RESUMEN
INTRODUCTION: Serologic testing can provide a safe and fast approach for assessing SARS-CoV-2 antibodies. These tests can be utilized as a complementary method in diagnosis and patients' follow-up, and can also be helpful in epidemiological studies. This study aimed to describe temporal changes in the incidence of COVID-19 IgM and IgG antibodies in emergency medical technicians (EMTs) within a specified time period. METHODS: All EMTs working for Tehran Emergency Medical Service (EMS) center during May to September 2020 were eligible for this study. Those EMTs who were suspected/probable/confirmed cases of COVID-19, based on WHO defined criteria and were willing to participate, entered the study. The EMTs underwent serology testing four weeks after the occurrence of exposure (in suspected cases) or onset of their symptoms (in probable/confirmed cases). Cases were further confirmed by RT-PCR and/or lung CT, and antibody testing was performed for the second and third time with 12-week intervals. Finger-stick blood sampling was utilized for the specimen collection in three different phases. Samples were then analyzed by a commercial immunochromatography-based kit for qualitative measurement of serum IgM and IgG antibodies against the COVID-19 S-protein antigen. RESULTS: Two hundred eighty-four participants met the inclusion criteria; their mean age was 35.9 (SD = 7.6) years and consisted of 244 (85.9%) males. COVID-19 was confirmed in 169 out of 284 participants. Subsequently, 142 and 122 participants were included in phases 2 and 3 of the study, respectively. The number of seronegative patients exceeded seropositive ones in all three phases. At baseline, 162 (57%) patients were seronegative, 27 (9.5%) were only positive for IgG, 3 (1.1%) were only positive for IgM, and 92 (32.4%) were positive for both antibodies; Seventy-eight (54.9%) were seronegative, and 31 (21.8%) were positive for both antibodies in the second phase; These values were 85 (69.6%) and 8 (6.6%) for the third phase, respectively. Among the people who were positive IgG in the first phase (80 people), 56.3% were still positive in the second phase and 27.5% in both subsequent phases. CONCLUSION: The results of our study show that there is a significant reduction in COVID-19 antibody seropositivity over time.