Expected outcome 1: Increased geometric mean (GMT) of anti-SARS-Cov-2 antibody titers 28 days after a booster dose of COVID-19 vaccine for each study group; measured by assay binding antibody detection of electrochemiluminescence and detection of neutralizing antibodies to SARS-CoV-2.;Expected Outcome 2: Frequency (%) of local and systemic adverse events requested within 7 days after each vaccine dose; for each group of participants.;Expected outcome 4: frequency (%) of serious adverse events throughout the study period for each group of participants.;Expected Outcome 3: Frequency (%) of unsolicited adverse events within 28 days after each vaccine dose; for each group of participants.

Expected outcome 1: Increased geometric mean (GMT) of anti-SARS-Cov-2 antibody titers 28 days after a booster dose of COVID-19 vaccine for each study group; measured by assay binding antibody detection of electrochemiluminescence and detection of neutralizing antibodies to SARS-CoV-2.;Expected Outcome 2: Frequency (%) of local and systemic adverse events requested within 7 days after each vaccine dose; for each group of participants.;Expected outcome 4: frequency (%) of serious adverse events throughout the study period for each group of participants.;Expected Outcome 3: Frequency (%) of unsolicited adverse events within 28 days after each vaccine dose; for each group of participants.