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 non-hospital healthcare 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 pre-symptomatic 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 reveal that high seroprevalence rate among staff can contribute to herd immunity within a workplace and protect against subsequent infection and spread within a facility.

Impact of prior infection on SARS-CoV-2 antibody responses in vaccinated long-term care facility staff

“…Neutralization assay. Standard plaque reduction neutralization test (PRNT) was performed as previously described [8]. Briefly, diluted sera were mixed with virus, incubated for one hour at 37°C, added to a Vero cell monolayer, incubated an additional hour at 37°C, then overlaid with tragacanth media and incubated for two days.…”

“…RBD and NP ELISA. Binding assays were performed as described previously [8]. Briefly, 96-well plates were coated with SARS-CoV-2 protein (RBD and NP from Sino Biological), blocked with non-fat dried milk, and diluted sera was added.…”

“…Due to the high number of cases in LTCFs prior to vaccines and other preventative measures, many staff and residents became infected during 2020 and 2021, with some facilities reporting infection and seroprevalence rates as high as 40% [8][9][10][11]. Therefore, there are two immunologically distinct populations of individuals receiving vaccines; those that are naïve, with no evidence of a prior infection (seronegative), and those that are pre-immune with either a documented prior infection, or serological evidence of prior infection (seropositive).…”

“…Staff at a local long-term care facility (LTCF), in parallel with their weekly SARS-CoV-2 nasal surveillance qPCR testing, provided blood samples for antibody analyses [8]. This facility experienced a SARS-CoV-2 outbreak in September 2020 prior to vaccine availability, resulting in infection and seroconversion of almost 35% of the staff members [8]. In January 2021, a Pfizer vaccine clinic was provided at their workplace, with the second dose provided three weeks later in early February.…”

“…As of January 30, 2022, 96% of staff and 97% of residents at this facility were fully vaccinated, slightly higher than Colorado statewide averages (92% and 93% of staff and residents respectively) [2]. We collected and analyzed sera from staff at this facility from August to December 2020 [8]. We found that during an outbreak at the facility, many staff (~30%) became infected and subsequently seroconverted, generating neutralizing, spike- and RBD-binding antibodies.…”