Why is there such a strong East-West gradient in incidence?

fact finder

Status: 05.07.2022 10:19 a.m

The incidences in the eastern federal states are currently well below those elsewhere. In conspiracy circles, this is explained by the lower vaccination rate. What are the real causes?

By Anna Behrend, NDR, and Pascal Siggelkow, ARD fact finder editors

The nationwide seven-day incidence in Germany has been increasing significantly for several weeks. Federal Health Minister Karl Lauterbach has already spoken of a corona summer wave. But when you look at the developments, you notice that there are huge differences between the federal states. While the incidences in Lower Saxony and Schleswig-Holstein have already risen above the 900 threshold, the values ​​in Thuringia and Saxony are still below 400.

In general, it is noticeable that all eastern German states – with the exception of Mecklenburg-Western Pomerania – are even well below the nationwide incidence of 687.7. And that despite the fact that Saxony, Brandenburg and Thuringia are among the tail lights when it comes to vaccination rates. In conspiracy circles, there are already stories about a “negative protection”, in particular from booster vaccinations, and the numbers would impressively prove the ineffectiveness of the vaccinations. But is there really a causal relationship between vaccination coverage and incidence?

Vaccination protects against severe disease and death

One thing is certain: unvaccinated people who become infected with the corona virus have a significantly higher risk of having to be treated in an intensive care unit or of dying than vaccinated people. This applies both to the Omicron variant currently in circulation and to the Delta variant that prevailed in autumn and winter last year. For the past year researchers from Jena, among others, examined excess mortality in the German federal states and related them to regional vaccination rates. The conclusion: Where the vaccination rate was high, excess mortality was comparatively low. Vaccination is therefore still a crucial factor in ensuring that the healthcare system is not overburdened.

Many vaccine breakthroughs with omicron variant

However, since the severity of the course of an infection is irrelevant for the incidence, it depends on the so-called vaccination breakthroughs – i.e. whether people become infected despite vaccination. In the case of the Delta variant, the available vaccines protect relatively well against a symptomatic infection. This could be one reason why the incidence figures in the delta wave last winter behaved in the opposite way regionally than they are now: At that time they were low where the vaccination rate was high.

The situation is somewhat different with the omicron variant: the vaccination protects against a severe course and death from Covid-19, but according to the RKI information it hardly protects against an infection itself.

In any case, the distinction between vaccinated and unvaccinated people in the development of the infection process is becoming less and less useful, says Johannes Knobloch, specialist in microbiology, virology and infection epidemiology at the University Hospital Hamburg-Eppendorf (UKE). Because of the extremely high number of cases, many unvaccinated people would have had at least one contact with Covid-19. “In the meantime, an additional infection has also occurred in many vaccinated people. In order to make a comprehensive statement, one would have to compare many subgroups with one another.”

Some studies indicate that vaccinated people infected with Omicron are slightly less contagious than unvaccinated people, but so far there is not enough data to make a conclusive assessment. The following applies to all virus variants: There is no scientific evidence that vaccination promotes infection with the coronavirus.

Incidence only of limited significance

However, if the vaccination rate in the omicron wave is not the decisive factor for the development of the incidence, how is it that the number of new infections in the eastern federal states is currently comparatively low?

First of all, it must be noted that the current infection numbers are only of limited significance. “You always have to remember that incidences are calculated based on reported laboratory results,” says Knobloch. As before, only positive PCR tests count in the RKI statistics. The willingness to be tested and the accessibility to PCR tests therefore currently have a very large influence on the measured incidence. Their significance is therefore “only very limited”. “The number of unreported cases is currently probably higher compared to all previous phases of the pandemic.”

According to experts, however, whether the wave of infection is rising or falling can still be seen from the incidence. However, caution is required when making regional comparisons, since the test strategies and thus the number of tests in the federal states differ greatly. Fewer tests also mean fewer infections are detected.

A high proportion of positive tests is therefore also an indication of a high number of unreported cases, such as in Bremen (86 percent) or Thuringia (55 percent). But it is by no means the case that all eastern federal states have a suspiciously high positive rate compared to the others. The different test procedures alone cannot therefore explain the currently low incidence there.

Eastern federal states already more infected?

If the incidence were actually much higher, one would also expect that this would also be noticeable in the intensive care units with a few days delay. Because a certain proportion of all those infected suffer a severe course and have to be treated in the hospital. However, the proportion of Covid patients in the intensive care units is currently not noticeably high in a national comparison in the eastern federal states.

So maybe most people in the low-incidence countries were already infected and are now immune? Anyone who becomes infected with the corona virus usually forms antibodies against it. How long and how well these protect against renewed infection with the virus has not yet been conclusively researched. However, assuming that there is protection for at least a while, one conceivable reason for the low incidence in certain federal states would be that more people have come into contact with the virus there in the past and are therefore less likely to become infected .

Due to a lack of studies, it is not known exactly how badly the population in which federal state is already infected with the virus. If one considers the number of new infections per 100,000 inhabitants since the beginning of the year as a rough approximation for the infection with the currently circulating omicron variant, the assumption that the population of the eastern federal states has already come into greater contact with it is not confirmed. Only Mecklenburg-Western Pomerania has had a rather high number of infections per capita since the beginning of the year in a national comparison.

Regional appearance of new variants

One reason why the incidences are rising so sharply nationwide is the omicron subtypes BA.4 and BA.5. Experts consider these to be more contagious than the previously known “sister variants”. BA.5 is now dominant in Germany with 65.7 percent, and the trend is clearly rising. It can therefore be assumed that the number of new infections will continue to rise in the coming weeks.

In which federal states the new subtypes are already rampant has not been published by the RKI since April due to too few samples. A comparison would no longer be meaningful due to excessive fluctuations. Nevertheless, it is of course possible that some federal states are already more affected by the new subtypes than others, which may yet be affected. Other factors such as population density and contact behavior also influence the spread of the virus and thus the seven-day incidence.

“We have been seeing different epidemiological courses in the various federal states since autumn of last year, so that we no longer expect parallel courses of the incidence curves in all federal states,” says Knobloch. “Newly occurring variants or sub-variants can only make themselves felt in other regions a few weeks later.”

How the virus spreads and what incidence is measured regionally depends on many factors. What is certain, however, is that there is no scientifically sound reason to assume that a high vaccination rate can be partly responsible for a high incidence. On the contrary: scientific data tends to indicate that vaccinated people infected with omicron infect other people somewhat less frequently than unvaccinated people.

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