Factors influencing SARS-CoV-2 IgG test sensitivity: A Bayesian analysis of seroconversion and reversion by time since infection, test, age and disease severity

BACKGROUND

Antibody testing is commonly used to assess past exposure to pathogens, but the interpretation is complex. We quantified test-specific SARS-CoV-2 seroconversion and reversion by time since PCR-confirmed infection, age and disease severity.

METHODS

We used Belgian data from laboratory SARS-CoV-2 testing, prescriptions, contact tracing and hospital surveillance collected between March 2020 and June 2021. Additionally, we gathered data for the Wantai and EuroImmun IgG serological tests from the scientific literature.

We used a hierarchical Bayesian model to estimate distributional parameters of a scaled Weibull-bi-exponential distribution for the time-varying sensitivity of qualitative serological test results obtained after PCR-confirmed SARS-CoV-2 infection. We accounted for disease severity (distinguishing between asymptomatic, symptomatic, and hospitalized cases), age (i.e., in terms of age groups 18-49, 50-64, and 65-74 years) and serological test used.

RESULTS

We included 44,262 serological test results: 10,864 obtained from published studies, 33,398 from Belgian laboratories. Seroconversion occurred during the six weeks following a PCR-confirmed infection. For the EuroImmun test, 82% (95%CrI: 80%-84%) of symptomatic individuals in the youngest age group seroconverted, compared to 95% (95%CrI: 95%-96%) for the Wantai test. In addition, seroconversion was associated with hospitalization (OR = 6.98, 95%CrI: 4.85-11.37, compared to asymptomatic infection) and older age (OR = 1.67, 95%CrI:1.43-1.92, for 65-74-year-olds compared to 18-49-year-olds). Reversion after initial seroconversion was strongly associated with the test used. At 50 weeks, the proportion of symptomatic individuals aged 18-49 years who remained seropositive was 63% (95%CrI: 56%-69%) for the EuroImmun test and 95% (95%CrI: 94%-96%) for the Wantai test. Slower reversion was associated with severe infection and older age.

CONCLUSION

Seropositivity after SARS-CoV-2 infection was significantly associated with the type of test used, age of the case and severity of the infection. More severe infection and older age resulted in increased and prolonged seropositivity.