1) Participants in the previous study A Survey of Vaccine-Induced COVID-19 antibody Titer to Verify Temporal Changes who received two doses of COVID-19 vaccination and completed antibody titer measurement on Day 29.2) Subjects who have provided written informed consent, are able to comply with the study protocol for the duration of the study, are able to make themselves available on scheduled study days as specified by the protocol, and are able to report on their health conditions such as symptoms.

1) Participants in the previous study A Survey of Vaccine-Induced COVID-19 antibody Titer to Verify Temporal Changes who received two doses of COVID-19 vaccination and completed antibody titer measurement on Day 29.2) Subjects who have provided written informed consent, are able to comply with the study protocol for the duration of the study, are able to make themselves available on scheduled study days as specified by the protocol, and are able to report on their health conditions such as symptoms.