This investigation was a collaboration of the participating college, the Georgia Department of Public Health (GDPH), and the U.S. Centers for Disease Control and Prevention (CDC). The protocol for this investigation was reviewed by their Institutional Review Boards and determined to be non-research and was conducted consistent with applicable federal law and CDC policy as defined in 45 CFR46.102(I)(2).
Setting and Participants
The investigation was conducted between February 22nd–April 20th, 2021 at a primarily residential college in Georgia, USA. The level of COVID-19 transmission in the surrounding county was moderate at the beginning of the investigation and low at the end [7]. At the beginning of the investigation period when vaccine eligibility was limited 14.6% of the total county population had received at least one dose of COVID-19 vaccine and 9.1% were considered fully vaccinated [7]. On March 25th vaccine eligibility was expanded to everyone ≥ 16 years. By the end of the investigation, 28.7% of the county population had received at least one dose of the COVID-19 vaccine, and 22.2% were considered fully vaccinated [7]. The vaccination rate at the college at the beginning of the investigation was 19.8% of staff and 5.4% of students. Mass vaccination events were held on campus the weeks of March 22nd and April 19th; by the end of the investigation, vaccination rates among staff and students were 75.6% and 62.2%, respectively.
Most students (90%) attending the college live in residence halls on the controlled-access campus. The school instituted several COVID-19 prevention strategies, including mask mandates, physical distancing in classrooms, enhanced facility cleaning, limiting campus access, and encouraging students to form small, mutually exclusive social “bubbles” of ≤ 5 students. Students attending the college (N = 1982), staff/faculty (N = 643), and affiliates (e.g., spouses of staff) associated with the college were eligible for the investigation. Any person who was in quarantine due to SARS-CoV-2 exposure or who had COVID-19 symptoms was not eligible for testing. In the two weeks before the investigation, seven COVID-19 cases were reported on campus (prevalence: 133.3/100,000).
Twice-weekly self-administered antigen screening testing
The Quidel QuickVue At-Home COVID-19 Test (hereafter self-administered antigen test) is a lateral flow assay that relies on qualitative detection of the SARS-CoV-2 nucleocapsid protein. Results are read visually on a test strip after ten minutes. At the time this investigation began this test was under FDA review for Emergency Use Authorization (EUA) for self-administration and received an EUA in March 2021 [8].
Participants were provided with self-administered antigen tests and manufacturer instructions and instructed to test twice weekly for seven consecutive weeks. During week three, nasal swabs for paired rRT-PCR testing were collected on the same day as antigen testing (Fig. 1).
Participants were asked to submit their self-administered antigen test results and a photo of their test strip through the college’s symptom screening online reporting system. Participants who tested positive by self-administered antigen test were counseled to obtain same-day confirmatory testing by RT-PCR and to self-isolate.
SARS-CoV-2 IgG antibody testing
To evaluate the number of SARS-CoV-2 infections the serial twice-weekly self-administered antigen testing missed, we examined SARS-CoV-2 specific IgG seroconversion, with paired baseline (week 1) and endline (week nine) serology testing (Fig. 1). The college provided participant COVID-19 vaccination records and positive SARS-CoV-2 test results within 90 days prior to and during the investigation.
Nurses collected blood specimens from consenting participants into K2-EDTA tubes. Plasma was separated from whole blood by centrifugation within 24 h of collection. Plasma specimens were aliquoted into Nalgene cryogenic vials, heat-treated at 56 °C (132.8°F) for 10 min, and frozen at − 80 °C. One plasma aliquot was tested using the qualitative VITROS anti-SARS-CoV-2 total antibody in vitro diagnostic test on the automated VITROS 3600 Immunodiagnostic System (Ortho Clinical Diagnostics), which measures total SARS-CoV-2 antibodies against the SARS-CoV-2 spike protein. An automatically calculated ratio of test sample signal to cutoff value (S/C) < 1.0 was interpreted as nonreactive, and S/C ≥ 1.0 was interpreted as reactive for anti-SARS-CoV-2 total antibody.
To delineate infection versus vaccine induced antibody responses among vaccinated participants, plasma specimens were analyzed with V-plex SARS-CoV-2 panel 2 IgG kit (Meso Scale Diagnostics [MSD]) as directed by the manufacturer. This multiplex assay detects antibodies against SARS-CoV-2 nucleocapsid (N), spike (S), and the spike receptor binding domain (RBD). Specimens that were positive for nucleocapsid antibodies, in addition to S and/or RBD, were classified as having infection induced antibody response; endline specimens positive only for S and/or RBD were classified as having vaccination induced antibody response only (i.e., no evidence of infection).
Paired rRT-PCR testing
During the third week of the investigation, participants were asked to submit a nasal swab specimen for rRT-PCR testing and to take a self-administered antigen test on the same day. Participants were directed to self-collect a bi-lateral anterior nasal swab for rRT-PCR testing. These swabs were stored in tubes with viral transport media in coolers with cold packs and transported daily to the Georgia Public Health Lab and stored at 4 °C until testing. Within 48 h of collection, specimen nucleic acid was isolated using the Perkin Elmer Chemagic Viral DNA/RNA assay on the Perkin Elmer Chemagic 360 instrument (Perkin-Elmer) and analyzed using the CDC Influenza SARS-2 (Flu SC2) Multiplex assay according to the manufacturer’s instructions for use [9]. Residual frozen specimens positive for SARS-CoV-2 by either test underwent viral culture. Specimens were cultured by limiting dilution in Vero E6/TMPRSS2 cells and were observed daily for cytopathic effects in 96-well plates, described previously [10]. Supernatant from cells that exhibited cytopathic effects was harvested and tested by rRT-PCR using the Flu SC2 Multiplex assay to confirm the presence of SARS CoV-2. A specimen was culture-positive if the first viral passage had a cycle threshold (Ct) value at least two Ct values lower than the clinical specimen.
Acceptability and use surveys
Participants were administered surveys during weeks two and eight of the investigation to assess characteristics and acceptability of twice-weekly self-administered antigen testing via an online survey platform (Qualtrics). Participation in the 2nd survey was low and results are not reported. Survey questions are included in Additional file 1.
Statistical analysis
Analyses were conducted using SAS (version 9.4; SAS Institute) and verified by an independent analyst. Frequencies of descriptive variables were calculated among groups of participants. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated for paired self-administered antigen testing compared to rRT-PCR testing and seroconversion. Percentages were calculated for testing circumstances and acceptability of testing methods from the surveys.