Methods:

The investigators performed this analysis as part of the prospective CHARM (COVID-19 Health Action Response for Marines) study. CHARM included predominantly male US Marine recruits, aged 18–20 years, following a 2-week unsupervised quarantine at home. After the home quarantine period, upon arrival at a Marine-supervised 2-week quarantine facility (college campus or hotel), participants were enrolled and were assessed for baseline SARS-CoV-2 immunoglobulin G (IgG) seropositivity, defined as a dilution of 1:150 or more on receptor-binding domain and full-length spike protein enzyme-linked immunosorbent assay (ELISA). Participants also completed a questionnaire consisting of demographic information, risk factors, reporting of 14 specific COVID-19–related symptoms or any other unspecified symptom, and brief medical history.

SARS-CoV-2 infection was assessed by PCR at weeks 0, 1, and 2 of quarantine and participants completed a follow-up questionnaire, which included questions about the same COVID-19–related symptoms since the last study visit. Participants were excluded at this stage if they had a positive PCR test during quarantine. Participants who had three negative swab PCR results during quarantine and a baseline serum serology test at the beginning of the supervised quarantine that identified them as seronegative or seropositive for SARS-CoV-2 then went on to basic training at Marine Corps Recruit Depot—Parris Island. Three PCR tests were done at weeks 2, 4, and 6 in both seropositive and seronegative groups, along with the follow-up symptom questionnaire and baseline neutralizing antibody titers on all subsequently infected seropositive and selected seropositive uninfected participants (prospective study period).