*4.4. Statistical Analysis*

We estimated crude age-specific COVID-19 seroprevalences and the confidence intervals in five age-groups (0–9, 10–19, 20–39, 40–59 and ≥60 years) for each prefecture city, the low- and high-risk cities, or Guangdong province using binomial approximation. We also estimated the age- and sex-weighted seroprevalences for low-risk, high-risk cities and Guangdong province using data on the population structure of Guangdong province based on the 2015 Guangdong One-Percent Population Sample Survey. The estimations were conducted with R version 3.6.3. For the neutralization titer, the IC50 was calculated using the three-parameter non-linear regression function in Prism 8.0 (GraphPad). The correlation between S/CO readout and neutralization titer were determined using Pearson's Correlation Test, also in Prism 8.0. The Pearson's correlation test was performed under the assumption that the data was sampled from a Gaussian distribution, with the *p*-value derived from a two-tailed test.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/pathogens10111505/s1, Table S1: Timeline and details of the public health response implemented after the emergence of COVID-19 in Guangdong province. Table S2: The seropositivity for antibodies to SARS-CoV-2 in the prefectural cities of Guangdong province identified in the present study between 11 March and 24 June 2020. Since the number of seropositive samples were too low to estimate seroprevalence at the individual city level, percentage of seropositivity were used instead. Table S3: The age-specific SARS-CoV-2 seroprevalence in Guangdong province, and for cities of low or high risk of COVID-19 activities according to the local official surveillance data. The crude seroprevalences for individual age groups were presented, while the overall age- and sexweighted seroprevalences for cities of low or high risk of COVID-19 activities, and that of Guangdong province, were presented. Table S4: Pseudovirus neutralization titers, expressed as 50% inhibitory concentration (IC50) of the SARS-CoV-2 IgG-positive samples. Figure S1: The population structure in Guangdong Province and sample structure in this study. File S1: Case definition of COVID-19 according to the Protocol on Prevention and Control of Novel Coronavirus Pneumonia (Edition 2 to 7). File S2: Daily case numbers of COVID-19 in Guangdong between 19 January and 1 July 2020. The daily numbers of asymptomatic cases prior to 1 April 2020 were not publicly accessible. File S3: Daily percentage of inbound and outbound events by rail, air and road traffic to and from Wuhan in the five days prior to the lockdown. Only the daily top 100-destination cities were reported by the website. Cities in Guangdong that were listed in daily the top-100 lists were highlighted.

**Author Contributions:** S.-S.W., B.J.C., N.H.L.L. and M.Z. conceived and designed the study. Y.C., W.G. and Z.Y. supervised the sample collection. Y.C., R.T., C.X., S.L., X.L. and X.H. were responsible for sample and data management as well as CLIA testing. C.X., H.L. and X.L. performed the PPNT testing. C.X., and N.H.L.L. performed the data curation, retrieval and analyses. S.-S.W., B.J.C., N.H.L.L., C.X. and M.Z. wrote the manuscript. S.-S.W., N.H.L.L. and C.X. verified the underlying data. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Guangzhou Institute of Respiratory Health Open Project (Funds provided by China Evergrande Group)—[Grant number 2020GIRHHMS12], the Project for Prevention and Control of COVID-19 from Guangzhou Institute of Respiratory Health and the High-Level University Talent Construction Program of Guangzhou Medical University to M.Z. and S.-S.W. [Grant numbers: 02-412-B205002-1015022, 02-412-B205002-1015023]. N.H.L.L. and B.J.C. are supported by the Theme-based Research Scheme project no. T11-712/19-N from the University Grants Committee of the Hong Kong Government.

**Institutional Review Board Statement:** This study was approved by the Institutional Review Board of the First Affiliated Hospital of Guangzhou Medical University (Ethics Number: 2020153).

**Informed Consent Statement:** The written informed consent was not required as the archived sera samples were used and anonymously handled in this study.

**Data Availability Statement:** The data available in this study are available on request from the corresponding author. The data are not publicly available due to ethical reasons.

**Acknowledgments:** We gratefully acknowledge the staff of Kingmed Diagnostics who provided assistance in the study. We also thank Jin-Cun Zhao from the State-Key Laboratory of Respiratory Disease for kindly providing the plasmids and cells for the pseudovirus neutralization assays. We also thank Min Kang and Tie Song, from Guangdong Centers of Disease Control and Prevention for their expert advice. We thank Zhanpeng Jiang for his assistance in making the figures, and Joyce Chen for providing administrative support to the project. We also thank Tingting Liang for collecting the necessary information.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

