Prevalence of unvaccinated and vaccinated: weaknesses in the calculation of LGL policy

– Once a week, the Bavarian State Office for Health and Food Safety (LGL) publishes the seven-day incidence of unvaccinated and vaccinated people. The calculation has weaknesses. A similar case at the Robert Koch Institute and a look at other federal states shows that calculations can theoretically be done differently.

In mid-August, LGL announced for the first time the seven-day incidence of unvaccinated and fully vaccinated individuals in Bavaria. Even then, critics argued that these values ​​could not be compared one by one. One argument: people with full vaccination protection would probably be tested much less frequently than people without immunization.

How many and how often vaccinated people get tested in relation to unvaccinated is unclear and highly individual dependent. However, the published occurrences of LGL have a completely different, concrete weakness in the calculation. There is apparently an under-registration of corona cases in vaccinated people.

Calculation of deposits

How does this underreporting occur? The calculation of the incidents is based on reporting cases with a reporting date for the last seven days, which are divided according to vaccination status. These are compared with the population figures for vaccinated and unvaccinated from the Robert Koch Institute’s (RKI) digital vaccination monitoring.

Vaccinated Covid-19 cases are considered individuals who had full vaccination protection at the time of infection. However, it is not only individuals who had not received a vaccination at the time of infection that count as unvaccinated Covid-19 cases. This also includes people with corona infection for whom there is no information on vaccination status from the health authorities. Persons with incomplete vaccination protection are not included in the calculation.

How does an unknown vaccination status arise?

LGL gives our editors an example. Admission of a person to a hospital in relation to Covid-19 must be notified. This notification is important in order to determine the incidence of hospitalizations to which additional safeguards are attached. In addition, the form for this notification also asks about the vaccination status of the hospitalized person. According to LGL, the information in reporting practices is often not completely filled out.

Underreporting of corona cases in vaccinated

The fact that persons with unknown vaccination status are initially included in the group of unvaccinated – information, if available, is added with a time delay – may lead to a higher incidence among unvaccinated and a lower incidence among vaccinated.

Now the crucial question is how high the proportion of Covid-19 cases is with unknown vaccination status. This is the only way to actually determine how strongly distorted the two ratios are. However, LGL can not give our editorial staff any more detailed insight into how high this under-reporting of the vaccinated can be.

When asked about this weak point in the calculation, LGL replied that after a thorough analysis of the available registration data and considerations, it was decided to include the group with unknown vaccination status among the unvaccinated. This is based on “that otherwise a not insignificant number of reporting cases would be excluded in the first instance due to lack of information in the evaluation”. LGL is always working to improve the data situation.

We asked the authority’s press office to clarify the information and specifically wanted to know how high the proportion of corona cases with unknown vaccination status is – also for example for the occurrences published last Thursday 11 November. Information from LGL: More accurate information is not possible, due to numerous inquiries, LGL can not offer individual evaluations.

Similar case at RKI

RKI’s calculation of vaccination effectiveness had a similar weakness, but they have now responded: First, in calculating vaccination efficiency, RKI has also counted persons with unknown vaccination status among the unvaccinated. Here, too, it had to be assumed that the number of corona cases in vaccinated people was underestimated and the vaccination effect was therefore overestimated. However, the authority adjusted its calculation at the end of September and now only takes into account those Covid 19 cases for which there is a complete indication of vaccination status.

Look at other states

There are no nationwide uniform calculation rules for the two indicators, and by no means do all federal states record the two occurrences. One reason why data is not collected in, for example, North Rhine-Westphalia is corona cases with missing or incomplete information on vaccination status. A spokesman for Lower Saxony justified the rejection on the grounds that the calculation was “quite complex and time-consuming”.

In addition to Bavaria, Thuringia and Saxony, among others, also register separate deposits. In Thuringia, corona cases are only taken into account if the information on vaccination status is complete. In Saxony, an incidence is calculated for “fully vaccinated persons” and one for “not (fully) vaccinated persons”. The “not (fully) vaccinated” also include all Covid 19 cases for which no vaccination status information is available because “not collected” or “cannot be determined”.

In Bavaria, LGL justifies its calculation in spite of the difficulties: “Nevertheless, the data collected in this way provide an opportunity to analyze general statements and trends about the relationship between the affected and unvaccinated population.”

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