key: cord-1017742-040hjhah authors: Carlton, Lauren H.; Chen, Tiffany; Whitcombe, Alana L.; McGregor, Reuben; Scheurich, Greg; Sheen, Campbell R.; Dickson, James M.; Bullen, Chris; Chiang, Annie; Exeter, Daniel J.; Paynter, Janine; Baker, Michael G.; Charlewood, Richard; Moreland, Nicole J. title: Charting elimination in the pandemic: a SARS-CoV-2 serosurvey of blood donors in New Zealand date: 2021-07-30 journal: Epidemiol Infect DOI: 10.1017/s0950268821001643 sha: 09646543c056c75b40f9e3d7cfbf896bb41823b5 doc_id: 1017742 cord_uid: 040hjhah New Zealand has a strategy of eliminating SARS-CoV-2 that has resulted in a low incidence of reported coronavirus-19 disease (COVID-19). The aim of this study was to describe the spread of SARS-CoV-2 in New Zealand via a nationwide serosurvey of blood donors. Samples (n = 9806) were collected over a month-long period (3 December 2020–6 January 2021) from donors aged 16–88 years. The sample population was geographically spread, covering 16 of 20 district health board regions. A series of Spike-based immunoassays were utilised, and the serological testing algorithm was optimised for specificity given New Zealand is a low prevalence setting. Eighteen samples were seropositive for SARS-CoV-2 antibodies, six of which were retrospectively matched to previously confirmed COVID-19 cases. A further four were from donors that travelled to settings with a high risk of SARS-CoV-2 exposure, suggesting likely infection outside New Zealand. The remaining eight seropositive samples were from seven different district health regions for a true seroprevalence estimate, adjusted for test sensitivity and specificity, of 0.103% (95% confidence interval, 0.09–0.12%). The very low seroprevalence is consistent with limited undetected community transmission and provides robust, serological evidence to support New Zealand's successful elimination strategy for COVID-19. New Zealand has a strategy of eliminating SARS-CoV-2 that has resulted in a low incidence of coronavirus-19 disease (COVID-19). The first case was reported on 26 February 2020, and the country entered a strict nationwide lockdown one month later for 49 days [1] . Through rigorous border control and managed isolation and quarantine facilities for new arrivals, New Zealand has since remained largely COVID-19 free. Globally, serological surveillance has been utilised throughout the pandemic to define the cumulative incidence, including estimations of missed cases and/or asymptomatic infection. Due to lockdowns and movement restrictions, blood donors have been used as a sentinel population in many settings [2, 3] . The aim of this study was to describe the spread of SARS-CoV-2 in New Zealand via a blood donor serosurvey. Though the pandemic response has been highly effective, PCR testing was initially restricted due to limited diagnostic reagents [4] and there have been occasional border incursions and small community outbreaks, including a cluster in August 2020 with no identified link to the border. Samples were collected by the New Zealand Blood Service via nine static collection centres and 36 mobile collection services over a 4-week period ( Antibodies to the Spike (S) protein and receptor-binding domain (RBD) persist for many months after infection, compared with antibodies to the nucleocapsid (N) protein [5, 6] , providing a rationale for the use of S protein-based assays in serosurveys. The overall serological testing algorithm was optimised for specificity given the low number of reported COVID-19 cases in New Zealand (2190 as of 6 January 2021) and the associated period prevalence of 0.04%, which limits the positive predictive value of tests with reduced specificity [7] . Samples were first screened with a widely used and well-validated two-step ELISA that comprises a single point dilution assay against the RBD followed by titration against trimeric S protein (Supplementary Appendix) [8, 9] . Samples above the cut-off were tested on two further immunoassays - interaction (sVNT), with the values highly correlated (Pearson r 0.7993, P < 0.0001) (Fig. 1 ). Further analysis of the 18 seropositive samples with a multiplex bead-based assay that detects antibody isotype reactivity to RBD, S and N proteins [5] revealed a pattern consistent with infections that occurred weeks or months prior; a dominance of RBD and S protein IgG with few samples positive for N protein IgG, nor IgA or IgM against any of the three antigens (Fig. 1 The very low seroprevalence of SARS-CoV-2 infection in New Zealand implies that undetected community transmission has been limited. This seroprevalence is broadly similar to a recent study conducted in the low prevalence city of Sydney in Australia [3] , and markedly lower than estimates of >10% from serosurveys in Europe and North America where the pandemic has been poorly controlled (https://serotracker.com). This study provides robust, serological evidence of New Zealand's successful elimination strategy ahead of vaccine roll-out and highlights the value of a nationwide blood donor service to monitor viral spread during the pandemic. Supplementary material. The supplementary material for this article can be found at https://doi.org/10.1017/S0950268821001643 Data. Data, in addition to those available in the Supplementary information, are available from the authors on request. Successful elimination of Covid-19 transmission in New Zealand Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Kenyan blood donors Seroprevalence of SARS-CoV-2-specific antibodies in Sydney after the first epidemic wave of 2020 New Zealand's science-led response to the SARS-CoV-2 pandemic Comprehensive analysis of SARS-CoV-2 antibody dynamics in New Zealand The first 12 months of COVID-19: a timeline of immunological insights Evaluation of serological tests for SARS-CoV-2: implications for serology testing in a low-prevalence setting A serological assay to detect SARS-CoV-2 seroconversion in humans Collaborative networks enable the rapid establishment of serological assays for SARS-CoV-2 during nationwide lockdown in New Zealand SARS-CoV-2 antibodies in the southern region of New Zealand Acknowledgements. This work was funded by the School of Medicine Foundation (University of Auckland), and the COVID-19 Innovation Acceleration Fund (Ministry of Business, Innovation and Employment). The 2018 Census data used in this study were supplied by Statistics New Zealand (Stats NZ) and accessed via its Integrated Data Infrastructure (IDI).Statistics New Zealand Disclaimer. Access to the data used in this study was provided by Stats NZ under conditions designed to give effect to the security and confidentiality provisions of the Statistics Act 1975. The results presented in this study are the work of the author, not Stats NZ or individual data suppliers.