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Seroprevalence of SARS‐CoV‐2 antibodies among rural healthcare workers

Abstract The objective of this longitudinal cohort study was to determine the seroprevalence of antibodies to severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2) in healthcare workers employed at healthcare settings in three rural counties in eastern South Dakota and western Minnesota from May 13, 2020, through December 22, 2020. Three blood draws were performed at five clinical sites and tested for the presence of antibodies against the SARS‐CoV‐2. Serum samples were tested for the presence of antibodies using a fluorescent microsphere immunoassay (FMIA), neutralization of SARS‐CoV‐2 spike‐pseudotyped particles (SARS‐CoV‐2pp) assay, and serum virus neutralization (SVN) assay. The seroprevalence was determined to be 1/336 (0.29%) for samples collected from 5/13/20 to 7/13/20, 5/260 (1.92%) for samples collected from 8/13/20 to 9/25/20, and 35/235 (14.89%) for samples collected from 10/16/20 to 12/22/20. Eight of the 35 (22.8%) seropositive individuals identified in the final draw did not report a previous diagnosis with COVID‐19. There was a high correlation (>90%) between the FMIA and virus neutralization assays. Each clinical site's seroprevalence was higher than the cumulative incidence for the general public in the respective county as reported by state public health agencies. As of December 2020, there was a high percentage (85%) of seronegative individuals in the study population.

impact rural areas. Notably, seasonal influenza has a shorter duration but higher degree of intensity in rural populations, and regional epidemics frequently begin in urban areas before spreading to rural communities. 8 Furthermore, rural communities often lack the medical resources and infrastructure necessary to handle large-scale outbreaks. 9 The need for serological testing is required to quantify the true percentage of individuals who were infected with SARS-CoV-2 in the United States. COVID-19 causes a wide range of respiratory symptoms with varying degrees of severity. The elderly and those with pre-existing conditions are at higher risk for experiencing a severe case of COVID-19. 10,11 This can be especially worrisome for rural communities where an estimated 17.5% of the population is age 65 or older in comparison with 13.5% in urban communities. 12 Younger individuals without pre-existing conditions may present with mild to no symptoms. Unlike symptomatic individuals who often seek medical attention, an asymptomatic individual may not be tested and therefore missed via conventional PCR-based testing protocols. Antibody testing allows for the identification of these asymptomatic and/or undiagnosed individuals.
Healthcare workers are a high-risk group for contracting COVID-19 due to their frequent exposure to infected individuals. In addition, healthcare workers have on-site access to phlebotomy services at their place of work. Therefore, individuals working in the healthcare facilities in eastern South Dakota and western Minnesota were tested for the presence of antibodies against SARS-CoV-2 over the course of an 8-month period.

| Study design
A longitudinal cohort study was conducted from May 13, 2020, through December 22, 2020, with healthcare workers in eastern South Dakota and western Minnesota. Following recruitment via email, potential participants were guided through an online informed consent process and completed an online questionnaire. Individuals were invited from five clinical sites and were asked to provide three blood samples throughout the course of the study. Participants' serum was tested for antibodies against SARS-CoV-2. Due to the COVID-19 pandemic, provisions were made to ensure the safety of participants and researchers. Each blood draw was performed by clinical staff phlebotomists at the discretion of the clinical site. After collection, the serum samples were turned over to study personnel and tested for SARS-CoV-2 antibodies.

| Participants
Human subjects' procedures were approved by the South Dakota State University Institutional Review Board before the start of the study, were in compliance with relevant laws and institutional guidelines, and were in accordance with the ethical standards of the Declaration of Helsinki. All participants provided informed consent before participating in the study.

| Study sites
Five clinical sites from three different counties were studied in South Dakota and Minnesota. Three clinical sites provide in-patient services, while two are out-patient services only. All three counties are considered nonmetro or rural areas based on the United States Department of Agriculture (USDA) classification. Two counties have a 2013 Rural-Urban Continuum Code of 9, which indicates completely rural and not surrounded by an adjacent metro area. The third county has a 2013 Rural-Urban Continuum Code of 5 meaning that the population is >20 000 and not surrounded by adjacent metro area. 13

| Online questionnaire
Participants provided demographic information including age, sex, race, height, and weight via an online questionnaire using Ques-tionPro (HIPPA certified). COVID-19-related questions also were asked, including whether they had been previously diagnosed with COVID-19, the date of diagnosis, and whether they had exposure to a known positive or suspected positive case of COVID-19.

| Serum collection
Upon collection, participant blood samples were deidentified and incubated at room temperature for 30 min to allow for clotting to occur. Samples were then centrifuged for 10 min at 1500g. Serum was separated from whole blood and stored at −40°C before being assayed.

| Fluorescent microsphere immunoassay (FMIA)
A quantitative assessment of serum IgG, IgA, and IgM antibodies against the SARS-CoV-2 nucleocapsid protein (NCP) was performed using an FMIA testing platform. To prepare the microspheres for antibody capture, a two-step carbodiimide coupling procedure was used to couple prokaryotic expressed and purified SARS-CoV-2-NCP antigen to Luminex™ microspheres as previously described. 14,15 Briefly, the full length (1257 bp) coding sequence corresponding to the Wuhan-Hu-1, nucleoprotein gene (Genbank MN908947.3) was chemically synthesized and cloned into the bacterial expression plasmid pET-28a (EMD Millipore/Novagen). DNA sequencing was used to confirm the identity and in-frame cloning of the SARS-CoV-2 protein with a 6× histidine fusion tag. The recombinant protein was expressed in Escherichia coli BL-21 cells and purified using nickel-NTA agarose resin (Qiagen). Using a series of titrations, the optimal coupling ratio was calculated to be 25 μg NCP antigen/3.1 × 10 6 microspheres.
For the performance of the FMIA, 50 μl of heat-inactivated serum (diluted 1:50 in PBS-BN) was added to 2.5 × 10 3 antigencoupled microspheres. Serum binding IgG, IgA, and IgM antibody isotypes were detected using a polyisotypic, anti-human, biotinylated secondary antibody (Invitrogen) followed by a fluorescent (streptavidin-phycoerythrin) reporter (Invitrogen) that was added to sample and control wells. Anti-NCP antibodies were quantified through a dual-laser instrument (Bio-Rad Bio-Plex 200) as previously described. 16 The median fluorescent intensity for 100 microspheres corresponding to each analyte was recorded for each well, their measurements were mathematically normalized against a serological reference standard to calculate a relative sample-to-positive (S/P) ratio. Determination of a diagnostic sensitivity and specificity threshold cut-off was calculated using a change-point analysis method determined by calculating the mean plus three standard deviations of the negative control, S/P ratios as described. 17 For serum samples tested more than once, the mean S/P ratio was calculated for diagnostic determination.

| Serum virus neutralization assay (SVN)
A fluorescent SVN using live virus was developed for the quantifi-  After 48 h post-transfection, the supernatant containing the released pseudovirus particle was harvested and stored at −80°C for future use or was used immediately with the assay.
The in-vitro serum neutralization assay was performed as previously described. 22  For each well, the luminescence for the highest dilution (1:2560) was divided by 2 to determine a 50% cut-off point. The highest serum dilution that resulted in a 50% neutralization titer (NT 50 ) was recorded for the samples.

| Public records search for cumulative incidence data
The cumulative case percentage for each county was accessed via the South Dakota Department of Health and Minnesota Department of Health online database. 23

| Serial blood draws show increasing seropositivity among study participants
Phase 1 blood draws occurred between 5/13/20 and 7/13/20, and 1/336 (0.3%) of healthcare workers were found to have antibodies against SARS-CoV-2 (Table 2). Phase 2 blood draws occurred between 8/13/20 and 9/25/20, and 5/260 (1.9%) healthcare workers were found to be seropositive ( Table 2). All of the individuals who tested positive during Phase 2 had tested negative during Phase 1.   (Table S1). Samples were deemed "positive" if they were positive for two out of the three assays (FMIA, SVN, SARS-CoV-2pp). In total, 53 samples were tested with 91.4% agreement between each of the neutralizing assays and between the neutralizing assays and the FMIA (Tables 3 and S1).

| Antibody testing shows higher seroprevalence than the county-level cumulative incidence reported by state health departments
The seroprevalence for each clinical site in Phase 3 was compared with the cumulative case rate for their respective county on the date of the blood draw. Each of the clinical sites had a higher seroprevalence among their healthcare workers than the cumulative case rate reported by the state department of health on the same date (Table 4).

| Asymptomatic or undiagnosed infection and occupational COVID-19 exposures
In Phase 1, the one participant who was seropositive also indicated having been diagnosed with COVID-19 prior to their first blood draw. Of the healthcare workers sampled in Phase 1, 15.1% had contact with patients confirmed of having an active case of COVID-19 (Table 5).

| Participant retention and loss to follow-up
The single seropositive individual from Phase 1 was lost to follow-up and did not return for subsequent draws. Overall, 76 participants from Phase 1 of testing were lost to follow-up ( Figure 1). Four of the five individuals who were found to have antibodies in Phase 2 returned for the final blood draw. of seropositive healthcare workers in the New York City Area also went undiagnosed. 26 The reason these workers went undiagnosed is not known but could be due to having an asymptomatic infection, were not tested, or had a false-negative test result.

| DISCUSSION
There was a >90% correlation between the FMIA and two neutralizing antibody assays. The SARS-CoV-2pp assay used a higher Two of three counties' where testing sites are located are considered Class 9 rural areas. The third county is considered a Class 5 rural area with a population of more than 20 000 people. 13 The overall seroprevalence from the five separate testing sites provided a more robust estimate compared with a single site that may be prone to isolated outbreaks.
A limitation of the study was the potential for participant bias as individuals who suspected they were exposed to SARS-CoV-2 may individuals lost to follow-up before the final blood draw. The retention rate for blood draws over the course of the study was 69.9% with 74.7% of those with a final blood draw also completing the final questionnaire.
In summary, serological testing will continue to be an important metric for understanding the true number of individuals who have been exposed to the SARS-CoV-2 virus and will be an essential tool for understanding the scope of COVID-19 spread in rural populations.