inclusion criteria: * signed informed consent * previously totally immunized, i.e., two doses for corona vac (sinovac) or chadox1 (az 1222, astra zeneca, and one dose for ad26.co2.s (janssen) vaccines at least three months before enrollment * negative rt-pcr for sars-cov-2 at the moment of triage * no previous history of laboratory confirmed covid-19

inclusion criteria: * signed informed consent * previously totally immunized, i.e., two doses for corona vac (sinovac) or chadox1 (az 1222, astra zeneca, and one dose for ad26.co2.s (janssen) vaccines at least three months before enrollment * negative rt-pcr for sars-cov-2 at the moment of triage * no previous history of laboratory confirmed covid-19

inclusion criteria: - signed informed consent - previously totally immunized, i.e., two doses for corona vac (sinovac) or chadox1 (az 1222, astra zeneca, and one dose for ad26.co2.s (janssen) vaccines at least three months before enrollment - negative rt-pcr for sars-cov-2 at the moment of triage - no previous history of laboratory confirmed covid-19

inclusion criteria: - signed informed consent - previously totally immunized, i.e., two doses for corona vac (sinovac) or chadox1 (az 1222, astra zeneca, and one dose for ad26.co2.s (janssen) vaccines at least three months before enrollment - negative rt-pcr for sars-cov-2 at the moment of triage - no previous history of laboratory confirmed covid-19