For those vaccinated with AstraZeneca, STIKO (Standing Committee on Vaccination) recommends a second dose with an mRNA vaccine.
Stimulating the immune system in a different way, heterologous vaccination, ensures an improved immune response that leads to antibodies that can tackle and defeat the pathogen with a better possibility. Mix-and-match vaccines would be more effective than two doses of Astrazeneca and would not bring any kind of interaction risk between the two drugs. The Robert Koch Institute recommends the second dose with mRNA vaccine to all those who have received the Astrazeneca vaccine as the first dose. However, it takes at least four weeks between the two doses.
You can receive the booster in medical offices and in vaccination centers. The third dose, which can be administered five months after the second, will instead be injected with the BioNTech or Moderna vaccine, therefore, vaccination with AstraZeneca will no longer be possible. People under 30 years old will only be able to get vaccinated with BioNTech.