A cohort study measuring SARS-CoV-2 seroconversion and serial viral testing in university students

Abstract

Background: To improve understanding of the antibody response to SARS-CoV-2 infection, we examined seroprevalence, incidence of infection, and seroconversion among a cohort of young adults living on university campuses during the fall of 2020.

Methods: At the beginning (semester start) and end (semester end) of an 11-week period, serum collected from 107 students was tested using the qualitative Abbott Architect SARS-CoV-2 IgG and AdviseDx SARS-CoV-2 IgG II assays. Results were matched to interim weekly surveillance viral testing and symptom data.

Results: With the SARS-CoV-2 IgG assay, 15 (14.0%) students were seropositive at semester start; 29 (27.1%) students were seropositive at semester end; 10 (9.3%) were seropositive at both times. With the AdviseDx SARS-CoV-2 IgG II assay, 17 (16.3%) students were seropositive at semester start, 37 (35.6%) were seropositive at semester end, and 16 (15.3%) were seropositive at both times. Overall, 23 students (21.5%) had positive viral tests during the semester. Infection was identified by serial testing in a large majority of individuals who seroconverted using both assays. Those seropositive at semester end more frequently reported symptomatic infections (56.5%) than asymptomatic infections (30.4%).

Conclusion: Differences between antibody targets were observed, with more declines in antibody index values below the threshold of positivity with the anti-nucleocapsid assay compared to the anti-spike assay. Serology testing, combined with serial viral testing, can detect seroconversions, and help understand the potential correlates of protection provided by antibodies to SARS-CoV-2.

Keywords: Antibody decline; IgG antibodies; Immune protection; SARS-CoV-2 serology; Seroconversion.

Fig. 1

Serology status of students at the beginning and end of the semester at two universities in Wisconsin. Student serology status at the beginning of the semester (first panel) and end of the semester (third panel), and interim serial SARS-CoV-2 viral testing results (middle panel). Seroconversion in students was evaluated based on serology results at the beginning and end of the semester and from viral testing results throughout the semester. A Data from the anti-N assay are shown. Numbers indicate the total number of individuals who either had positive viral test (orange connecting bars) or consistently tested negative (green) during the semester. B Data from the anti-S assay are shown. Numbers indicate the total number of individuals who either had positive viral test (red connecting bars) or consistently tested negative (blue) during the semester

Fig. 2

Semester start and end IgG serology for students with a confirmed positive viral test during the semester. A Line plots show trajectories of anti-N IgG index values from baseline to endpoint for students who had baseline negative serology and positive viral tests reported during the semester. The horizontal dotted line at index value of 1.4 indicates the threshold for positivity. B Number of days between positive viral test date and endpoint serology collection date, based on endpoint serology status as determined by the anti-N assay. The whiskers indicate range and boxes indicate median with interquartile range (seropositive and seronegative medians are not significant, ns, p = 0.17, Mann Whitney U Test). C Line plots show trajectories of anti-S IgG values (in arbitrary units/mL). The horizontal dotted line at 50 AU/mL indicates the threshold for positivity. Black dots represent two students who were seropositive at semester start but did not have confirmed positive viral tests until after serum collection. D Number of days between positive viral test date and endpoint serology collection date, based on endpoint serology status as determined by the anti-S assay as in B

Fig. 3

Semester start and end IgG serology index values for students with only negative viral tests during the semester. A Students with baseline positive serology and negative viral tests reported during the semester. Anti-N IgG index values are shown. B Students with negative baseline serology and negative viral tests reported during the semester. The horizontal dotted line at index value of 1.4 indicates the threshold for positivity. C Anti-S results from students with negative viral tests reported during the semester. D As in B, anti-S results in students with negative test results reported during the semester. Horizontal dotted line at 50 AU/mL represents the threshold of positivity

Fig. 4

Percent of students who were seronegative and seropositive at the end of semester who reported symptoms with their SARS-CoV-2 infection. Symptoms were reported at the time of specimen collection for SARS-CoV-2 viral testing. Percentage of total reporting symptoms in students seronegative at the end of the semester (black, n = 8) and seropositive at the end of the semester (grey, n = 15) are shown in bars. Percentage of students reporting each symptom are indicated inside each bar, Fisher’s exact test was used for statistical comparisons between seropositive and seronegative students