Infection rate in Guangzhou after easing the zero-COVID policy: seroprevalence results to ORF8 antigen

On Dec 7, 2022, the Chinese government announced ten measures indicating the end of the zero-COVID policy, which was in effect for more than 2 years.1XinhuaChina focus: COVID-19 response further optimized with 10 new measures.https://english.news.cn/20221207/ca014c043bf24728b8dcbc0198565fdf/c.htmlDate: 2022Date accessed: February 14, 2023Google Scholar Most of the stringent preventive measures, such as mandatory PCR testing, are no longer required. However, easing of restrictions has contributed to the emergence of new outbreaks predominantly by the SARS-CoV-2 omicron lineages BA5.2 and BF.7 in many cities, such as Beijing.2WHOTAG-VE statement on the meeting of 3 January on the COVID-19 situation in China.https://www.who.int/news/item/04-01-2023-tag-ve-statement-on-the-3rd-january-meeting-on-the-covid-19-situation-in-chinaDate: 2023Date accessed: February 14, 2023Google Scholar With suspension of mass testing in mainland China after adjustment of the zero-COVID policy, empirical data on the epidemic growth and transmissibility of omicron variants are scarce. Until now, the number of infected cases and transmission rates were usually estimated using modelling with partial data.3Leung K Lau EHY Wong CKH Leung GM Wu JT Estimating the transmission dynamics of SARS-CoV-2 omicron BF.7 in Beijing after adjustment of the zero-COVID policy in November-December 2022.Nat Med. 2023; (published online Jan 13.)https://doi.org/10.1038/s41591-023-02212-yCrossref Scopus (10) Google Scholar Previously, we reported that presence of the ORF8 antibody in blood is one of the most accurate serological markers of SARS-CoV-2 natural infection.4Hachim A Kavian N Cohen CA et al.ORF8 and ORF3b antibodies are accurate serological markers of early and late SARS-CoV-2 infection.Nat Immunol. 2020; 21: 1293-1301Crossref PubMed Scopus (137) Google Scholar, 5Hachim A Gu H Kavian O et al.SARS-CoV-2 accessory proteins reveal distinct serological signatures in children.Nat Commun. 2022; 132951Crossref Scopus (9) Google Scholar Because the ORF8 gene is only expressed during SARS-CoV-2 replication, this serology test can confirm whether the patient has previously been infected with the virus and help to estimate the attack ratio. During Jan 5–14, 2023, we collected serum samples from 1500 patients aged 1–99 years at The First Affiliated Hospital of Guangzhou University of Chinese Medicine (Guangzhou, China), around 1 month after the zero-COVID policy was ended. Serum samples were tested using ELISA, with ORF8 protein used as the detection antigen (appendix p 1). The demographic information of patients and ELISA results are shown in the table. The overall positive rate of ORF8 ELISA was 61·5%. The samples were further stratified into five age groups with positive rates of 66·2% in group 1–17 years, 71·3% in group 18–39 years, 54·1% in group 40–59 years, 55·8% in group 60–79 years, and 64·5% in group 80–99 years.Table 1Summary of ELISA test results per age groupNumber of samplesSampling date (2023)Median age, years (IQR)MaleFemalePositive rateAttack ratio (credible interval)All age groups1500Jan 5–1452 (34–71)736 (49%)764 (51%)922 (61·5%)80·7% (72·2–86·8)1–17 years74Jan 5–1412 (8–15)45 (61%)29 (39%)49 (66·2%)85·1% (72·0–92·4)18–39 years422Jan 5–1330 (25–35)184 (44%)238 (56%)301 (71·3%)87·5% (83·3–90·7)40–59 years381Jan 5–1250 (45–54)196 (51%)185 (49%)206 (54·1%)74·3% (63·7–82·5)60–79 years412Jan 5–1469 (65–74)206 (50%)206 (50%)230 (55·8%)73·9% (66·7–79·9)80–99 years211Jan 7–1485 (82–88)106 (50%)105 (50%)136 (64·5%)75·2% (60·3–85·6)Samples were defined as positive if the optical density was higher than 0·17 from the ORF8 ELISA test. Open table in a new tab Samples were defined as positive if the optical density was higher than 0·17 from the ORF8 ELISA test. After the zero-COVID policy ended, the age-specific infection attack ratio and disease transmission measures (ie, initial reproduction number and doubling time) were estimated by an age-specific renewal equation. We assumed an initial proportion of susceptibility was 98% on Dec 7, 2022, providing that stringent zero-COVID control measures were implemented in Guangzhou and that there had been no previous large scale COVID-19 outbreaks. The Markov chain Monte Carlo method was used to calculate the posterior estimates and corresponding credible intervals (CIs; appendix). We estimated that the overall infection attack ratio on the day of the exit strategy was 5·7% (95% CI 4·8–6·8). With an estimated doubling time of 5·3 days (95% CI 4·8–5·8), the overall infection attack ratio rapidly increased to 15·2% (12·3–18·7) at 1 week and 32·9% (26·3–40·2) at 2 weeks after the imposed day of the exit strategy. Among five age groups, individuals aged 1–17 and 18–39 years had higher infection attack ratio than those aged 40–99 years (table; appendix p 1). By day 30 after the exit strategy, the infection attack ratios were 80·7% (95% CI 72·2–86·8) for all age groups, 85·1% (72·0–92·4) for age 1–17 years, 87·5% (83·3–90·7) for age 18–39 years, 74·3% (63·7–82·5) for age 40–59 years, 73·9% (66·7–79·9) for age 60–79 years, and 75·2% (60·3–85·6) for age 80–99 years. Our research indicates that the COVID-19 epidemic spread fast in mainland China after the zero-COVID control measure was ended. Importantly, these results show that more than 80% of the studied population had already recovered from SARS-CoV-2 omicron infection. Herd immunity to the circulating omicron variants likely is high in Guangzhou and potentially in other urban areas in China with high transmission networks and this wave of transmission in China's urban areas is ceasing. We declare no competing interests. This research was funded by the Health and Medical Research Fund Commissioned Research on COVID-19 (COVID1903003), Emergency Key Program of Guangzhou Laboratory (EKPG22–30–6), Chinese University of Hong Kong Direct Grant (2021·015), Research Grants Council Collaborative Research Fund (C6036–21GF, C4139–20G), University Grant Committee Research Matching Grant (8601629), Group Research Scheme (The Chinese University of Hong Kong), and Visiting Scientist Scheme (Lee Kong Chian School of Medicine). We thank S H Ho Foundation for the monetary donation to develop the ORF8 ELISA. Download .pdf (.55 MB) Help with pdf files Supplementary appendix