentrum auf dem Messegelände auf ihre Impfung. Die bundesweite Sieben-Tage-Inzidenz ist in den vergangenen Tagen deutlich gefallen. (zu dpa: DRK-Präsident Czaja wirbt für Impfterminschwänzer-Abgabe) Foto: Michael Kappeler/dpa +++ dpa-Bildfunk +++

A large part of the adult population in Germany came into contact with Corona at least twice by the turn of the year 2021/2022. Around 90 percent had previously been vaccinated twice or had had an infection before or after the initial vaccination – in both cases one is considered to have had basic immunization. These are the first results of the study “Corona Monitoring nationwide – Wave 2” by the Robert Koch Institute (RKI).

Almost a third of those over the age of 18 had built up hybrid immunity, i.e. had been vaccinated twice by the turn of the year and had further antigen contact through boosters or infection. An estimated ten percent of the adult population had been infected with Sars-Cov-2 by the time examined.

The study is based on a nationwide sample of over 11,000 participants aged 14 and over from 6,760 households. The RKI experts carried it out between November 2021 and February 2022, with the results mainly relating to the period up to the turn of the year – i.e. at a time when the omicron variant replaced the delta variant during the wave of infections.

The researchers wanted to investigate how many people had been infected and how many had been vaccinated by then – and thus also check whether the data collected, for example on new infections or the monitoring of the vaccination rate, is reliable. Apparently they are: According to the researchers, the results of the study are consistent with those of other epidemiological studies in Germany from the second half of 2021.

Volunteers were asked to take blood from their fingertips themselves – a kit was sent to them. The researchers checked the samples in the laboratory for IgG antibodies to find out whether the people had been infected with Sars-Cov-2 or had been vaccinated against the virus.

In addition, the participants were asked to fill out a twelve-page questionnaire and state, among other things, whether they had been vaccinated – if so, how often, when and with what type of vaccine. They should also answer whether they have undergone a corona infection detected with a PCR test – if so, when. In addition to German, the questionnaire was also available in Arabic, Farsi, English, Polish, Bulgarian and Romanian.

The results of the study indicate many corona antibodies in adolescents and adults by the turn of the year 2021/22. Most of the positive antibody detection was due to vaccinations.

Hybrid immunity, i.e. three contacts with Sars-Cov-2 through vaccination and/or infection, had built up around 14 percent among 14 to 17-year-olds. Among the 18 to 59 year olds it was almost 27 percent and among the over 60 year olds almost 37 percent.

Eleven percent of 14- to 17-year-olds had been infected, and in the age group over 60 the proportion of those infected was seven percent. The study does not analyze whether there is a causal connection to the fact that the corona measures in force at the time led to the low number of infections.

However, the study has its limits: The scientists assume that people who have been vaccinated and who have also endorsed and complied with measures to contain the pandemic took part. The fact that more vaccinated people are taking part in corona studies is a well-known phenomenon that has also been observed in other studies.

The fact that the proportion of those who were vaccinated at least once, at 94 percent, was also higher than the data from the digital vaccination rate monitoring for Covid 19 vaccination at around 86 percent at the turn of the year 2021/22 suggests that it is overestimated. The proportion of infected people is underestimated in the study.

Certain uncertainties in the questionnaire data and the laboratory tests may have contributed to the underestimation or overestimation, mainly because the level of antibodies in the blood decreases over time, in some faster than in others. Some people do not build antibodies or lose them quickly.

Protection against infection and disease cannot therefore be derived from a positive antibody status against Sars-Cov-2 alone – this is also emphasized by the researchers of the study. They hope that in the future blood tests will provide more information about immunity or which corona variants you have already come into contact with.

The data is several months old and does not reflect the current situation in Germany. There are other tools for this that are updated daily or weekly, according to the researchers. They conclude that there are large vaccination gaps, especially when it comes to booster vaccinations. That would currently be 72 percent for the adult population.

The researchers will continue to evaluate. Among other things, analyzes are planned as to which population groups are affected by a high risk of infection and why; which factors influence the willingness to be vaccinated or how good or difficult access to the healthcare system is; how the pandemic has affected mental health, but also on the long-term consequences of Covid-19 disease. The results are to be published gradually over the course of the year.