The prevalence of antibodies to SARS-CoV-2 among blood donors in China

Abstract

In this study, we investigate the seroprevalence of SARS-CoV-2 antibodies among blood donors in the cities of Wuhan, Shenzhen, and Shijiazhuang in China. From January to April 2020, 38,144 healthy blood donors in the three cities were tested for total antibody against SARS-CoV-2 followed by pseudotype SARS-CoV-2 neutralization tests, IgG, and IgM antibody testing. Finally, a total of 398 donors were confirmed positive. The age- and sex-standardized SARS-CoV-2 seroprevalence among 18-60 year-old adults (18-65 year-old in Shenzhen) was 2.66% (95% CI: 2.24%-3.07%) in Wuhan, 0.033% (95% CI: 0.0029%-0.267%) in Shenzhen, and 0.0028% (95% CI: 0.0001%-0.158%) in Shijiazhuang, respectively. Female sex and older-age were identified to be independent risk factors for SARS-CoV-2 seropositivity among blood donors in Wuhan. As most of the population of China remained uninfected during the early wave of the COVID-19 pandemic, effective public health measures are still certainly required to block viral spread before a vaccine is widely available.

Fig. 1. Flowchart of screening and confirmatory procedure.

A total of 38,144 blood donations were tested for SARS-CoV-2 total antibody (TAb). Those reactive were further confirmed by neutralization assay and tested for IgG antibody against receptor-binding domain (RBD) of the spike protein and nucleoprotein of SARS-CoV-2 (IgG-RBD and IgG-N), and IgM antibody against SARS-CoV-2. Three-hundred ninety-eight donors were finally confirmed for SARS-CoV-2 seropositivity. The numbers of donations tested for specific antibodies in three cities are shown in the gray box. The crude seroprevalence of antibodies to SARS-CoV-2 among blood donors was calculated by the number of confirmed positive donors divided by the total number of tested donors: 2.22% (395/17,794, 95% CI: 2.01–2.45%) in Wuhan, 0.029% (2/6810, 95% CI: 0.0081–0.11%) in Shenzhen, and 0.0074% (1/13,540, 95% CI: 0.0013–0.042%) in Shijiazhuang, respectively. R, reactive; NR, nonreactive; IgG-RBD, IgG antibody against receptor-binding domain (RBD) of the spike protein of SARS-CoV-2; IgG-N, IgG antibody against nucleoprotein of SARS-CoV-2.

Fig. 2. Weekly seroprevalence of SARS-CoV-2 antibody during different periods from January to April 2020 in the cities of Wuhan, Shenzhen, and Shijiazhuang.

The number of donors tested for total antibody (TAb) every week (the black numbers on the top of each histogram) is shown in histograms. The number of confirmed positive cases is shown in red numbers on the top of each histogram. The confirmed seropositive rate (number of confirmed positive donors/number of donors tested for TAb) in each week is shown in red lines. The first donor confirmed positive by the pseudotype lentivirus-based neutralization tests in Wuhan was donated on January 20, the fourth week of 2020. Lockdown of Wuhan City started on January 23 and on April 8, all the travel restrictions in Wuhan were lifted. The period of study in Wuhan is divided into three stages: prelockdown (Jan 15–Jan 22), lockdown (Jan 23–Apr 7), and lifting restrictions (Apr 8–Apr 30). The confirmed seroprevalences of the three stages varied: only one from 2164 donors was confirmed in the first stage (0.046%, 95% CI: 0.082–0.26%); 169 donors with confirmed serological evidence were identified from 5587 donors, suggesting a seroprevalence of 3.02% (95% CI: 2.60–3.51%) in the lockdown stage. After April 8, we further tested a total of 10,043 donors, and found out that 225 were confirmed SARS-CoV-2 seropositive (2.24%, 95% CI: 1.97–2.55%). The peak of seroprevalence (5.21%, 5/96) occurred in the stage of lockdown. The seroprevalence of the three stages, after age–sex standardization with the population distribution in the city of Wuhan of 18–60-year-old adults, was 0.022% (95% CI: 0.005–1.494%), 3.54% (95% CI: 2.71–4.37%), and 2.71% (95% CI: 2.09–3.32%), respectively.

Fig. 3. Relationships between ppNAT titer and ELISA-binding titers among all TAb-positive samples.

a The relationship between ID50 (maximal dilution ratio at half-infection inhibition) of pseudotype lentivirus-based neutralization tests (ppNAT) and different groups of signal to the cutoff ratio (S/CO) of total antibody (TAb). The horizontal gray line shows the cutoff value of ppNAT (ID50 = 20). The neutralization titer successively elevated with increasing S/CO of TAb among all samples (p < 0.001, Group 1–5 vs. 5–10: p < 0.001; Group 1–5 vs. 10–15: p < 0.001; Group 1–5 vs. >15: p < 0.001; Group 5–10 vs. 10–15: p = 0.103; Group 5–10 vs. >15: p < 0.001; Group 10–15 vs. >15: p = 0.001). Median titers of the four groups are 11.0, 66.0, 97.0, and 233.9, respectively. b The relationship between ppNAT ID50 titer and titer of IgG antibody against receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 (IgG-RBD). The correlation coefficient (r) by Spearman rank correlation analysis between IgG-RBD and ID50 was as high as 0.844 (p < 0.000001). The dotted lines show the cutoff value of ppNAT (ID50 = 20) and IgG-RBD assay (S/CO = 1.0). c The relationship between ppNAT ID50 titer and titer of IgG antibody against nucleoprotein of SARS-CoV-2 (IgG-N). The correlation coefficient (r) between IgG-N and ID50 was 0.725 (p < 0.000001). The dotted lines show the cutoff value of ppNAT (ID50 = 20) and IgG-N assay (S/CO = 1.0). d The relationship between ppNAT ID50 titer and titer of IgM antibody against SARS-CoV-2. The correlation coefficient (r) between IgM and ID50 is 0.660 (p < 0.000001). The dotted lines show the cutoff value of ppNAT (ID50 = 20) and IgM assay (S/CO = 1.0). p-values by Kruskal–Wallis test followed by Bonferroni’s multiple-comparison post hoc test (two-sided) are indicated in (a), and the Spearman correlation coefficient (r) and p-values by Spearman’s test are indicated in (b–d). The error band in gray shows the 95% confidence intervals of the fitted line by Spearman rank correlation analysis in (b–d).

Fig. 4. Relationship between specific antibody titer and gender or age.

a Relationship between the titer of IgG antibody against receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 (IgG-RBD) and four different age groups. No statistical significance is shown among different age groups (p = 0.259). The horizontal gray line shows the cutoff value of IgG-RBD assay (S/CO = 1.0). Median titers of the four groups are 11.2, 12.6, 13.8, and 14.0, respectively. b Relationship between the titer of IgG antibody against nucleoprotein of SARS-CoV-2 (IgG-N) and four different age groups. A significant difference is shown among different age groups (p = 0.001). The IgG-N titer of age group 18–25 is lower than that of group 36–45 (p = 0.005) and 46–60 (p = 0.001).The horizontal gray line shows the cutoff value of IgG-N assay (S/CO = 1.0). Median titers of the four groups are 1.8, 2.8, 3.4, and 4.3, respectively. c Relationship between the titer of ppNAT ID50 (maximal dilution ratio at half-infection inhibition) and four different age groups. A significant difference is shown among different age groups (p = 0.013). Median titers of the four groups are 117, 163, 172, and 232, respectively. The ID50 titer of age group 18–25 is lower than that of group 36–45 (p = 0.027) and group 46–55 (p = 0.022). d Relationship between the titer of specific IgG or ID50 of pseudotype lentivirus-based neutralization tests (ppNAT) and gender. There was no difference in the titers of IgG-RBD, IgG-N, or neutralizing antibodies between males and females (p = 0.060, 0.190, and 0.247). Median titers of the six groups are 12.2, 13.5, 2.8, 3.2, 153, and 165, respectively. p-values by Kruskal–Wallis (two-sided) test are indicated in (a), by Kruskal–Wallis test followed by Bonferroni’s multiple-comparison post hoc test (two-sided) in (b), and (c), and by Mann–Whitney U test (two-sided) in (d).