Incidence of SARS-CoV-2 infection according to baseline antibody status in staff and residents of 100 Long Term Care Facilities (VIVALDI study)
Authors: Krutikov Maria , Palmer Tom , Tut Gokhan , Fuller Chris , Shrotri Madhumita , Williams Haydn , Davies Daniel , Irwin-Singer Aidan , Robson James , Hayward Andrew , Moss Paul , Copas Andrew , Shallcross Laura 
Published: March 2021

Background

SARS-CoV-2 infection represents a major challenge for Long Term Care Facilities (LTCFs) and many residents and staff are now sero-positive following persistent outbreaks. We investigated the relationship between the presence of SARS-CoV-2 specific antibodies and subsequent infection in this population.

Methods

Prospective cohort study of infection in staff and residents in 100 LTCFs in England between October 2020 and February 2021. Blood samples were collected at baseline (June 2020), 2 and 4 months and tested for IgG antibodies to nucleocapsid and spike protein. PCR testing for SARS-CoV-2 was undertaken weekly in staff and monthly in residents. The primary analysis estimated the relative hazard of a PCR-positive test by baseline antibody status, from Cox regression adjusted for age and gender, and stratified by LTCF.

Findings

Study inclusion criteria were met by 682 residents and 1429 staff. Baseline IgG antibodies to nucleocapsid were detected in 226 residents (33%) and 408 staff (29%). A total of 93 antibody-negative residents had a PCR-positive test (0.054 per month at risk) compared to 4 antibody-positive residents (0.007 per month at risk). There were 111 PCR-positive tests in antibody-negative staff (0.042 per month at risk) compared to 10 in antibody-positive staff (0.009 per month at risk). The adjusted hazard ratios for reinfection in staff and residents with a baseline positive versus negative antibody test were 0.13 (95% CI 0.05-0.40) and 0.39 ((95% CI: 0.19-0.77) respectively. Of 12 reinfected participants with data on symptoms, 11 were symptomatic. Antibody titres to spike and nucleocapsid were comparable in PCR-positive and PCR-negative cases.

Interpretation

The presence of IgG antibodies to nucleocapsid was associated with substantially reduced risk of reinfection in staff and residents for up to 10 months after primary infection.

Funding

UK Government Department of Health and Social Care

Research in context

Evidence before this study

We performed a systematic search of MEDLINE (Ovid) and MedRxiv on 18 January 2021 for studies in LTCFs that described the risk of infection in individuals who were seropositive for SARS-CoV-2 compared to individuals who were seronegative. Search terms were deliberately broad to improve capture of relevant literature and included “SARS-CoV-2”OR “COVID-19” OR “coronavirus” AND “care home” OR “nursing home” OR “long term care facility” with no date or language restrictions. We did not identify any publications that focussed on risk of reinfection in seropositive individuals, but subsequent to our search one study has been published using data from two LTCFs in London, UK. This study reported a 96% reduction in the odds of reinfection in individuals who were seropositive compared to those who were seronegative based on 4-month follow-up in 161 participants. We found 10 studies that performed seroprevalence surveys in either staff or staff and residents in LTCFs in 8 cohorts. Five of these were carried out in response to SARS-CoV-2 outbreaks within the care homes, either as part of the subsequent investigation or as post-infection surveillance. The largest of these, which enrolled both staff and residents, was performed in 6 LTCFs and performed longitudinal antibody testing.

Added value of this study

We undertook a cohort study in staff and residents from 100 LTCFs in England to investigate whether individuals with evidence of prior SARS-CoV-2 infection could be infected twice. Staff and residents were offered up to three rounds of antibody testing and antibody results were linked to PCR test results which were obtained weekly from staff and monthly from residents through the national SARS-CoV-2 testing programme. This study, which was conducted in >2000 staff and residents, suggests that antibodies provide high levels of protection against reinfection for up to 10 months. Almost all cases of reinfection were symptomatic, but no cases required hospital treatment. Amongst those with detectable baseline antibodies, quantitative antibody titres against spike protein and nucleocapsid were comparable between cases of reinfection and those who did not become reinfected.

Implications of all available evidence

Despite high background rates of infection in LTCFs, the overall risk of reinfection was low in this population. This is broadly consistent with findings from large cohort studies of hospital staff, but, importantly, extends the evidence of substantial protection to frail elderly, who are vulnerable to severe outcomes of SARS-CoV-2 due to age-related changes in immunity (immune-senescence) and high levels of comorbidity. The low risk of reinfection in our study suggests identification of immune correlates of protection in this population will require pooling of data across multiple cohorts.

As vaccination coverage in residents approaches 100% in England, it will be important to understand whether vaccination and natural infection provide comparable levels of protection against infection. Such insights will inform future policy decisions regarding re-vaccination schedules in LTCF, and the longer-term need for non-pharmaceutical interventions to prevent SARS-CoV-2 transmission, such as asymptomatic testing and visitor restrictions.