Updated 2020-02-21

COVID-19: the distinction between a disease and an epidemic

Education and information are crucial factors in the management of infectious disease. The distinction between the disease and the epidemic is key.

Updated 21 February 2020

In infectious disease the uncertainties can broadly be divided into two completely different but related issues.

1. The Disease Process: This helps to address the question:

    1. What will happen to me if I get sick?

2. The Epidemic Process: This deals with the questions:

    1. How likely am I to get sick now?
    2. Will that change in the future?

What do we know about the disease?

This illness is causing pneumonia. This study published in the Lancet on January 30th, 2020 describes the clinical and epidemiological features of 99 patients from the early cohort in Wuhan2. A further analysis of the early Wuhan cohorts was published on 4th February 2020. A more recent overview has been published again in the Lancet on February 14 20207.

An important recent study of 9 pregnant women did not suggest infection transferred to their babies. Obviously, this will be very important information8.

The mortality remains relatively low. This data is inevitably crude. It does not allow for ill patients who are dying but equally we have no real handle on the true number of infections in the population. Mortality is by definition hard data (with caveat explained below) so as the incidence of currently undetected and milder cases increases the true mortality rate is likely to fall.

For example the Hong Kong's Centre for Health Protection (CHP) publishes regular updates on the epidemic. By 12 AM on 20th February 2020 there had been 76,702 confirmed cases and 2,247 deaths1. This gives a crude mortality of 2.9%, but it is possible that some of the infected and seriously ill people will die over the next few weeks. This would increase this crude mortality rate. However, what happens if there was actually a large number of people in the community who had mild illness and were never diagnosed? Modelling by international experts currently put the likely outbreak at significantly higher than the 76,702 confirmed cases6. A team from HKU published an epidemic projection in the Lancet on 31 January. Their estimate is of 75,815 cases in Wuhan alone by January 25th 2020 with seeding in other cities3. Paradoxically this would make the true mortality significantly lower. This is one dilemma of evolving epidemics. Mortality data early in an epidemic will invariably be unreliable and usually overstates the true mortality.

China has changed the criteria by which they report this disease. They have moved away from reporting only positive tests and are now using clinical diagnosis. This makes sense. The situation early in epidemics is often difficult. As we have explained in a previous article mortality rates can be over or under reported. The sudden surge in cases must be interpreted in this context. The death rate has doubled but the incidence has increased by a factor of 10. Counterintuitive but this reduces the case fatality rate by a factor of five and may explain some of the higher mortality in Hubei. There may be other factors which need to be considered.

The WHO gave an early estimate of Mortality of 2%. The CFR (Case Fatality Rate) in Hubei before the change of criteria was 3%, but this is likely to include significant under reporting in view of the acknowledgement of limited testing capacity. The mortality in the Rest of the World excluding China (Small data set) is currently 0.9% (11/1,237). We have provided a more detailed analysis of the difficulties in extrapolating data from early in epidemics.

This disease continues to appear to be closer to Influenza than SARS in severity at a similar point in the epidemic. Most deaths are occurring in people over 60 and/or in people with other existing health problems. There seems to be a significant increased risk in males to females. Highest fatality rates are for people over 80 which is the same for all infections. There have so far been no fatalities in children and under 40 years of age the mortality is 0.2%. The high mortality in health care workers is likely to be explained by high viral load. Health care workers have always died in epidemics. They are working with the sickest people and we know that in many infections the amount of virus which an individual is exposed to is related to the severity of the disease that they suffer.

We postulated that it is likely that some of the illness resulting from the Coronavirus, now known as COVID-19, will be due to an immunological reaction to the infection. The first published scientific literature in the Lancet reporting the first 41 patients in Wuhan supports this hypothesis5. This raises several potential options for treatment. Again, as we understand more about the disease the mortality is likely to fall.

This article includes a number of academic papers in the Lancet providing excellent medical research from China. We will have a much better understanding of the demographics of the individuals affected and the natural evolution of the disease over the next few weeks.


The Epidemic Process 

In order to understand the likelihood of an illness spreading it is necessary to understand the factors which influence this spread.

What determines whether a virus spreads?

The main factors influencing the evolution of an epidemic of illness are:

The Mode of Spread: This illness is spread by droplets and close personal contact. There have been some reports of the virus spreading via the stool. This is important because illnesses which spread in this way can potentially be contained by public health measures such as:

  • Isolation of infected individuals and social distancing such as closing schools
  • Hand washing and personal hygiene
  • Masks

We wish to stress our support for these appropriate health measures.

The Incubation Period: This refers to the time between an individual being infected and when they first develop symptoms. The WHO reports the mean incubation to be 5-6 days though in some cases may be up to 14 days. The mean epidemic doubling time is 6-7.4 days. Influenza by comparison is typically two days. This means that every week that passes we have twice as much data as the previous week.

The Infectivity in the Incubation: We do not yet know for certain when this virus becomes infective. We now have definite evidence of person to person transmission, including within Hong Kong. The current increase in incidence certainly suggests a virus which is likely to be reasonably infectious. It is important to understand that infectivity and severity may be inversely related. The more cases that are occurring means that the disease severity (the Case Fatality Rate) generally reduces.

The Immunity of Individuals: This is influenced by many factors including age and the presence of other illness. An individual can also enhance factors which influence their immunity. This may include healthy diet, exercise, adequate rest, avoiding stress, maintaining a positive psychological outlook in addition to immunization against other infectious illnesses including influenza.

The way in which these factors subsequently impact the size of an epidemic is discussed here.

Many media stories confuse these two distinct concepts. They refer to the number of deaths (mortality) or severity of the illness and then conflate this with infectivity. The importance of this illness to our population will be determined by the evolution of the epidemic. This will become much clearer as we accumulate data from contacts of confirmed cases. SARS affected over 8,000 people with 744 deaths in 2003. Influenza kills between 250-500,000 people annually (WHO estimate). The CDC estimates that there have been between 14,000-36,000 deaths from influenza in the USA so far this Winter compared to the 2,247 currently reported from COVID-194.

This is not to reduce the importance of this new virus nor the appropriate public health responses but to place the current data in context. We are not suggesting that COVID-19 is the same as or even a type of influenza, it is not.  Accepting the limits of projections made early in an epidemic this illness currently seems to be more severe than seasonal influenza. Exactly how severe will take longer to establish but early indicators suggest this disease will be less severe than first thought. The impact it will have on the population will be determined by the evolution of the epidemic.

The information which we obtain from the cruise ship in Japan and the clusters in France and Hong Kong will give useful information. The source passenger on the cruise ship was on the cruise between January 20th and 25th. The subsequent cluster of infections suggests that the illness is reasonably infectious. Any of the passengers on the ship who had any symptoms were tested. The same applies to contacts of infected individuals in the clusters in France and Hong Kong. The severity of illness in these passengers and contacts is very important. If significant numbers of these passengers have a mild illness this would be in favour of the illness being more infectious but less severe than first thought for the reasons explained here.

All infectious illness has greatest impact on the young, the old, the poor and the immunocompromised. This is one of the reasons why the WHO has declared a Global Health Emergency.

Hong Kong has a world class public health system and quite rightly public health measures have been introduced in order to protect the most vulnerable members of the community.

We will continue to update the article as data becomes available and information changes.

To learn more about the explanations behind the current public health measures, please click here.



1.(2020). Retrieved 5 February 2020, from https://www.chp.gov.hk/files/pdf/statistics_of_the_cases_novel_coronavirus_infection_en.pdf

2. Chen, N., Zhou, M., Dong, X., Qu, J., Gong, F., & Han, Y. et al. (2020). Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. The Lancet

3. Huang, C., Wang, Y., Li, X., Ren, L., Zhao, J., & Hu, Y. et al. (2020). Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. The Lancet

4. Preliminary In-Season 2019-2020 Flu Burden Estimates. (2020). Retrieved 5 February 2020, from https://www.cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm

5. Wang, F., & Zhang, C. (2020). What to do next to control the 2019-nCoV epidemic?. Retrieved 5 February 2020, from https://www.thelancet.com/pb-assets/Lancet/pdfs/S0140673620303007.pdf

6. Wu, J., Leung, K., & Leung, G. (2020). Nowcasting and forecasting the potential domestic and international spread of the 2019-nCoV outbreak originating in Wuhan, China: a modelling study. Retrieved 5 February 2020, from https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30260-9/fulltext

7. Heymann, D. L., & Shindo, N. (2020). COVID-19: what is next for public health? Retrieved February 14, 2020, from https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30374-3/fulltext

8. Chen, H., Guo, J., Wang, C., Luo, F., Yu, X., Li, P. J., et al. (2020). Clinical characteristics and intrauterine vertical ... Retrieved February 14 2020, from https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30360-3/fulltext

Topics: Hong Kong Health, Preventative Healthcare/Medicine, COVID-19

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