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SARS-CoV-2 seropositivity and subsequent infection risk in healthy young adults: a prospective cohort study

Our take โ€”

This was a prospective cohort of US Marine Corps recruits who were followed for reinfection following a supervised quarantine period prior to basic training. A total of 3249 participants enrolled into the study; 3168 (98%) continued into the 2-week supervised quarantine period; and 3076 completed follow up for 6 weeks after the supervised quarantine. Participants who had SARS-CoV-2 antibodies (seropositive) at enrollment were less likely to experience (re)infection over follow up than participants who were seronegative at enrollment. Additionally, among participants who tested positive over follow up, those who were seropositive at enrollment had lower viral loads than those who were seronegative at enrollment. Although there may have been an underestimation of participants with prior infection of SARS-CoV-2 or the possibility that incident infections were undetected persistent infections, the results demonstrate high, though not complete, protection conferred from prior infections in this cohort.

Study design

Prospective Cohort

Study population and setting

This analysis leveraged data from the COVID-19 Health Action Response for Marines study (CHARM). This was a prospective cohort of US Marine Corps (USMC) recruits arriving at the Marine Corps Recruit Depot in Parris Island (MCRDPI) to initiate basic training. The USMC enforced a two-week self-quarantine prior to arriving to MCRDP followed by a USMC-supervised quarantine in either a hotel or college. Participants were eligible to enroll into the study at arrival to the MCRDPI if they were 18 years or older. At enrollment and weeks 1 and 2 of supervised quarantine, PCR testing was conducted, serum samples were collected, and a questionnaire was administered. Participants were defined as seronegative if they had a PCR negative test at enrollment and weeks 1 and 2 and a negative serology test at enrollment. Follow up visits were held 2, 4, and 6 weeks after completion of the supervised quarantine, which included questionnaires, PCR testing to assess current infection status, and collection of sera samples to test for antibodies from a current or prior infection. A total of 3249 participants enrolled into the study; 3168 (98%) continued into the 2 week supervised quarantine period; and 3076 completed follow up for 6 weeks after the supervised quarantine.

Summary of Main Findings

Among enrolled participants, 225 were seropositive (had SARS-CoV-2 antibodies) at enrollment. Among participants who were seropositive at enrollment and completed follow up, 19 had at least one positive PCR test during the 6-week post quarantine follow-up. Among participants determined to be seronegative at enrollment 2247 participants tested positive during the 6 week post supervised quarantine follow up. The incidence rate ratio was 0.18 (95% CI 0.11โ€“0.28; p<0ยท001) comparing those who were seropositive at enrollment compared to those who were seronegative at enrollment. Participants who were seropositive at enrollment and who tested positive over follow up had lower viral loads than those who tested positive over follow up but were seronegative at enrollment (ORF1ab gene cycle threshold difference 3.95 [95% CI 1.23โ€“6.67]; p 0.004). Among seropositive participants, baseline neutralizing titers were detected in 83% (45/54) of participants who did not test positive over follow up and in 32% (6/19) of participants who tested positive over follow up.

Study Strengths

This study consisted of a closely monitored prospective cohort to allow for longitudinal assessment and an environment which supports close observation.

Limitations

There are likely several limitations to this study, including the underestimation of participants with prior infection of SARS-CoV-2. Participants needed to have 3 negative PCR tests prior to being released from quarantine, however it is possible that individuals who tested positive over follow up may have had an undetected persistent infection instead of having experienced a separate, unique reinfection.

Value added

This study consisted of a closely monitored prospective cohort to assess reinfection following a supervised quarantine. Close follow up consisting of PCR test, serological samples, and questionnaires allowed for assessment of infection, viral loads, levels of neutralizing titers, and symptom presentation.

This review was posted on: 9 August 2021