Durable Antibody Responses in Staff at Two Long-Term Care Facilities, during and Post SARS-CoV-2 Outbreaks

Abstract

SARS-CoV-2 has had a disproportionate impact on nonhospital health care settings, such as long-term-care facilities (LTCFs). The communal nature of these facilities, paired with the high-risk profile of residents, has resulted in thousands of infections and deaths and a high case fatality rate. To detect presymptomatic infections and identify infected workers, we performed weekly surveillance testing of staff at two LTCFs, which revealed a large outbreak at one of the sites. We collected serum from staff members throughout the study and evaluated it for binding and neutralization to measure seroprevalence, seroconversion, and type and functionality of antibodies. At the site with very few incident infections, we detected that over 40% of the staff had preexisting SARS-CoV-2 neutralizing antibodies, suggesting prior exposure. At the outbreak site, we saw rapid seroconversion following infection. Neutralizing antibody levels were stable for many weeks following infection, suggesting a durable, long-lived response. Receptor-binding domain antibodies and neutralizing antibodies were strongly correlated. The site with high seroprevalence among staff had two unique introductions of SARS-CoV-2 into the facility through seronegative infected staff during the period of study, but these did not result in workplace spread or outbreaks. Together, our results suggest that a high seroprevalence rate among staff can contribute to immunity within a workplace and protect against subsequent infection and spread within a facility. IMPORTANCE Long-term care facilities (LTCFs) have been disproportionately impacted by COVID-19 due to their communal nature and high-risk profile of residents. LTCF staff have the ability to introduce SARS-CoV-2 into the facility, where it can spread, causing outbreaks. We tested staff weekly at two LTCFs and collected blood throughout the study to measure SARS-CoV-2 antibodies. One site had a large outbreak and infected individuals rapidly generated antibodies after infection. At the other site, almost half the staff already had antibodies, suggesting prior infection. The majority of these antibodies bind to the receptor-binding domain of the SARS-CoV-2 spike protein and are potently neutralizing and stable for many months. The non-outbreak site had two unique introductions of SARS-CoV-2 into the facility, but these did not result in workplace spread or outbreaks. Our results reveal that high seroprevalence among staff can contribute to immunity and protect against subsequent infection and spread within a facility.

Keywords: adaptive immunity; coronavirus; neutralizing antibodies; surveillance studies.

FIG 1

SARS-CoV-2 vRNA surveillance testing at two LTCFs. (a) Total number of staff tested weekly as part of vRNA nasal surveillance testing. (b) Number of positive vRNA tests recorded each week at sites A and B. (c) vRNA positivity expressed as percent positive at each site. Timing of sera collections relative to surveillance testing are indicated by red circles and arrows.

FIG 2

SARS-CoV-2 polyclonal antibodies bind spike and RBD. (a) Polyclonal immune sera from sites A and B were evaluated for their ability to bind recombinant spike (solid) and RBD (dash) protein; n indicates the number of samples tested each week. (b) Level of spike and RBD binding as determined by absorbance reading. Dashed line represents Youden cutoffs.

FIG 3

Polyclonal antibodies neutralize SARS-CoV-2 virus. (a) Polyclonal immune sera from sites A and B were evaluated for the ability to neutralize SARS-CoV-2 virus; n indicates the number of samples tested each week. (b) Neutralizing antibody levels over time. PRNT₅₀ represents the serum dilution factor required to neutralize 50% of virus. The dashed line represents limit of detection (20). Nonneutralizing samples are graphed at half the limit of detection (10).

FIG 4

Spike binding, RBD binding, and neutralizing antibody levels are highly correlated. Samples from site A (a to c) and site B (d to f) were graphed by spike and RBD binding levels (a and d), spike binding and neutralization titers (b and e), and RBD binding and neutralization titers (c and f). Spike and RBD dashed lines represent Youden cutoffs. PRNT₅₀ represents the serum dilution factor required to neutralize 50% of virus. PRNT₅₀ dashed line represents limit of detection (20). Nonneutralizing samples are graphed at half the limit of detection (10). Two-tailed, nonparametic Spearman correlation is noted in the graphs.

FIG 5

Trends in binding and neutralizing antibody levels vary over time. Individuals at site B who were infected during the course of the surveillance study were sampled up to 180 days postinfection. (a) Spike binding; (b) RBD binding; and (c) neutralizing antibody levels are graphed by days post first vRNA positive test. (d) Samples are stratified by days postinfection, and graphed by spike binding, RBD binding, and neutralization titers. Arrows show trend of data over time. Spike and RBD dashed lines represent Youden cutoffs. PRNT₅₀ represents the serum dilution factor required to neutralize 50% of virus. PRNT₅₀ dashed line represents limit of detection (20). Nonneutralizing samples are graphed at half the limit of detection (10). Two-tailed, nonparametic Spearman correlation is noted in the graphs.

FIG 6

Two seronegative individuals at site A became vRNA positive with unique strains. (a and b) Spike binding, RBD binding, and neutralizing antibody levels relative to timing of surveillance vRNA testing indicated these two individuals were seronegative prior to infection (N, SARS-CoV-2 negative; P, SARS-CoV-2 positive). (c) Viral RNA from positive surveillance testing was deep sequenced and the consensus sequence was compared to the WA01 SARS-CoV-2 reference sequence. Single nucleotide polymorphisms (SNPs) shared between both site A sequences relative to the reference are shown as black lines. Unique SNPs between the site A sequences are shown as red lines.