High prevalence of SARS-CoV-2 antibodies in care homes affected by COVID-19: Prospective cohort study, England

Abstract

Background: We investigated six London care homes experiencing a COVID-19 outbreak and found high rates of SARS-CoV-2 infection among residents and staff. Here we report follow-up investigations including antibody testing in the same care homes five weeks later.

Methods: Residents and staff in the initial investigation had a repeat nasal swab for SARS-CoV-2 RT-PCR and a blood test for SARS CoV-2 antibodies using ELISA based on SARS-CoV-2 native viral antigens derived from infected cells and virus neutralisation.

Findings: Of the 518 residents and staff in the initial investigation, 186/241 (77.2%) surviving residents and 208/254 (81.9%) staff underwent serological testing. Almost all SARS-CoV-2 RT-PCR positive residents and staff were seropositive five weeks later, whether symptomatic (residents 35/35, 100%; staff, 22/22, 100%) or asymptomatic (residents 32/33, 97.0%; staff 21/22, 95.5%). Symptomatic but SARS-CoV-2 RT-PCR negative residents and staff also had high seropositivity rates (residents 23/27, 85.2%; staff 18/21, 85.7%), as did asymptomatic RT-PCR negative individuals (residents 61/91, 67.0%; staff 95/143, 66.4%). Neutralising antibody was detected in 118/132 (89.4%) seropositive individuals and was not associated with age or symptoms. Ten residents (10/79 re-tested, 12.7%) remained RT-PCR positive but with higher RT-PCR cycle threshold values; 7/10 had serological testing and all were seropositive. New infections were detected in three residents and one staff.

Interpretation: RT-PCR provides a point prevalence of SARS-CoV-2 infection but significantly underestimates total exposure in outbreak settings. In care homes experiencing large COVID-19 outbreaks, most residents and staff had neutralising SARS-CoV-2 antibodies, which was not associated with age or symptoms.

Funding: PHE.

Fig. 1

Flow diagram of residents and staff in 6 London care homes experiencing a COVID-19 outbreak during the pandemic who consented to follow-up testing including blood sampling for SARS-CoV-2 antibodies four to six weeks later. ‘Ever-symptomatic’ indicates that symptoms were experienced at some point during the follow-up period. *Three individuals in this group became SARS-CoV-2 PCR positive at follow-up RT-PCR testing conducted simultaneously with SARS-CoV-2 antibody testing.

Fig. 2

Proportion of indicated study populations with positive (maroon), equivocal (yellow) or negative (green) native viral antigen lysate assay. a) Seroconversion for all cohort and staff and resident sub groups. b) Seroconversion by sex for whole cohort. c) Seroconversion by acute RT-PCR result and presence of symptoms for residents and staff. All N = 394; Staff N = 208; Residents N = 186.

Fig. 3

Age group analysis native viral antigen lysate assay. Left panel: Seroconversion percentage by age group for whole cohort, N = 394. Right panel: Native viral antigen assay index value for seropositive individuals by initial RT-PCR status and age group, N = 307. Bars indicate median and 95% confidence interval. Dashed line indicates assay positive cut-off. Statistical analysis using Chi-square test of proportion (p = 0.43) and Kruskal-Wallis with Dunn's multiple comparisons test (p = 0.07).

Fig. 4

Virus neutralising antibody titre analysis. a) Virus neutralising antibody titre positive and negative percentage by age group for whole cohort. N = 132 b) Virus neutralisation titre by sex. Bars indicate median and 95% confidence interval. c) Virus neutralisation titre by symptom status during the initial testing period. Bars indicate median and 95% confidence interval. d) Virus neutralisation titre by age group. Box and whisker plot with bars indicating full range of results. N = 118. Statistical analysis a) Chi-square test, P = 0.27; b Mann-Whitney U Test, P = 0.69; c) Mann-Whitney U Test, P = 0.10 d) Kruskal Wallis with Dunn's multiple comparisons test adjustment, P = 0.40.