Study Cohort

In total, 1267 eligible participants completed the baseline questionnaire and were enrolled (November 2020 to January 2021). Of these, 1008 (79.6%) completed baseline SARS-CoV-2 molecular and serological testing (Fig. S2; Table 1). As noted in the previous study publication, the proportions of patients in each workplace exposure risk group was highest in the low-risk group, followed by the medium-to-low-risk and medium-to-high-risk groups, and lowest in the high-risk group [9].

Table 1 Participant baseline demographics and clinical characteristicsLongitudinal Molecular and Serological Testing of Study Population

Overall, at baseline, 6.4% (65/1008) had evidence of current (i.e., positive PCR test) or prior (i.e., IgG positive) SARS-CoV-2 infection. Of those who provided nasal samples and tested positive for SARS-CoV-2 infection via PCR test, 70.0% (14/20) were symptomatic (Table 2). Of those who provided nasal samples for PCR testing during follow-up visits, 0.4% (3/781) tested positive at month 3, 0.4% (3/733) tested positive at month 6, and 0% (0/673) tested positive at month 9.

Table 2 SARS-CoV-2 molecular and serological testing at baseline and across follow-ups—full analysis seta

At baseline, 4.5% (45/1008) of participants who provided blood specimens for IgG testing tested positive for prior SARS-CoV-2 infection (Table 2), of whom, 28 (62.2%) had a negative IgG at one of the follow-up visits. Of those 28, 7 were IgG-negative for the first time at month 3 (25.0%), 14 at month 6 (50.0%), and 7 at month 9 (25.0%). At baseline, 95.5% (963/1008) of participants had negative IgG tests. Of those, 41 (4.3%) had a positive subsequent test. Of those 41, positive tests occurred for 26 (63.4%) at month 3, 12 (29.3%) at month 6, and 3 (7.3%) participants at month 9. Of the 26 participants who had negative baseline IgG and positive IgG at month 3, 18 (69.2%) had a subsequent negative test at one of the follow-up visits. Of those 18, 9 (50%) tested negative at month 6, and 9 (50%) tested negative at month 9. Of those who tested negative at baseline but positive at month 6 (n = 12), 54.6% (n = 6) tested negative at month 9 (Table 2).

Persistence of IgG Seropositivity by Subgroup

IgG against N protein was detected in a total of 86 participants at one point during the duration of the study including baseline, among whom 52 subsequently demonstrated antibody waning below detectable level (Table 3). Persistency of IgG seropositivity by subgroup was also reported; however, no statistical comparisons were made. Participants in medium-to-high- and high-risk groups were numerically more likely to retain IgG positivity at a subsequent visit (12 of 22 and 8 of 15, respectively) than those in low- and medium-to-low-risk groups (7 of 29 and 7 of 20, respectively). Those aged 18–29 years were more likely to retain IgG positivity (5 out of 8) than other age groups, and those between ages 40 and 49 years were least likely (12 of 14 participants had subsequent negative tests). In terms of sex, women had a greater likelihood than men to have a subsequent negative result after testing IgG-positive (40 versus 19 participants for females; 12 versus 15 participants for males; Table 3).

Table 3 SARS-CoV-2 IgG testing according to occupational risk of exposure to SARS-CoV-2 and demographicsAgreement Between Self-Report and PCR Results versus IgG Results for SARS-CoV-2 Infections

IgG against N protein was detected at the subsequent scheduled follow-up visit (i.e., not the same visit) among 37.6% (38/101) of those who self-reported having COVID-19 and 50.0% (13/26) of those who had a positive PCR test at either baseline, 3, or 6 months. IgG against N protein was detected at a subsequent scheduled follow up visit among only 3.0% of those who self-reported not having COVID-19 and 2.6% of those who had a negative PCR test (Table 4).

Table 4 SARS-CoV-2 IgG testing after either a self-report COVID-19 infection or positive PCR test according to occupation risk of exposure to SARS-CoV-2 and demographics throughout study

Among 36 participants who were negative for PCR and positive for IgG at baseline, 24 (66.7%) provided a blood specimen/sample at a subsequent visit at month 3, among whom 79.1% (19/24) maintained a detectable IgG level. Among 43 participants who tested negative for PCR and positive for IgG at month 3, 38 (88.4%) attended the subsequent visit at month 6, among whom 47.4% (18/38) maintained a detectable IgG level. Among the 37 participants who tested negative for PCR and positive for IgG at month 6, 34 (91.9%) returned for a subsequent visit at 9 months and among whom 41.2% (14/34) had a detectable IgG level. Out of 20 participants who tested positive for PCR at baseline, 2 participants (20%) never had a detectable level of IgG when tested across visits (baseline or months 3, 6, or 9).

Time to First Infection

Participants in the high-risk occupation subgroup had a greater likelihood of infection as compared with participants in low-, medium-to-low-, or medium-to-high-risk subgroups based on the time to first infection event for each group (Fig. 1). The likelihood of an infection event was greater for those at high risk of SARS-COV-2 infection in their occupational environment.

Fig. 1

Kaplan–Meier curves of time to first infection by level of risk to SARS-CoV-2 in occupation—full analysis set SARS-CoV-2 severe acute respiratory syndrome coronavirus 2

Vaccination Status

At baseline, 0.7% (7/1008) of participants had received a first dose of vaccine against SARS-CoV-2. At 9 months, 73.2% (630/861) and 70.0% (603/861) received a first or second SARS-CoV-2 vaccination dose, respectively.