The aim of the project was:

• selection of the best serological test to identify antibodies directed against SARS‑CoV‑2,
• determining the percentage of people who developed antibodies in the group of people infected with SARS-CoV-2,
• determining the percentage of seropositive people among symptomatic and asymptomatic people previously infected with SARS‑CoV‑2,
• determining the percentage of people who developed antibodies in the group of people who did not suffer from illness themselves, but had direct contact with infected people (household members of infected people),
• determining changes in the level of antibodies over time after falling ill or after contact with an infected household member.

The additional purpose of the study was:

• determination of changes in the level of antibodies over time after vaccination against SARS‑CoV‑2

In the SARS (+) group, IgG antibodies were found in 83.95% within 4‑8 weeks from the onset of the disease. As time went on, the number of people who were positive for antibodies decreased. Within 10-12 months from the onset of the disease, only 69.57% of convalescents had the presence of antibodies. The maximum level of antibodies was found in 16.42% in the first period and 5.80% in the last observation period.

In the group of SARS (-) people, healthy household members living with infected people, IgG antibodies were found in 6.78% within 4‑8 weeks from the onset of the disease. As time went on, the number of people who were antibody positive increased. In the period of 10‑12 months of observation, the antibodies were found in 38.89% of people, which may indicate a subsequent asymptomatic infection. The maximum level of antibodies was found in 0.85% in the first period and 5.56% in the last observation period.

The observation of the natural course of the variability of the level of antibodies after contact with SARS‑CoV‑2 is interrupted by the administration of the vaccine.

Within 4-8 weeks of vaccination, IgG antibodies were found in 85.26%, and 58.67% of people achieved their maximum level (120 RU / ml).

In the next observation period (3-4 months), IgG antibodies were found in 98.47%, and 86.46% of people achieved their maximum level.

In the next period (5‑6 months), IgG antibodies were found in 99.29%, but their level began to decrease significantly: the maximum level of antibodies was found only in 63.13% in the 5‑6 month and 49.28% in the 7‑8 month of observation.

The visible increase in the number of people with the maximum level of antibodies (78.87%) in the last observation period (11-12 months) results from additional vaccination (administration of a booster dose) and a small number of observations during this period.

1. Serological tests accepted for clinical use (CE-IVD certified) do not seem to differ significantly in terms of sensitivity and specificity.
2. In the group of people infected with SARS‑CoV‑2, the percentage of seropositive persons was 83.95% in the period of 4-8 weeks from the onset of the disease, and 69.57% in the period of 10-12 months from the onset of the disease.
3. In the group of people who did not get sick themselves, but had direct contact with infected people (household members of infected people), the percentage of seropositive people was 6.78% in the 4-8 weeks after the onset, and 38.89% in the 10-12 months after the onset of the disease.
4. In the group of people infected with SARS‑CoV‑2, the level of antibodies after the onset of the disease decreased with time: the maximum level of antibodies was found in 16.42% within 4‑8 weeks from the onset of the disease and 5.80% within 10‑12 months after the onset of the disease.
5. In the group of people who did not get sick themselves, but had direct contact with infected people (household members of infected people), the level of antibodies after contact with an infected household member increased over time: the maximum level of antibodies was found in 0.85% within 4-8 weeks from the disease and 5.56% in the period of 10-12 months from the onset of the disease.
6. The percentage of seropositive persons after vaccination against SARS-CoV-2 was 85.26% one month after vaccination, reaching the value of 98.47% at month 3 after vaccination and remaining at this level until the booster dose was administered in months 5-6.
7. The maximum level of antibodies after vaccination was achieved by 58.67% of people 3-4 weeks after vaccination and 86.46% of people after the next month. This percentage decreased to 63.13% in 5-6 months and 49.28% in 7-8 months after vaccination.