https://ojs.wpro.who.int/ 1WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001 Original Research P neumonia caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Wuhan, China was first reported to the World Health Organization (WHO) on 31 December 2019.1 As of 16 August 2020, the virus, which causes coronavirus disease 2019 (COVID-19), had spread globally and infected more than 21 million people, with more than 700 000 deaths.2 The outbreak of COVID-19 was declared a public health emergency of international concern by the WHO on 30 January 2020, following international spread of the disease. Malaysia’s preparedness and response plan was instituted as early as February 2020. It included public health activities, intensified diagnostic capacity and early, appropriate treatment of confirmed COVID-19 cases.3 The first cases of COVID-19 in Malaysia were detected on 25 January 2020 in three travellers from China,4 and the first case in a Malaysian citizen was confirmed as the ninth case in early February 2020.4 Localized clusters started to emerge in February, the largest cluster being linked to a religious gathering in Sri Petaling, which resulted in a major increase in the number of local cases and contributed to imported cases in neighbouring countries.5 By 16 March, every state and federal territory in Malaysia had reported cases of COVID-19. Malaysia implemented a movement control order (MCO) on 18 March 2020 to contain the spread of the virus.4 The government initiative included closing international borders, shutting down certain economic sectors and restricting social movement within and between states to protect the population.6 Many of the initial confirmed cases were con- nected to a wet market in Wuhan, and the SARS-CoV-2 a Petaling District Health Office, Selangor, Malaysia. Published: 21 April 2021 doi: 10.5365/wpsar.2020.11.4.001 Descriptive epidemiology of the first wave of COVID-19 in Petaling District, Malaysia: Focus on asymptomatic transmission Rama Krishna Supramanian,a Lavanyah Sivaratnam,a Arifah Abd Rahim,a Noor Dalila Inche Zainal Abidin,a Ong Richai,a Zazarida Zakiman,a Salina Md Taib,a Lee Soo,a Syed Hafeez Syed Ibrahim Jamalullai,a Muhammad Nur Asraf Khirusalleha and Mohamed Paid Yusofa Correspondence to Rama Krishna Supramanian (email: ramakrishna@moh.gov.my) Background: COVID-19 was first detected in Malaysia on 25 January 2020. Multiple clusters were detected in Petaling District, with the first locally transmitted case reported on 8 February. Descriptive analyses of the epidemiology of the COVID-19 outbreak in Petaling are presented, from the first case to the end of the first wave. Methods: All laboratory-confirmed COVID-19 cases reported to the Petaling District Health Office between 1 February and 26 June 2020 were analysed. Socio-demographic characteristics, symptoms, date of onset, date of exposure, travel history and history of comorbidities were obtained by phone interviews using one of two investigation forms. The descriptive analysis was conducted according to time, place and person. Results: There were 437 COVID-19 cases, for an incidence rate of 24/100 000 population. Ten (2.3%) deaths and 427 recovered cases were recorded. Of the 437 cases, 35.5% remained asymptomatic and 64.5% were symptomatic. Common symptoms included fever (43.8%), cough (31.6%) and sore throat (16.2%); 67.3% had no comorbidities, 62.5% reported close contact with a confirmed case, and 76.7% were local infections. Transmission occurred in four main groups: religious gatherings (20.4%), corporations (15.1%), health facilities (10.3%) and a wholesale wet market (6.4%). In 31.9% of confirmed cases, an epidemiological link to an asymptomatic case was found. Conclusion: Transmission of the disease by asymptomatic cases should be emphasized to ensure continuous wearing of face masks, hand hygiene and social distancing. Further research should be conducted to better understand the transmission of SARS-CoV-2 from asymptomatic cases. WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001 https://ojs.wpro.who.int/2 Krishna et alDescriptive epidemiology of the first wave of COVID-19 in Petaling District, Malaysia Case definition The definition of a confirmed case of COVID-19 is a person with a reverse transcriptase polymerase chain reaction (RT-PCR) positive result, regardless of their symptoms. Only cases that met this case definition were included. People under investigation for COVID-19 are defined as having fever OR acute respiratory infection (sudden onset of shortness of breath, cough and/or sore throat) AND travel to or residence in an affected country (China, Islamic Republic of Iran, Italy, Japan, Republic of Korea) within 14 days before illness onset OR close contact within 14 days before illness onset with a confirmed case of COVID-19. Epidemiological investigation Each notified case was verified by the Petaling DHO before an epidemiological investigation was undertaken to determine the source of infection, including contact tracing, active case detection and prevention and control measures, including quarantine. The primary objective of investigation was to identify the source of infection and close contacts of confirmed cases. Information on socio-demographic characteristics, symptoms, date of illness onset, date of exposure, travel history and co- morbidities was obtained by phone interview with cases and contacts using one of two investigation forms. The data were then shared with the Selangor State Health Department. Date of exposure was defined as the last date of contact with a known case of COVID-19 or last date of travel, if any, while date of onset was defined as the date the person self-reportedly developed any symptoms related to COVID-19. Details of close con- tacts were retrieved during case investigations and sent to the contact tracing team for further action. All cases and contacts were monitored daily for the next 14 days. All relevant data were captured within the COVID-19 surveillance system of Malaysia’s Ministry of Health (MOH). Data management Most notifications of confirmed cases were received from the Surveillance Unit of the Selangor State Health Department; some were received by phone, fax or email from hospitals and accredited laboratories. The MOH has a surveillance system for notification and monitor- ing of infectious diseases known as the Communicable Diseases Control Information System or eNotifikasi,12 pathogen was indicated to be zoonotic in origin. Reports have confirmed person-to-person transmission via res- piratory droplets, as the virus was shown to spread in Wuhan by close contact with positive cases, without exposure to live animals.7 The average incubation period for COVID-19 is 5 days but may be up to 14 days. The common reported symptoms include fever, cough, short- ness of breath, fatigue and other flu-like symptoms.8,9 Asymptomatic cases have also been documented.10 The first COVID-19 case in Petaling was documented on 3 February 2020 and was later confirmed to be the first case in a Malaysian citizen.4 Following notification of a confirmed COVID-19 case, the District Health Of- fice (DHO) implements control and prevention measures and conducts a thorough epidemiological investigation to identify the source of infection or index case. To break the chain of transmission, confirmed COVID-19 cases are isolated and treated in designated COVID-19 hospitals, while contacts are traced and identified for mandatory COVID-19 laboratory testing and a 14-day at-home quarantine. Early detection of cases among close contacts is crucial for early containment to prevent further seeding of community transmission. In early March 2020, the number of cases in Petaling increased due to a localized cluster of COVID-19 infections in one corporation, with more than 90 cases confirmed within 3 weeks.11 The outbreak then increased exponentially, triggering a more rigorous control response from the Petaling DHO. As analysis of the COVID-19 cases in Petaling may provide critical information to help control the spread of similar infectious diseases at district and national levels, the objective of this paper is to describe the epidemiology of the COVID-19 epidemic in Petaling District between 1 February and 26 June 2020. METHODS Study design This descriptive study is based on an exploratory analy- sis of all cases of COVID-19 notified to the Petaling DHO from the beginning of the outbreak in early February 2020 to the end of June 2020. Study setting This study was conducted in the Petaling District, Sel- angor, Malaysia, a highly urbanized area with a dense population of over 2 million people. WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001https://ojs.wpro.who.int/ 3 Descriptive epidemiology of the first wave of COVID-19 in Petaling District, MalaysiaKrishna et al and COVID-19 was added as a notifiable disease to this system at the end of March 2020 to ensure manda- tory reporting of suspected and confirmed cases of COVID-19 to the nearest DHO. Reporting is compulsory under the Malaysia Prevention and Control of Infectious Disease Act 1988.13 As all case records contain na- tional identification numbers, all cases are recorded in the system without duplication. The inclusion criteria for this study were confirmed COVID-19 cases according to the case definition notified to the Petaling DHO between 1 February 2020 and 26 June 2020. Data analysis The socio-demographic and clinical characteristics of all confirmed cases of COVID-19 were summarized with descriptive statistics. An epidemic curve of all cases was constructed by plotting the number of cases (y- axis) against the self-reported date of symptom onset (x-axis). For asymptomatic cases, the date of onset was considered to be the last date of known exposure. Ethical approval The study protocol was approved by the Medical Re- search and Ethics Committee, MOH Malaysia (NMRR- 20–1540–55803 [IIR]). RESULTS Between 1 February 2020 and 26 June 2020, there were 437 confirmed cases of COVID-19 in Petaling District. The total population of Petaling District in the 2010 census14 was 1 812 633. Therefore, the incidence rate of COVID-19 infection was 24/100 000 population. The baseline characteristics of the confirmed cases are presented in Table 1. All 437 cases were admitted to the hospital for isolation and treatment. Ten cases (2.3%) died due to complications, and the other 427 cases were eventually discharged. Of all cases, 76.7% were local and 23.3% were imported. The mean age was 41 years, and 25.6% were in the 21–30 years age group. The gender distribu- tion was relatively even, with 53.8% male and 46.2% female cases. Malaysian citizens accounted for 92%, and 64.5% of cases were symptomatic. The most com- monly observed symptoms were fever (43.8%), cough (31.6%) and sore throat (16.2%). The total number of close contacts of confirmed COVID-19 cases was 7081. Among 160 close contacts who were later confirmed positive, 51 (31.9%) were close contacts of asymp- tomatic primary cases, and 109 (68.1%) were close contacts of symptomatic primary cases. A total of 294 cases (67.3%) had no comorbidi- ties, while 70 (16%) had hypertension and 46 (10.6%) had diabetes mellitus. Of all cases, 62.5% had reported close contact with a confirmed COVID-19 case, and 76.7% were classified as locally transmitted infections. In Petaling, four main clusters of cases were identi- fied: at a religious gathering (20.4%), in a corporation (15.1%), in health facilities (10.3%) and at a wholesale wet market (6.4%). Other clusters included sporadic local and imported cases. Fig. 1 shows the dates of symptom onset for cases of COVID-19 in Petaling District between January and June 2020. The first cluster of COVID-19 was detected in a corporation in early February, which peaked in mid-February. A total of 66 cases were reported from this cluster. The highest peak of cases occurred in mid- March; the infection rate then tapered off and ended in mid-April. Most cases during the peak were linked to a mass religious gathering (89 cases). The third peak, seen at the end of April, involved vendors at a whole- sale wet market, with a total of 28 cases reported. The epidemic curve in Fig. 1 shows a pattern indicating person-to-person transmission. DISCUSSION We report the epidemiological characterization of the initial COVID-19 outbreak in the most densely populated district of the State of Selangor, Malaysia. Most of the reported cases were aged 21–30 years (25.6%), and the distribution of cases by gender was similar. The age distribution of the cases in this study is consistent with that in the initial outbreak reported in China, i.e. mainly young adults.15 About 65% of the cases were symp- tomatic, the three most commonly reported symptoms being fever, cough and sore throat. The pathogenesis of SARS-CoV-2 includes both upper and lower respiratory tract infections,16 and the earliest outbreak in the epi- centre, Wuhan, also included symptoms of respiratory tract infection in most reported cases.17 Respiratory viruses are highly contagious when patients are sympto- matic. In the outbreak reported here, more than half the WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001 https://ojs.wpro.who.int/4 Krishna et alDescriptive epidemiology of the first wave of COVID-19 in Petaling District, Malaysia cases were locally transmitted and had reported close contact with a confirmed COVID-19 case. COVID-19 is transmitted primarily in respiratory droplets7 and by physical contact.18 Evidence of human-to-human trans- mission among close contacts has been found since the beginning of the pandemic, in mid-December 2019.19 Furthermore, the clusters of COVID-19 cases in Petaling District involved gatherings, further spreading the virus in the community.20 In this outbreak, about 32% of cases had an epidemiological link to an asymptomatic case. As similar viral loads have been reported in symp- tomatic and asymptomatic cases,17 community trans- mission of SARS-CoV-2 by asymptomatic cases is likely. The salivary glands could act as a potential reservoir for COVID-19; thus, infectious salivary droplets could be transmitted to a susceptible host from the mouth when an asymptomatic carrier is speaking, sneezing or even breathing, or from the eyes, and directly inhaled into the lungs.21 Similar evidence of transmission from asympto- matic carriers to close contacts has been reported.22,23 In view of the novelty of SARS-CoV-2, accumulation of evidence on transmission from asymptomatic people has contributed to understanding the dynamics and public health implications of the disease. In our study, almost one third of close contacts who became infected were contacts of asymptomatic cases. As asymptomatic individuals appear to be a common source of infection, strict monitoring of close contacts of asymptomatic cases is essential to contain potential outbreaks. The fundamental characteristics of first-wave cases and the associated epidemic curves in Petaling Table 1. Baseline characteristics of COVID-19 cases in Petaling District N % Total number of cases 437 Attack rate 0.024 Age (years, mean, SD) 41, 17.7 Age group 0–10 15 3.4 11–20 28 6.4 21–30 112 25.6 31–40 77 17.6 41–50 59 13.5 51–60 77 17.6 >60 69 15.8 Gender Male 235 53.8 Female 202 46.2 Nationality Malaysian 402 92.0 Non-Malaysian 35 8.0 Symptom status Symptomatic 282 64.5 Asymptomatic 155 35.5 Symptoms (n = 282) Fever 187 43.8 Cough 137 31.6 Sore throat 71 16.2 Headache 22 5.0 Loss of taste and smell 21 4.8 Myalgia 18 4.1 Gastrointestinal disturbances 12 2.7 Comorbidities or risk factors None 294 67.3 Hypertension 70 16.0 Diabetes mellitus 46 10.6 Dyslipidaemia 22 5.0 Heart disease 16 3.7 Bronchial asthma 10 2.3 History of close contact with a confirmed COVID-19 case Yes 273 62.5 No 164 37.5 Total number of close contacts Symptomatic index cases 4568 64.5 Asymptomatic index cases 2513 35.5 Confirmed COVID-19 cases among close contacts Symptomatic index cases 109 68.1 Asymptomatic index cases 51 31.9 Type of infection Local 335 76.7 Imported 102 23.3 N % Clusters Religious gathering 89 20.4 Corporation 66 15.1 Health facilities 45 10.3 Wholesale wet market 28 6.4 Others 209 47.8 Local council area subdivision Petaling Jaya 178 40.7 Subang Jaya 100 22.9 Shah Alam 157 36.0 Others 2 0.5 Status Alive 427 97.7 Dead 10 2.3 WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001https://ojs.wpro.who.int/ 5 Descriptive epidemiology of the first wave of COVID-19 in Petaling District, MalaysiaKrishna et al District indicate that 282 (64.5%) cases were symp- tomatic, with appropriate dates of onset of illness. The epidemic curve of all clusters in Petaling District between January and June 2020 (Fig. 1) indicates that the outbreak had a propagated source pattern of spread. This trend is consistent with person-to-person spread in outbreaks of this newly introduced zoonotic viral pathogen that subsequently became capable of human-to-human transmission due to high mutation and recombination rates.24 As shown in the epidemic curve, the outbreak in Petaling District had multiple surges of cases, resulting from several main clusters, including a corporation, a religious gathering, health facilities, a wholesale wet market and sporadic cases. The index case in the corporation cluster was believed to have been infected while travelling in Indonesia before the onset of symptoms. Subsequently, while symptomatic, the index case attended a meeting at the office, and transmission occurred to other workers. The religious gathering was attended by more than 19 000 people from various countries. It not only became a catalyst for subsequent spread of COVID-19 in Petaling District but also resulted in massive transmission throughout Ma- laysia and abroad.25 The gathering involved sharing of communal spaces, such as prayer halls, collective eating from shared plates and sharing of sleeping areas, which increased the opportunities for transmission among participants. Transmission of COVID-19 in these two main reported clusters in Petaling District went beyond household contacts, and contact tracing revealed up to five generations of contacts. The epidemic curve shows that cluster transmission accounted for more than half of the confirmed COVID-19 cases in this outbreak; a similar phenomenon has been seen in other cities.18 Early implementation of the MCO in response to the COVID-19 pandemic played a vital role in controlling the outbreak and preventing disease transmission within the community. Closure of all universities, schools, places of worship and non-essential sectors during the MCO helped to break the chain of transmission in the community by prohibiting mass movement and gather- ings nationwide. This federal response was successful in lowering the epidemic curve in Petaling District. The enhanced or targeted MCO, a cordon sanitaire imple- mented on 10 May 2020 by the federal government, slowed the COVID-19 outbreak in Petaling District dur- ing the wholesale wet market cluster. Overall, this study provides valuable information on the first wave of the COVID-19 outbreak in Petaling District and the general epidemiological measures taken to curb the outbreak. Additionally, this study included a large number of cases, as Petaling is part of the state of Selangor, which had the second-largest number of confirmed COVID-19 cases in Malaysia during this pe- Fig. 1. COVID-19 epidemic curve, with all clusters in Petaling District between January and June 2020 WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001 https://ojs.wpro.who.int/6 Krishna et alDescriptive epidemiology of the first wave of COVID-19 in Petaling District, Malaysia Funding The authors received no financial support for the re- search, authorship or publication of this article. References 1. Novel coronavirus (2019-nCoV): situation report, 1. Geneva: World Health Organization; 2020. Available from: https://apps.who.int/ iris/handle/10665/330760, accessed 9 February 2021. 2. Coronavirus disease (COVID-19): situation report, 209. Ge- neva: World Health Organization; 2020. Available from: https:// apps.who.int/iris/handle/10665/333897, accessed 9 February 2021. 3. Elengoe A. COVID-19 outbreak in Malaysia. Osong Public Health Res Perspect. 2020;11(3):93–100. doi:10.24171/j. phrp.2020.11.3.08 pmid:32494567 4. Shah AUM, Safri SNA, Thevadas R, Noordin NK, Rahman AA, Sekawi Z, et al. COVID-19 outbreak in Malaysia: Actions taken by the Malaysian Government. Int J Infect Dis. 2020;97:108–16. doi:10.1016/j.ijid.2020.05.093 pmid: 32497808 5. Aziz NA, Othman J, Lugova H, Suleiman A. Malaysia’s approach in handling COVID-19 onslaught: Report on the movement control order (MCO) and targeted screening to reduce community infec- tion rate and impact on public health and economy. J Infect Public Health. 2020;13(12):1823–9. doi:10.1016/j.jiph.2020.08.007 pmid:32896496 6. Prime Minister’s Office. Restriction of movement order (PMO of- ficial website; 2020). 7. Rothan HA, Byrareddy SN. The epidemiology and pathogen- esis of coronavirus disease (COVID-19) outbreak. J Autoimmun. 2020;109(2):102433. doi:10.1016/j.jaut.2020.102433 pmid: 32113704 8. Huang C, Wang Y, Li X, Ren L, Zhao J, Hu Y, et al. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, Chi- na. 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Kuala Lumpur: Government of Malaysia; 1988. Avail- able from: https://www.moh.gov.my/index.php/database_stores/ attach_download/317/19, accessed 9 February 2021. riod of the pandemic. Nevertheless, the study had some limitations, such as lack of data on the severity and clinical outcomes of cases. Furthermore, the data were retrospective and self-reported by patients and may be inaccurate due to recall bias. CONCLUSION This study provides key findings in the Petaling COVID-19 outbreak that are consistent with those re- ported in other studies. Most cases had a history of close contact with confirmed COVID-19 cases, confirming human-to-human transmission. The study also confirms that asymptomatic cases can transmit the disease to others. This should be emphasized to the community to ensure continuous wearing of face masks, hand hygiene and social distancing in public. Public health efforts should focus on surveillance for local transmission of cases and swift control measures to avert widespread community transmission. Active case detection and quarantine of close contacts of confirmed cases is a key prevention and control strategy to prevent spread of the disease, while strict monitoring of close contacts of asymptomatic infected cases is just as important as for symptomatic cases. Further research should be conducted to better understand the transmission of SARS-CoV-2 from asymptomatic cases. Acknowledgements We acknowledge the Director-General of Health of Malaysia’s Ministry of Health, Datuk Dr Noor Hisham Abdullah, for his permission to publish this paper. We express our gratitude to the Petaling District Health Office for sharing valuable knowledge and advice for the writing of this paper. Our appreciation also goes to the staff of the Petaling District Health Office for their help and cooperation in providing general feedback to improve the paper. Conflicts of interest The authors declare that they have no conflict of interest in this publication. WPSAR Vol 12, No 2, 2021 | doi: 10.5365/wpsar.2020.11.4.001https://ojs.wpro.who.int/ 7 Descriptive epidemiology of the first wave of COVID-19 in Petaling District, MalaysiaKrishna et al 14. Population distribution and basic demographic characteris- tics 2010. Kuala Lumpur: Department of Statistics; 2011:1– 133. 15. The epidemiological characteristics of an outbreak of 2019 novel coronavirus diseases (COVID-19) in China. CCDC Wkly; Vol 2; 2020. 16. Chen J. Pathogenicity and transmissibility of 2019-nCoV. A quick overview and comparison with other emerging viruses. Microbes Infect. 2020;22(2):69–71. doi:10.1016/j.micinf.2020.01.004 pmid:32032682 17. Lai CC, Shih TP, Ko WC, Tang HJ, Hsueh PR. 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