Researchers conduct a SARS-CoV-2 serosurvey of blood donors in New Zealand

Various strategies are being deployed across the globe to curb the ongoing coronavirus disease 2019 (COVID-19) pandemic. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the novel causal agent of the COVID-19. This virus is highly contagious and is transmitted via primarily via respiratory droplets from infected individuals.

The approaches undertaken by New Zealand have been reported to have effectively reduced the incidence of COVID-19. In New Zealand, the first COVID-19 infection was reported on February 26, 2020. After a month of the first incidence, the country implemented a strict lockdown for 49 days. They followed an arduous border control, skillfully managed the quarantine facilities for new arrivals, and had also effectively managed isolation programs. As a result, New Zealand remained mostly free from COVID-19. It must, however, be noted that initially, owing to limited diagnostic reagents, rigorous PCR testing was not conducted. Additionally, small community outbreaks and border intrusions were also reported.

Serological surveillance has been found to be most effective and is being used to determine the cumulative incidence and for assessing the number of asymptomatic COVID-19 cases. In the current scenario, owing to national lockdowns and restricted movements, blood donors have been used as a sentinel population in many settings. New research has been released on the medRxiv* preprint server, which focuses on the transmission and prevalence of SARS-CoV-2 in New Zealand, through a blood donor serosurvey.

In the current study, samples were obtained through static collection centers and mobile collection services run by the New Zealand Blood Service. The samples were collected from December 3, 2020, to January 6, 2021, from individuals who were between 16 and 88 years of age. In total, 9,806 samples were analyzed. From the 2018 New Zealand census, scientists determined an overview of participants' demographic details. Spatially, the participants were likely to be from sixteen districts, out of twenty, represented by the health board. This study was also evaluated by the Health and Disability Ethics Committee.

Researchers found that in comparison to the antibodies against nucleocapsid (N) protein, the antibodies specific to Spike (S) protein and receptor-binding domain (RBD) were retained several months post-COVID-19. Owing to this, S protein-based assays were being used in serosurveys. In the present research, the serological testing algorithm was optimized for specificity because of the low number of COVID-19 cases reported in New Zealand. Further, optimization was essential because the prevalence of seropositive individuals was low (0.04%). This declined the positive predictive value of serological tests and also decreased specificity.

In this study, the samples were initially screened using 2-step ELISA. This test is based on a single-point dilution assay against the RBD, after which titration against trimeric S protein was conducted. The blood samples which were above the cut-off were further assessed using two immunoassays, namely, the EuroImmun SARS-CoV-2 IgG ELISA (EuroImmun AG, Lübeck, Germany) and the cPass surrogate Viral Neutralization Test (sVNT) (GenScript, New Jersey, USA). The samples were regarded as seropositive only after obtaining positive results on both commercial assays. The sensitivity and specificity of these assays were evaluated using Receiver Operator Characteristic (ROC) curves, which are based on prior analyses that include 413 pre-pandemic negatives, 99 COVID-19 cases confirmed via PCR test.

Researchers of the present study have reported that among the 9,806 studied samples, 18 were found to be positive for both Spike IgG (EuroImmun) and antibodies that can inhibit the RBD-hACE-2 interaction (sVNT) with a high degree of correlation (Pearson r 0.7993, p < 0.0001). Next, these 18 seropositive samples were further analyzed using the multiplex bead-based assay. This assay determined the antibody isotype reactivity to RBD, S, and N proteins, whose pattern was found to be similar to the infection that had occurred several weeks or months earlier. Predominantly, most of the samples showed a high concentration of RBD and S protein IgG. However, very few samples reported the presence of N protein IgG, IgA, or IgM against the three antigens (S, N, and M).

The present study reported that among the 18 seropositive samples, six corresponded to donors with previously confirmed COVID-19 infections. Another four seropositive samples constituted the donors who had traveled to high-risk countries, such as the United Kingdom and Europe, in 2020. Thereby, these four individuals were infected outside New Zealand. The last eight seropositive samples were from seven different district health regions, where the estimated crude seroprevalence was 0.082%. To estimate the true prevalence, the Rogan-Gladen estimator was used with the Lang-Reiczigel CI method to assess the test's sensitivity. In this study, a true seroprevalence of 0.103% (95% CI 0.09-0.12%) was estimated. Further, the research also revealed that during the study period, undiagnosed infections prevailed.

Researchers of the present study observed that the very low seroprevalence of SARS-CoV-2 infection in New Zealand indicates reduced community transmission. A similar incidence has also been reported in Australia. This study has been the first report that provided serological evidence of New Zealand's successful strategy to control COVID-19 disease before the vaccination program.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Priyom Bose

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.


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