inclusion criteria: * aged 4 years old * with legal guardian signing the informed consent * able to participate in all planned visits and comply with all research procedures (such as completing diary cards of adverse events and participating visits) * with clear vaccination records including no sars-cov-2 vaccine history, with 1 varicella vaccine and 2 measles-containing vaccines history * the last attenuated vaccine was administered ≥28 days ago, and other vaccines administered ≥14 days ago * with an axillary temperature ≤37.5℃ at the time of vaccination

inclusion criteria: * aged 4 years old * with legal guardian signing the informed consent * able to participate in all planned visits and comply with all research procedures (such as completing diary cards of adverse events and participating visits) * with clear vaccination records including no sars-cov-2 vaccine history, with 1 varicella vaccine and 2 measles-containing vaccines history * the last attenuated vaccine was administered ≥28 days ago, and other vaccines administered ≥14 days ago * with an axillary temperature ≤37.5℃ at the time of vaccination

inclusion criteria: aged 4 years old with legal guardian signing the informed consent able to participate in all planned visits and comply with all research procedures (such as completing diary cards of adverse events and participating visits) with clear vaccination records including no sars-cov-2 vaccine history, with 1 varicella vaccine and 2 measles-containing vaccines history the last attenuated vaccine was administered ≥28 days ago, and other vaccines administered ≥14 days ago with an axillary temperature ≤37.5℃ at the time of vaccination

inclusion criteria: aged 4 years old with legal guardian signing the informed consent able to participate in all planned visits and comply with all research procedures (such as completing diary cards of adverse events and participating visits) with clear vaccination records including no sars-cov-2 vaccine history, with 1 varicella vaccine and 2 measles-containing vaccines history the last attenuated vaccine was administered ≥28 days ago, and other vaccines administered ≥14 days ago with an axillary temperature ≤37.5℃ at the time of vaccination