Inclusion Criteria:  People who lived in the two blocks (V and Y) of Ngo Gia Tu apartment, located in District 10, Ho Chi Minh City, Vietnam, from July to September 2021 18 years of age and older Ability to understand and willingness to sign a written informed consent document.  Exclusion Criteria:  People who were diagnosed with primary or secondary immunodeficiencies induced by diseases or medical treatments (immunosuppressants, chemotherapy, radiation therapy, ect.)