Seroprevalence of SARS-CoV-2 anti-nucleocapsid total Ig, anti-RBD IgG antibodies, and infection in Thailand: a cross-sectional survey from October 2022 to January 2023

Abstract

Seroprevalence studies on SARS-CoV-2 are essential for estimating actual prevalence rates of infection and vaccination in communities. This study evaluated infection rates based on total anti-nucleocapsid immunoglobulin (N) and/or infection history. We determined the seroprevalence of anti-receptor binding domain (RBD) antibodies across age groups. A cross-sectional study was conducted in Chonburi province, Thailand, between October 2022 and January 2023. Participants included newborns to adults aged up to 80 years. All serum samples were tested for anti-N total Ig and anti-RBD IgG. The interviewer-administered questionnaires queried information on infection history and vaccination records. Of 1459 participants enrolled from the Chonburi population, ~ 72.4% were infected. The number of infections was higher in children aged < 5 years, with evidence of SARS-CoV-2 infection decreasing significantly with increasing age. There were no significant differences based on sex or occupation. Overall, ~ 97.4% of participants had an immune response against SARS-CoV-2. The anti-RBD IgG seroprevalence rate was lower in younger vaccinated individuals and was slightly increased to 100% seropositivity at ages > 60 years. Our findings will help predict the exact number of infections and the seroprevalence of SARS-CoV-2 in the Thai population. Furthermore, this information is essential for public health decision-making and the development of vaccination strategies.

Figure 1

Map of Thailand showing the blood sampling sites in 11 districts, Chonburi province, Thailand. The number of blood samples collected for the individual district is indicated.

Figure 2

The timeline of infected cases in this study. The left Y-axis represents the number of SARS-CoV-2 infections in this study (bar graph). The right Y-axis represents the number of SARS-CoV-2 infections in Thailand reported by the Ministry of Public Health (area graph). The timeline of infected cases and the duration of the SARS-CoV-2 variant strain outbreaks have been reported in previous studies^,.

Figure 3

The number of infections measured by anti-N total Ig and/or history of infection (a) in all participants, (b) in the general population, and (c) in the healthcare worker group.

Figure 4

The overall seroprevalence measured by anti-RBD IgG in (a) all participants, (b) in the general population, and (c) among healthcare workers. Anti-RBD concentrations were classified into four ranges: < 7.1 BAU/mL, seronegative; 7.1– < 100 BAU/mL, low titer, 100– < 1000; BAU/mL medium titer, and ≥ 1000 BAU/mL, high titer.