Coronavirus disease-2019 (COVID-19) is an infectious disease caused by a newly discovered coronavirus called Severe Acute Respiratory Coronavirus 2 (SARS-CoV2) [1]. The SARS-CoV2 belongs to a family of viruses that cause illness with various symptoms such as pneumonia, fever, breathing difficulty and lung infections [2]. The virus is one of the main pathogens of respiratory infection alongside others such as SARSCoV and Middle East Respiratory Syndrome Coronavirus (MERSCoV) that caused severe respiratory syndrome in humans [3]. The term 2019 novel coronavirus was first used to refer to the virus that affected the lower respiratory tract of patients with pneumonia in a local Huanan South China Seafood market in Wuhan City, China on 29 December 2019 [4] [5]. Afterwards, the World Health Organization (WHO) announced the official name of the 2019 novel coronavirus to be coronavirus disease (COVID-19) [1].

The SARS-CoV2 has been understood to be zoonotic, however, no specific animal association has been identified [6]. Human to human transmission has been proven through small droplets expelled from the nose or mouth when a person with COVID-19 coughs, sneezes, or speaks [7]. The virus has been found to adhere to surfaces which act as fomites for the transmission of the disease [8]. The SARS-CoV2 presents in different ways which range from severe illness and mortality to mild symptoms and asymptomatic [9]. Early patterns of COVID-19 transmission have shown a trend similar to SARS and MERS coronaviruses however, unlike other coronaviruses, asymptomatic cases can still transmit the infection, which raises public health concerns [10] [11].

The COVID-19 outbreak is increasing and spreading rapidly globally, with over 20 million cases and 800,000 thousand deaths reported in 216 countries as on 28^th August 2020 [12]. Since the report of the index case in Nigeria in February 2020 by the Nigeria Center for Disease Control (NCDC), daily records of confirmed cases have been reported in all states in the country, including Ondo State. As on 28^th August 2020, a total of 53, 477 cases and 1011 deaths have been reported in Nigeria, while Ondo State ranked 11^th among the states with high confirmed cases in the country [13].

This paper presents the outcome of the epidemiological investigation of COVID-19 outbreak in Ondo State, Nigeria from March to August 2020 to further guide the outbreak response activities.

The finding in this investigation that a high proportion of the confirmed COVID-19 cases were males and between the age group of 20 to 59 years is consistent with previous studies in Oyo State, Nigeria [3] and Wuhan, China [17]. The high proportion of COVID-19 infection among the male age group could be attributed to the fact that this is an economically active age group, suggesting a potential role of socio-economic or work-related activities rather than immunological capacity. In the Nigeria system, men are more likely to engage in economic activities outside of the household and potentially become more exposed to SARS-CoV-2 infection than women [18]. Also, Bwire [19], De La Vega et al., [20] and Isere et al., [11] in their studies reported poor compliance and attitude towards COVID-19 preventive measures and protocol such as frequent hand washing, wearing of face mask, and stay at home orders among men compare to women.

Similarly, our investigation shows a high proportion of infection among healthcare workers which is consistent with a previous investigation by Usman et.al., [3]. This might be attributed to poor compliance to COVID-19 IPC protocol by health care workers due to key issues such as knowledge gaps and access to PPEs in the health care settings. This is of serious public health importance as it could potentially enhance disease transmission [21] and further weaken a health system that already struggles with insufficient human resources for health [18]. Also, the high proportion of infection among healthcare workers could also be attributed to poor level of preparedness for the COVID-19 outbreak in the health care settings, and this is also in consonance with the report of Elhadi et al., [22].

The epidemic curve of the outbreak suggests a propagated pattern of transmission of the disease. This is consistent with previous COVID-19 outbreak reports and studies in other regions of Nigeria NCDC [23], Usman et al., [3], Elimian et.al., [18] and in China by Chen et al., [24]. The pattern of the outbreak is suggestive of person-to-person transmission which further explains the findings in this investigation that most of the cases (88.8%) occurred in urban settings where crowded conditions, clustered settlement pattern of the houses and large-scale human interactions due to daily commercial activities could have facilitated the spread of the outbreak [25] [26].

The most common COVID-19 symptoms reported in our investigation were fever and cough. Grant et al., [27] and Tahvildari et al., [28] have reported similar findings from previous studies. Furthermore, the lower proportions of fever and cough reported in this study compared to other studies could be attributed to the fact that our investigation is a population-based study where symptoms were self-reported alongside testing, unlike previous studies where testing was limited to patients on admission at the hospital in the early phase of the outbreak [27] [28]. More importantly, fever which is one of the common symptoms reported in this investigation is often common in endemic febrile illnesses (malaria and Lassa fever) in Ondo State [18]. Hence, co-infection with these diseases could be responsible for the high report of fever as a symptom in this investigation compared to other symptoms.

The low case fatality rate (CFR) found in this study is consistent with that previous reports in the Nigeria [23] and other Africa countries such as South Africa and Senegal [12]. This could be associated with the mild to asymptomatic clinical presentation of most cases in this study who are more likely to recover from the infection [11] [29]. Also, our investigation shows a significantly high mortality rate among cases in the older age group who were mostly male compared to females. This sex-biased mortality could be attributed to sex differences in immune response as females are known to mount more robust innate, cell-mediated, and humoral immune response than males [30]. Sex chromosome constitution could also play an important role in disease outcome. Females have two copies of X chromosomes while males have one [31] [32] [33]. The X chromosome contains a high density of immune-related genes. Males and females show a distinct difference in immune system responses with females eliciting stronger immune responses to pathogens [31] [32] [33].

This difference in immune system responses may be a major contributing factor to viral load, disease severity, and mortality [30]. Also, differences in sex hormone milieus could be a determinant of viral infections as estrogen has immunoenhancing effects while testosterone has immunosuppressive effects [30]. The sex-specific severity of COVID-19 infection, however, indicates that further research on understanding the sex differences is needed [30].

Some key limitations of this investigation include misclassification bias as most cases were asymptomatic and their clinical presentations were not consistent with the surveillance operational case definition. However, a consistent review of the operational case definition using daily data generated from the field investigation assisted in minimizing misclassification bias. In addition, the data presented in this investigation is a preliminary finding of the outbreak investigation which might change with time as the outbreak is still ongoing. Finally, the epi-curve was generated using the week of report as opposed to the typical use of the date of onset of symptoms. This was because of the asymptomatic clinical status of most of the confirmed cases during the investigation. In addition, social desirability bias is suspected as community perception about the COVID-19 disease might have stigmatized respondents. However, probing questions were asked to ensure correct responses where possible.

The outcome of this investigation indicates that high transmission among urban residence and health care workers are key public health concerns in the response to the COVID-19 outbreak in Ondo State, Nigeria. Furthermore, high case mortality among the older age groups requires public health intervention. Thus, we recommend intensified risk communication, enhanced surveillance activities, and use of community structures such as community and religious leaders, market and commercial vehicles associations, Ward Development Committee (WDC) and Village Development Committee (VDC) to ensure compliance with public health COVID-19 preventive measures particularly in the urban areas and among those facing a high risk of death. Furthermore, there is a need to prioritize public health interventions including training and vaccination among the vulnerable groups including health care workers who serve as front liners during case investigation, testing and case management.

Author EEI conceived the study and statistical analysis plan. Author AMA and EEI did the statistical analysis and drafted the first draft of the manuscript under the supervision of ATF, SF, AGF, TOO and WA. All authors contributed to the interpretation of results and writing of the manuscript.

The study has no funder who played any role in any stage of the study.

No ethical approval required given that the study was included in an outbreak investigation.

The authors wish to appreciate all health care workers, surveillance officers, public health administrators and other stakeholders on the frontline in response to the COVID-19 outbreak in Ondo State especially field officers who through which the data for this study were generated.

The authors declare no conflicts of interest regarding the publication of this paper.

Isere, E.E., Adejugbagbe, A.M., Fagbemi, A.T., Fagbemi, S., Famokun, A.G., Omoju, T.O. and Adegbenro, W. (2021) Outcome of Epidemiological Investigation of COVID-19 Outbreak in a South West State of Nigeria, March to August 2020. Open Journal of Epidemiology, 11, 163-177. https://doi.org/10.4236/ojepi.2021.112015