If you ask yourself this question, you might think about an antibody test to make a decision for or against the booster. But experts believe that this is only useful to a limited extent. Why? We answer important questions on this topic.

It strengthens the immune system again against the Sars-CoV-2 virus. This is where the term booster comes from.

Actually, the term booster is not quite correct, writes immunologist Carsten Watzl on Twitter. It seems that the vaccination restores something that has been lost. But the booster not only restores the condition after the second vaccination: immunity is better after the booster than after the second vaccination.

That also makes sense, says the expert. Because every time the immune system comes into contact with the pathogen or the vaccine, the immunity becomes stronger, better and more durable.

The ulterior motive seems to make sense: to know how high the immune protection built up by the previous Corona vaccinations is. And thus be able to assess whether the booster vaccination is necessary or (still) dispensable.

Here, antibody tests could provide clarity, says Andreas Bobrowski, Chairman of the Professional Association of German Laboratory Physicians. But he also makes it clear that it makes no sense to recommend these tests across the board. However, for those who are susceptible to infections or generally immunocompromised, such a test may provide important information on how well the vaccinations have worked.

The Robert Koch Institute (RKI), however, has a clear message: it is not recommended to do an antibody test before the booster vaccination to check whether there is still protection against Covid-19. One reason for this is that it is not yet known exactly at what antibody level one can assume sufficient protection.

According to Andreas Bobrowski, an antibody test usually checks the so-called anti-spike antibodies (IgG) that are formed as a result of the vaccination. It is important that such a test gives meaningful and comparable results. This is done with a standard of the World Health Organisation WHO, which is given in BAU/ml (BAU = Binding Antibody Units).

However, there are no safe limits for the BAU/ml value at which one is still considered protected. They are not (yet) known, Watzl writes on Twitter. The question of whether it is necessary to determine the antibodies before booster vaccination, he answers like the RKI: “No."

Yes, there are. Looking at data from his laboratory and from Israel, among others, Bobrowski says: "Below a value of 21.8 BAU/ml, there is no measurable protection by anti-spike antibodies. Above that, however, there is a large grey area where it is not really known when the protection is sufficiently good.

"In my estimation, a value of 500 is high enough that you don't need a third vaccination immediately," the laboratory doctor said. Anything above 1,000 BAU/ml can be considered good protection, he said. This coincides with what Carsten Watzl writes on Twitter after looking at the studies on the subject: "Antibody values above 1,000 BAU/ml seem to correlate with quite good protection against symptomatic infection." Correlate means to be related.

No, the RKI clarifies. However, many people "mistakenly" assume that a booster should not be administered if the antibody level in the body is high due to basic immunisation. Even if immunity is still present, there are no safety concerns with regard to the booster. The protection only gets better.

The discussion about these threshold values is about protection against symptomatic infection, writes immunologist Watzl. "Protection against severe disease can still be high." Therefore, his advice: don't panic with low antibody levels, but better make a booster appointment.

As a rule, these tests are not covered by public health insurance. So you have to pay the costs yourself. Antibody tests that check the effect of previous vaccinations cost a good 20 euros.

Yes, but not always to the same extent. In Germany, boosters are only used with so-called mRNA vaccines. Specifically, these are the vaccines from Biontech (Comirnaty) and Moderna (Spikevax).

Those who received their basic immunisation with one of the vector vaccines from AstraZeneca or Johnson & Johnson receive a different type of vaccine as a booster anyway, which, according to the data available so far, gives the immune protection an enormous boost.

Here, data from the USA, published as a preprint and pointed out by the SPD health expert Karl Lauterbach on Twitter, show that the vaccine is not the same: A mixture - such as basic immunisation with Biontech and booster with Moderna - does indeed produce small differences in the booster effect.

But they are small: the average effect on antibody levels with the combination Biontech/Moderna was 17.3, with Biontech/Biontech it was 14.9. Conversely, with Moderna/Biontech it was 9.7, with Moderna/Moderna 7.9.

It is therefore irrelevant which mRNA vaccine is used for the booster, says Carsten Watzl in view of these data when asked. Especially since in this study booster vaccinations were given with the full dose of Moderna (100 micrograms). For the booster in Germany, however, half the dose (50 micrograms) should be used.

This would make the differences smaller or even disappear, explains the Secretary General of the German Society for Immunology.

The Standing Commission on Vaccination (Stiko) recommends anyway that the mRNA vaccine that was already injected during the basic immunisation should be used for the booster vaccination.

In addition, according to the latest recommendation of the Stiko, Moderna should no longer be used for people under 30 years of age. As the data show, however, no one has to fear disadvantages in the booster effect if he or she receives Biontech a third time.

For comparison: If mRNA vaccinated people are given the Johnson & Johnson vaccine as a booster, the antibody booster effect is on average only a factor of 6.2 (in combination with Biontech) or 4.7 (combination with Moderna), according to this study.

On the other hand, those who received the vaccine from Johnson & Johnson as their first vaccination benefited greatly from the mRNA booster: with Biontech, the factor was 32.8, with Moderna even 56.1.

Thus, these data also provide evidence why it makes sense to boost a basic vector vaccine immunisation with an mRNA vaccine. The interval between the booster and the basic immunisation was at least twelve weeks in the study.

Theoretically, you can vaccinate earlier. According to Watzl, anything between four and eight months is "probably okay" for people with healthy immune systems. People with a severely weakened immune system can already get their third dose after 28 days, according to the Stiko's recommendation. This also applies to all those who received the Johnson & Johnson vaccine as their first vaccination.

Original text: Tom Nebe, dpa

Translation: Mareike Graepel