Epidemiology

The Epidemiology of COVID-19

Overview

Between 31^st December 2019 and 3^rd January 2020, 44 cases of pneumonia with an unknown cause were reported in Wuhan, Hubei Province, China. The outbreak was later found to be caused by SARS-CoV-2 and linked to attending a seafood market in Wuhan (WHO, 2020a). By the 20^th January, there were 282 confirmed COVID-19 cases, the majority (258) of which were in Hubei Province. There were 20 other cases in China outside of Hubei Province, 2 cases in Thailand, and one case each in Japan and Republic of Korea, 18 of which had travelled from Wuhan and 2 were close contacts with cases (WHO, 2020a). A city-wide lockdown was implemented in Wuhan on 23^rd January, and later in other cities, to contain the outbreak (Ainslie et al., 2020). The number of confirmed COVID-19 cases continued to increase in mainland China, reaching 9720 on 31^st January and 106 cases reported across 19 other countries, with a total of 213 deaths, leading the WHO to declare COVID-19 an international public health emergency. The majority of cases outside of China had travel history to Wuhan, however some cases of local transmission were observed, suggesting that human-to-human transmission was occurring (WHO, 2020f). The outbreak in China peaked in early February, with 3225 new cases per day on 4^th February (WHO, 2020g) before slowing in mid-end of February (399 new confirmed cases on 20^th February) (WHO, 2020h). No new cases of transmission within Wuhan were reported by 19^th March (Flaxman et al., 2020). Imai et al. (2020) used mathematical modelling to estimate the rate of human-to-human transmission of SARS-CoV-2. The reproduction number (R) of SARS-CoV-2 was estimated to 2.6, meaning that on average one COVID-19 patient would infect 2.6 other people. Uncontrolled, this leads to exponential growth of the number of infections, and infection control measures are needed to bring R to less than 1 to decrease the number of infections (Imai et al., 2020).  Further modelling indicated that the social distancing policies used in China reduced the R number below 1 (Flaxman et al., 2020). 

Cases were reported in an increasing number of countries in Asia, North America, Europe and the Middle East over February (6009 cases and 86 deaths), including an outbreak in a cruise ship where 705 people were infected and 6 died (WHO, 2020i). Bhatia et al. (2020) estimated that 63-73% of COVID-19 cases exported from China were undetected leading to widespread human-to-human transmission outside of China. Outbreaks began to appear in northern Italy, particularly in the Lombardy region, which was associated with local transmission rather than travel from China. The number of confirmed cases in Italy rose steeply from 9 cases on 22^nd February (WHO, 2020j) to 4636 cases and 197 deaths on 7^th March (WHO, 2020k) and 17660 cases and 1268 deaths by 14^th March, an increase of 2547 cases and 252 deaths in one day (WHO, 2020l). 

The outbreaks in Italy were then linked to an increase in cases seen in other European countries towards the end of February due to travel from the outbreak areas, including the UK (ECDC, 2020c). In the UK, cases began to increase in March, from 23 cases on 1^st, 167 on 7^th, 802 on 14^th, 3983 on 21^st and 14547 on 28^th, with 759 deaths (WHO 2020l-p). 

Mathematical modelling was used to estimate the number of cases and deaths of COVID-19 in Great Britain based on different public health measures. It was estimated that without any control measures, 81% of the Great Britain population would be infected causing 510,000 deaths. The health system would also be overwhelmed by demand, likely resulting in more deaths. ‘Mitigation’ of the outbreak, where case isolation, home quarantine and social distancing of over 70s is implemented, was predicted to only reduce the number of deaths by half and still lead to the healthcare system being overwhelmed (8 times greater demand than the number of beds available). The most optimal policy was ‘suppression’ i.e. social distancing of the entire population, school closure and self-isolation of suspected cases and their household contacts. Social distancing would need to be maintained until a treatment or vaccine was made available to avoid a second peak in infections (Ferguson et al., 2020). Mandatory social distancing measures and closure of non-essential businesses were ordered in the UK on 23^rd February in response to these findings. The number of cases in the continued to rise, reaching 41907 cases on 5^th April and 4313 deaths, rises of 3735 and 708, respectively, in one day (WHO, 2020q). The continued increase is due to a lag of 1-2 weeks between infection and symptoms appearing, and 2-3 weeks between transmission and death (Flaxman et al., 2020). The number of cases and deaths in the UK are likely to be significantly underestimated due to only hospitalised patients being tested. Preliminary investigations of the public health measures used across 11 European countries, including Italy and the UK, indicates that the R number has been reduced substantially and was on average 1.43 (Flaxman et al., 2020).

The crude mortality rate of COVID-19 (the number of people that have died as a ratio to the number of confirmed infections) was projected to be around 1.4% over all ages, ranging from 0.32% for those under 60 years and 6.4% in those aged over 60 (Verity et al., 2020). This does not take into account that a number of the infected cases have not reached the final clinical outcome (recovery or death), and the number of cases that remain untested or asymptomatic, which will affect the mortality rate. 

In contrast to COVID-19, SARS only resulted in around 8000 cases but had a greater fatality rate of around 7%. Similarly, MERS was more frequently fatal, with a 33% mortality rate and only caused around 2500 cases (Peeri et al., 2020). 

Epidemiology Resources

Carolan, K., Crossley, M., Verran, J. Amos, M., Whitton, N. and Redfern, J. (n.d.) Simfection [Online] Available from: http://www.simfection.org.uk/ [Accessed 06/04/2020].

• Interactive software demonstrating the spread of infections and how vaccination can alter the rate of transmission.

Centers for Disease Control and Preventioin (n.d.) Solve the Outbreak [Online] Available from: https://www.cdc.gov/mobile/applications/sto/index.html#game [Accessed 06/04/2020].

• Interactive game based on analysing clues and data to solve disease outbreaks for secondary school students. 

Imperial College London (2020) COVID-19 Reports [Online] Available from: http://www.imperial.ac.uk/mrc-global-infectious-disease-analysis/covid-19/ [Accessed 06/04/2020].

• Reports from MRC Centre for Global Infectious Disease Analysis, Imperial College London, modelling the global epidemiology of COVID-19. The reports have influenced the UK government policies contain the outbreak. Topics include: estimating the number of infections, predicting the impact of non-pharmaceutical measures on mortality and healthcare demand, COVID-19 symptom progression and transmissibility of the infection.

Mres (2020) Epidemic Simulation [Online] Available from: https://scratch.mit.edu/projects/376656449/ [Accessed 06/04/2020].

• Online epidemic simulation game demonstrating the spread of disease in a population and how the rate of spread can be altered by social distancing. 

Public Health England (2020) COVID-19: Track Coronavirus Cases [Online] Available from: https://www.gov.uk/government/publications/covid-19-track-coronavirus-cases [Accessed 06/04/2020]. 

• Dashboard showing the number of confirmed COVID-19 cases and fatalities in the UK and breakdown by local authority in England. 

World Health Organization (2020) Coronavirus (COVID-19) [Online] Available from: https://experience.arcgis.com/experience/685d0ace521648f8a5beeeee1b9125cd [Accessed 06/04/2020].

• Dashboard showing the global number of confirmed COVID-19 cases and fatalities.

World Health Organization (2020) Coronavirus Disease (COVID-19) Situation Reports [Online] Available from: https://www.who.int/emergencies/diseases/novel-coronavirus-2019/situatio... [Accessed 06/04/2020].

• Daily global COVID-19 epidemiological reports on from WHO. 

European Centre for Disease Prevention and Control (2020) Considerations Relating to Social Distancing Measures in Response to COVID-19 – Second Update. Stockholm: ECDC.

• Guidance and background on social distancing for public health authorities.

Ainslie, K.E.C., Walters, C., Fu, H., Bhatia, S., Wang, H., Baguelin, M., Bhatt, S., Boonyasiri, A., Boyd, O., Cattarino, L. et al. (2020) Report 11: Evidence of Initial Success for China Exiting COVID-19 Social Distancing Policy After Achieving Containment [Online] Available from: https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-... [Accessed 31/03/2020].

• Report from Imperial College London’s MRC Centre for Global Infectious Disease Analysis on the impact of social distancing on the spread of COVID-19. This gives evidence to support the current social distancing interventions in the UK and other countries. 

Department of Education and Public Health England (2020) Coronavirus (COVID-19): implementing Social Distancing in Education and Childcare Settings [Online] Available from: https://www.gov.uk/government/publications/coronavirus-covid-19-implemen... [Accessed 01/04/2020].

• Guidance from the Department of Education and Public Health England on how to implement social distancing n education and childcare facilities, what parents can do and which children can continue to attend school and childcare facilities.

Public Health England (2020) Guidance on Social Distancing for Everyone in the UK [Online] Available from: https://www.gov.uk/government/publications/covid-19-guidance-on-social-d... [Accessed 01/04/2020].

• Guidance on how to implement social distancing for the general public.