The goal of this observational study is to evaluate the protective effect of immune barrier on secondary infection after COVID-19 (coronavirus disease 2019) vaccination or COVID-19 virus Omicron B A. 5.2 strain infection by dynamically monitoring the COVID-19 antibody titer, cellular immune function and the occurrence of secondary infection of healthy participants, mainly medical staff in our hospital, to understand the cross protective effect of COVID-19 antibody on different variants of Omicron, and explore the best time to use COVID-19 vaccine to strengthen immunity after Omicron mutant infection.

This study is a single center, prospective, non-intervention study. It is planned to recruit 300 medical and nursing participants, administrative and logistics participants or other participants who can cooperate with the follow-up for 48 weeks to voluntarily join the group.

Participants will be investigated about the vaccination history of COVID-19 vaccine, whether they have ever been infected with COVID-19, the time of initial infection with COVID-19, and following clinical symptoms.The COVID-19 antibody titer of peripheral blood will be used for baseline.

The participants will be divided into positive group and negative group according to the COVID-19 antibody titer results,then they wii be followed up for 48 weeks.If the participants have no symptoms related to COVID-19 infection, COVID-19 antibody and COVID-19 nucleic acid will be tested every 4-8 weeks according to the epidemic situation (the specific visit time is initially set as 0 week, 6 ± 2 weeks, 12 ± 4 weeks, 20 ± 4 weeks, 28 ± 4 weeks, 36 ± 4 weeks, 48 ± 4 weeks). There will be about 7 visits in total, according to the epidemic situation and the time of secondary infection of the participants). If the nucleic acid turns to be positive, we will consider it as the asymptomatic infection, then we will collect nasal/pharyngeal swabs for gene sequencing of virus strains, and terminate the study.

If the participants get fever, fatigue, sore throat, cough, nasal congestion, runny nose, muscle soreness or headache and other symptoms related to COVID-19 infection during the study period, they need to test the COVID-19 antigen or COVID-19 nucleic acid within 48 hours. If it is positive, we will collect nasal/pharyngeal swabs for gene sequencing of virus strains, and terminate the study in advance.

Inclusion Criteria:

1. No age limit, no gender limit;
2. Medical staff, administrative and logistics staff who work in the Third Affiliated Hospital of Sun Yat-Sen University or other participants who can cooperate with the follow-up for 48 weeks;
3. The participants need to be sure whether they have ever been infected with COVID-19, and need to be clearly remember the time when they first infected with COVID-19.

Exclusion Criteria:

1. Have the following serious respiratory diseases: such as asthma, bronchiectasis, chronic obstructive pulmonary disease, pulmonary interstitial disease, tuberculosis and other respiratory diseases that may interfere with symptom observation;
2. Those with other serious diseases or disease history that affect immune function, including but not limited to uncontrolled and unresectable malignant tumors,hematological diseases, cachexia, active bleeding, severe malnutrition, mental diseases, autoimmune diseases, HIV, etc.
3. Those who have long-term assignment plans and cannot return to the hospital regularly for follow-up.

The participants need to be sure whether they have ever been infected with COVID-19, and need to be clearly remember the time when they first infected with COVID-19.