Various strategies are being used around the world to curb the ongoing coronavirus disease 2019 (COVID-19) pandemic. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a new causative agent of COVID-19. The virus is highly contagious and is transmitted primarily by respiratory droplets from infected individuals.
New Zealand’s approach has reportedly been successful in reducing the incidence of COVID-19 effectively. In New Zealand, the first COVID-19 infection was reported on 26 February 2020. After a month from the first incident, the country implemented a strict 49 day lockdown. They follow difficult border controls, skillfully manage quarantine facilities for new arrivals, and also effectively manage isolation programs. As a result, New Zealand has remained largely free of COVID-19. However, it should be noted that at first, the diagnosis was limited reagent, rigorous PCR testing is not carried out. In addition, small community outbreaks and border intrusions were also reported.
Serological surveillance has been shown to be most effective and is used to determine the cumulative incidence and to assess the number of asymptomatic COVID-19 cases. In the current scenario, due to national lockdowns and limited movement, blood donors have been used as population guards in many settings. New research has been released in medRxiv* preprint server, which focuses on SARS-CoV-2 transmission and prevalence in New Zealand, via blood donor serosurvey.
In the current study, samples were obtained through a static collection center and a mobile collection service run by the Blood Service of New Zealand. Samples were collected from 3 December 2020 to 6 January 2021, from individuals aged between 16 and 88 years. In total, 9,806 samples were analyzed. From the 2018 New Zealand census, scientists determined a detailed overview of the participants’ demographics. Spatially speaking, the participants were most likely to come from sixteen districts, out of the twenty, represented by the health council. This study was also evaluated by the Health and Disability Ethics Committee.
The researchers found that compared with antibodies to the nucleocapsid protein (N), specific antibodies to the Spike protein (S) and receptor binding domain (RBD) were maintained several months after COVID-19. Because of this, protein S-based assays are used in serosurveys. In this study, the serological testing algorithm was optimized for specificity due to the low number of COVID-19 cases reported in New Zealand. Furthermore, optimization is important because the prevalence of seropositive individuals is low (0.04%). This decreases the positive predictive value of the serologic test and also decreases the specificity.
In this study, samples were initially filtered using 2-step ELISA. This test is based on a one-point dilution test against RBD, after which a titration is performed against the trimeric S protein. Blood samples above the cut-off were then assessed using two immunoassays, namely, EuroImmun SARS-CoV-2 IgG ELISA (EuroImmun AG, Lübeck, Germany) and cPass replacement Viral Neutralization Test (sVNT) (GenScript, New Jersey, USA) . Samples were considered seropositive only after obtaining positive results in both commercial tests. The sensitivity and specificity of this test were evaluated using a Receiver Operator Characteristic (ROC) curve, which is based on a previous analysis covering 413 pre-pandemic negatives, 99 confirmed cases of COVID-19 via PCR testing.
Investigators of this study have reported that among 9,806 samples studied, 18 were found to be positive for Spike IgG (EuroImmun) and antibodies that inhibit the RBD-hACE-2 (sVNT) interaction with a high degree of correlation (Pearson r 0.7993, p <0.0001). Furthermore, these 18 seropositive samples were analyzed using a multiplex bead-based assay. This test determines the reactivity of the antibody isotypes to the RBD, S, and N proteins, whose patterns are found to be similar to those of infections that occurred weeks or months earlier. Most of the samples showed high concentrations of RBD and S. IgG protein. However, very few samples reported the presence of N IgG, IgA, or IgM proteins against three antigens (S, N, and M).
The study reports that among 18 seropositive samples, six were associated with donors with previously confirmed COVID-19 infection. The other four seropositive samples were donors who had traveled to high-risk countries, such as the UK and Europe, in 2020. Thus, all four people were infected outside of New Zealand. The last eight seropositive samples came from seven different health districts, where the crude seroprevalence estimate was 0.082%. To estimate true prevalence, the Rogan-Gladen estimator was used with the CI Lang-Reiczigel method to assess the sensitivity of the test. In this study, it was estimated that the true seroprevalence was 0.103% (95% CI 0.09-0.12%). Furthermore, the research also revealed that during the study period, undiagnosed infections occurred.
Investigators from this study observed that the very low seroprevalence of SARS-CoV-2 infection in New Zealand indicates reduced community transmission. Similar incidents have also been reported in Australia. This study is the first report to provide serological evidence of the success of New Zealand’s strategy to control COVID-19 before the vaccination program.
* Important Notice
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be construed as conclusions, guidelines for health-related clinical / behavioral practice, or are treated as established information.