1. serial of neutralizing antibody at 2, 4, 6, 9, 12 month after COVID-19 vaccination
2. investigate the factors on antibody titer.

1. serial of neutralizing antibody at 2, 4, 6, 9, 12 month after COVID-19 vaccination 1) semi-quantitative measurement of neutralizing antibody at 2, 4, 6, 9, 12 month after COVID-19 vaccination 2) investigate the duration maintaining acceptable range (over 30% inhibition)
2. investigate the factors on antibody titer at 2 month after vaccination 1) potential factors: age, sex, body weight, height, use of medication, use of antipyretics (NSAID or tyrenol), side effect of COVID-19 vaccination 2) correlation between antibody titer and potential factors
3. Compare 4 diagnostic ELISA kit at 2 month after vaccination 1) measure neutralizung antibody using ELISA kits from 4 different companies 2) investigate correlation coefficient of antibody titer between companies

Inclusion Criteria:

• healthcare works after completion of COVID-19 vaccination
• 20-60 years
• sign the agreement of study

Exclusion Criteria:

• serious underlying diseases

healthcare works aged 20-60 years after completion of COVID-19 vaccination

participants have no serious disease.