Reports of vaccinated people getting COVID-19 have been appearing in the news over and over in recent weeks, but that's neither a reason to get angry, nor an argument to reject vaccines. Even if someone experiences mild symptoms after a vaccination or tests positive for the coronavirus, the vaccine is still effectively doing what it's supposed to do: prevent severe disease progression and death.

How effective are the currently approved vaccines?

All of the vaccines approved by the health authorities of the European Union (EMA) and the United States (FDA) have shown high efficacy.

For example, mRNA vaccines from BioNtech-Pfizer and Moderna achieve about 95% efficacy, while AstraZeneca's vector vaccine achieves 76%, according to recent data. However, this means that it's still possible to become infected after a vaccination.

Still, if efficacy is considered according to the severity of the course of the disease, all vaccines perform significantly better than going without. In the event of infection, for instance, vaccines usually prevent the type of severe reaction that requires people to be put on ventilators — or even kills them.

Why have COVID deaths occured after vaccination?

In Germany, however, there have been some cases in which seniors in nursing homes still had severe COVID-19 courses even after vaccination. Some even died.

The Robert Koch Institute (RKI), Germany's central authority for infectious diseases, writes that two circumstances in particular can lead to illness after vaccination — namely, if the infection occurred shortly before vaccination or in the days immediately thereafter.

The reason? The body needs about two weeks to fully build up immunity. Thus, complete protection is only achieved two weeks after a patient has received his or her second vaccination shot.

What do sterilizing immunity and functional immunity mean?

When it comes to vaccination, doctors distinguish between sterilizing and functional immunity. Sterilizing means that a vaccinated person can no longer infect anyone. The first evaluations of BioNTech-Pfizer vaccines administered in Israel  showed that people who had been vaccinated twice had a 92% lower risk of infecting other people.

In this respect, it's safe to say the BioNTech-Pfizer vaccine offers a fairly high sterilizing immunity against the most common coronavirus variants. Though conclusive figures are not yet available for other vaccines, efficacy is likely comparable.

Nevertheless, only when there is clear evidence that vaccinated individuals cannot infect others, or when a sufficiently high "herd immunity" has been reached, can authorities exempt vaccinated people from obligations such as wearing face masks or observing physical distancing rules.

Still, though we have not yet reached that point, all of the vaccines currently available have been shown to provide functional immunity, i.e., prevent or reduce the risk of severe symptoms. And even in rare cases of disease occurring, vaccines have significantly shortened the course of illness.

How long does a COVID-19 vaccine protect against infection?

So far, there is no clear answer to this question for any of the vaccines. Before vaccines are approved, researchers must determine how safe they are. And in phase III of vaccine development, experts focus on functional efficacy.

But only with time will we learn how long the immunity from the COVID vaccines lasts. That's why the period after vaccines are licensed is also known as Phase IV of vaccine development. We are now in the middle of this phase. Reliable statements about the duration of immunity will only be possible after several months or even years.

What are the effects of mutations and variants?

Each vaccine responds differently to viral mutations. At the moment, vaccine developers are concerned about the P.1 variant found in Brazil. This mutant variation has infected numerous people even though they had already been previously infected or vaccinated. The South African variant B.1.351 can also infect people who have been vaccinated, as medical experts were able to prove with the AstraZeneca vaccine.

But here, too, physicians assume vaccines will at least lead to milder courses of the disease and reduce the number of patients who require intensive care medical treatment.

This has to do with the fact that immunity consists of two components — antibody immunity against the spike protein, in which individual components can mutate, and T-cell immunity, which continues to protect even when the antibodies are no longer as effective.