https://ojs.wpro.who.int/ 1WPSAR Vol 12, No 1, 2021 | doi: 10.5365/wpsar.2020.11.2.006 Outbreak Investigation Report C oronavirus disease 2019 (COVID-19) was first detected in Wuhan, China, on 31 December 2019.1 The first confirmed case of COVID-19 imported from Wuhan to Hong Kong SAR (China) was recorded on 23 January 2020,2 and on 30 January 2020 the World Health Organization (WHO) declared the COVID-19 outbreak a public health emergency of international concern.3 Between January and May 2020, 1084 confirmed cases of COVID-19 were reported in Hong Kong SAR (China). The local epidemic progressed through four phases: (1) preparedness and imported infection from mainland China, (2) local transmission, (3) imported infection from overseas countries associated with local transmission, and (4) controlled imported infection with limited local transmission.4 During the second phase – local transmission (4 February to 3 March) – we reported a local family cluster of six confirmed COVID-19 cases among 29 people who attended a Chinese New Year family dinner gathering in a restaurant on 26 January 2020 (the second day of Chinese New Year). METHODS We conducted an epidemiological investigation of a confirmed case of COVID-19. On 10 February 2020, we received notification of a confirmed case of COVID-19 involving a 37-year-old female (patient 1) who had devel- oped fever, cough and sore throat from 2 February 2020. She was admitted to a public hospital on 10 February 2020 and her nasopharyngeal aspirate tested positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ribonucleic acid (RNA) using real-time reverse transcription polymerase chain reaction. Symp- tomatic contacts were isolated in public hospitals for SARS-CoV-2 testing and management. Asymptomatic close contacts were quarantined in quarantine centres, while other contacts who were asymptomatic were put under medical surveillance. RESULTS Patient 1 was home based, had no recent travel history outside Hong Kong SAR (China) and denied having any contact with confirmed COVID-19 cases. Contact tracing revealed that her husband (patient 2), who resided with her, developed fever and cough on 30 January 2020. In addition, her father (patient 3), who did not reside with her, developed fever and cough from 3 February 2020. Patients 2 and 3 were admitted for isolation on 10 Febru- ary 2020 and tested positive for SARS-CoV-2. Patients 1–3 shared a Chinese New Year dinner with 26 other relatives on 26 January 2020. Between 31 January and 8 February 2020, three more relatives were found to be symptomatic and tested positive for SARS-CoV-2 (patients 4–6). The 29 attendees at the dinner lived in various separate residences, and the family dinner was the only common exposure among all six confirmed cases dur- ing the incubation period. The dinner, which was held in a restaurant, lasted for about 7 hours and included mahjong playing. Three of the six confirmed cases had played mahjong. The 29 diners were seated at two tables in the same room, in a partitioned area within the restaurant. None of the attendees were symptomatic during the gathering. The restaurant where the outbreak occurred was closed permanently because of business considerations before the notification of a COVID-19 case a Communicable Disease Branch, Centre for Health Protection, Department of Health, Hong Kong SAR (China). Published: 16 February 2021 doi: 10.5365/wpsar.2020.11.3.006 Coronavirus disease 2019 (COVID-19) outbreak during a Chinese New Year dinner in a restaurant, Hong Kong SAR (China), 2020 Tsz-sum Lam,a Chi-hong Wong,a Wing-hang Lam,a Ho-yeung Lam,a Yonnie Chau-kuen Lam,a Emily Chi-mei Leunga and Shuk-kwan Chuanga Correspondence to Tsz-sum Lam (smo_epi1@dh.gov.hk) WPSAR Vol 12, No 1, 2021 | doi: 10.5365/wpsar.2020.11.2.006 https://ojs.wpro.who.int/2 Lam et alCOVID-19 outbreak during a Chinese New Year dinner in a restaurant, Hong Kong SAR (China) COVID-19.5 Mahjong is generally played by four people sitting around a square table in close proximity for hours, with the distance between players usually being less than 1 m (a distance at which transmission of respiratory droplets is possible). A Chinese dinner is commonly shared by 12 diners sitting close together at a round table, but in this particular instance, 29 diners were seated at two tables that usually accommodated 24 people, further reducing the distance between people. This investigation had limitations. Information on viral load might have indicated who was more likely to be the heavier spreader, but no laboratory investigation of viral load was conducted. Also, it was not possible to undertake contact tracing of the wait staff or conduct environmental investigations. It appears that some people can be positive for COVID-19 for 1 to 3 days before they develop symp- toms.6 Although the source of the family cluster could not be identified, our findings support pre-symptomatic transmission and effective human-to-human transmis- sion of COVID-19 through social activities. Non-pharma- ceutical interventions (e.g. social distancing) have been associated with reduced transmission of COVID-19 in on 10 February 2020. Hence, it was not possible to un- dertake contact tracing of the wait staff or environmental investigations. Contact tracing identified one domestic helper (pa- tient 7) who did not join the family dinner but shared a bedroom with patient 4 (symptom onset on 31 January); patient 7 developed a fever and cough from 2 February and tested positive for SARS-CoV-2 (Fig. 1) (four other members of that household were sent to a quarantine centre and were not infected). In summary, the cluster was six (from the fam- ily cluster) plus one (the domestic helper) confirmed cases, comprising three males and four females aged between 32 and 75 years (median: 37 years). None of the seven patients had a travel history outside Hong Kong SAR (China) and they all denied having any contact with confirmed COVID-19 cases during the incubation period; all seven were discharged home uneventfully. DISCUSSION WHO advises maintaining social distancing of at least 1 m (3 feet) as a basic protective measure against 5 4 3 2 1 16 Ja n 2 02 0 5 F eb 20 20 6 F eb 20 20 7 F eb 20 20 8 F eb 20 20 9 F eb 20 20 10 F eb 20 20 11 F eb 20 20 22 F eb 20 20 25 Ja n 2 02 0 26 Ja n 2 02 0 27 Ja n 2 02 0 28 Ja n 2 02 0 29 Ja n 2 02 0 30 Ja n 2 02 0 31 Ja n 2 02 0 1 F eb 20 20 2 F eb 20 20 3 F eb 20 20 4 F eb 20 20 family dinner N um be r of c as es Date of symptom onset primary case secondary case Fig. 1. Epidemic curve of Chinese New Year restaurant dinner COVID-19 outbreak WPSAR Vol 12, No 1, 2021 | doi: 10.5365/wpsar.2020.11.2.006https://ojs.wpro.who.int/ 3 COVID-19 outbreak during a Chinese New Year dinner in a restaurant, Hong Kong SAR (China)Lam et al 2. Latest situation of cases of COVID-19 in Hong Kong. Hong Kong SAR (China): Centre for Health Protection, Department of Health; 2020. Available from: https://www.chp.gov.hk/files/pdf/local_situa- tion_covid19_en.pdf, accessed 22 October 2020. 3. WHO Statement on the second meeting of the International Health Regulations (2005) Emergency Committee regarding the outbreak of novel coronavirus (2019-nCoV). Geneva: World Health Organi- zation; 30 January 2020. Available from: https://www.who.int/ news-room/detail/30-01-2020-statement-on-the-second-meeting- of-the-international-health-regulations-(2005)-emergency-commit- tee-regarding-the-outbreak-of-novel-coronavirus-(2019-ncov), ac- cessed 22 October 2020. 4. Lam HY, Lam TS, Wong CH, Lam WH, Leung CME, Au KWA, et al. The epidemiology of COVID-19 cases and the successful contain- ment strategy in Hong Kong-January to May 2020. Int J Infect Dis. 2020 Sep;98:51–8. doi:j.ijid.2020.06.057 pmid:32579906 5. WHO coronavirus disease (COVID-19) advice for the public. Gene- va: World Health Organization; 2020. Available from: https://www. who.int/emergencies/diseases/novel-coronavirus-2019/advice-for- public, 22 October 2020. 6. WHO coronavirus disease 2019 (COVID-19) situation report 73. Geneva: World Health Organization; 2 April 2020. Available from: https://www.who.int/docs/default-source/coronaviruse/situation- reports/20200402-sitrep-73-covid-19.pdf?sfvrsn=5ae25bc7_2, accessed 22 October 2020. 7. Cowling BJ, Ali ST, Ng TWY, Tsang TK, Li JCM, Fong MW, et al. Impact assessment of non-pharmaceutical interventions against coronavirus disease 2019 and influenza in Hong Kong: an obser- vational study. Lancet Public Health. 2020 May;5(5):e279–88. doi:S2468-2667(20)30090-6 pmid:32311320 Hong Kong SAR (China).7 The Centre for Health Protec- tion appeals to the public to properly maintain social distancing at all times during the COVID-19 pandemic. Acknowledgements The authors would like to thank the staff of the Com- municable Disease Branch and Public Health Laboratory Services Branch of the Centre for Health Protection, Hong Kong SAR (China) for their contributions to the outbreak investigation. Funding None. Conflicts of interest No potential conflicts of interest were reported by the authors. References 1. Novel coronavirus (2019-nCoV) situation report 1. Geneva: World Health Organization; 21 January 2020. 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