Introduction:
Severe acute respiratory syndrome, coronavirus type 2 (SARS-CoV-2) has become a global threat for every healthcare system, and the coronavirus disease 2019 (COVID-19) pandemic has resulted in over 3 million deaths worldwide. SARS-CoV-2 brings symptoms such as fever, cough, shortness of breath, headache, sore throat and loss of taste or smell. Diagnostic of COVID-19 may include specific RT-PCR for viral ribonucleic acid detection, and ELISA testing for virus-derived spike protein or nucleocapsid.

Aim:
The aim of this study was to measure the antinucleocapsid level of SARS-CoV-2 IgG to identify the number of asymptomatic cases of COVID-19 after infection in a population of workers from a production company.

Material and methods:
Human anti-SARS-CoV-2(N) IgG ELISA kit was used to determine serum IgG level. Study includes 107 individuals (48% female, 52% male) in different ages (18–60 years).

Results and discussion:
Of 107 tested individuals in 80 (74.7%) cases SARS-CoV-2(N)-specific IgG antibodies were detected, with an average antibody concentration of the whole study group 4.08 µg/mL (n = 107 with the range 0.59–7.91 µg/mL; n = 80 were included in the study with the sensitivity of the method above 2.344 µg/mL). In only 9 cases, SARS-CoV-2 infection was confirmed before using the PCR test. Our data underscore the need for a population study in Poland to test the proportion of asymptomatic IgG positive for SARS-CoV-2 individuals.

Conclusions:
This study indicates that within studied sample large proportion of asymptomatic people have undergone SARS-CoV-2 infection and suggests that isolation of only symptomatic patients would not stop the transmission of the virus.