The Distinction Between a Disease and an Epidemic

Many media stories confuse these two distinct concepts the infectivity in the incubation and the immunity of individuals. They refer to the number of deaths (mortality) or the number of patients in intensive care and then conflate this with infectivity.

by OtandP Healthcare

Education and information are key factors in the management of infectious disease. The distinction between the disease and the epidemic is key. During SARS we saw two different epidemics. The epidemic of the disease itself and the epidemic of anxiety. The latter was significantly more infectious.

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:

What will happen to me if I get sick?

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

How likely am I to get sick now?

Will that change in the future?

What do we know about the disease?

Like SARS this illness is causing pneumonia and the mortality remains relatively low. This data is inevitably crude as it does not allow for ill patients who are already dying. Equally we have no real handle on the true number of infections in the population. Mortality is by definition hard data, therefore as the incidence of currently undetected and milder cases increases the true mortality rate is likely to fall.

For example if the media report that there have been 20 deaths and 500 confirmed cases this gives crude mortality of 4%. But it is possible that another 10 of the infected people will die over the next few weeks. This would make the mortality 6%. However, what happens if there were actually large numbers of people in the community who had mild illness and were never diagnosed? Modelling by international experts currently put the likely outbreak at around 4000. This would make the true mortality 0.5%. This is one dilemma of evolving epidemics. Mortality data early in an epidemic will invariably be unreliable.

This illness appears to be significantly less severe than SARS at a similar point in the epidemic.

It is likely that much of the illness will be due to an immunological reaction to the infection. As the virus occurs in clusters outside of China it is likely that stories will appear about different drug treatments and the impact they will have on certain populations. Most fatalities will occur in people who are already ill. If this changes and we see an increase in mortality then expect a dramatic increase in scare stories and anxiety.

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. This is important because illnesses which spread in this way can 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

The Incubation Period

This refers to the time between an individual being infected and when they first develop symptoms. Early modelling suggests that the incubation may be 5-7 days. Influenza by comparison is typically two days. This means that if the illness is spreading widely we would expect the incidence to increase over a relatively short period of time though not as rapidly as influenza. This also means that we would expect to see more cases and develop more hard data over the next 10 days.

The Infectivity in the Incubation

We do not yet know for certain when this virus becomes infective. The current increase in incidence suggests a virus which is unlikely to be highly contagious before symptoms develop.

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 and maintaining a positive psychological outlook.

Many media stories confuse these two distinct concepts. They refer to the number of deaths (mortality) or the number of patients in intensive care and then conflate this with infectivity.

SARS was an example of a very severe illness but it actually wasn’t very infectious except when in close proximity to a very ill patient. This is why so many health care workers were affected relative to the population as a whole. The health care workers in China are likely to have been infected by a very ill patient. It is very likely the viral load (amount of virus in the patient) will be a factor in both spreading the illness and the severity of the disease. This is the case with many infectious illnesses including SARS.

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 8000 people with 744 deaths in 2003. Influenza kills between 250 – 500,000 people annually (WHO estimate).

This is not to reduce the importance of this new virus nor the appropriate public health response, but to place the current data in context.

Accepting the limits of projections made early in an epidemic this illness currently seems to be a little more severe than seasonal influenza but less infectious.

All infectious illness has greatest impact on the young, the old, the poor and the immunocompromised. Hong Kong has a world class public health system and quite rightly public health measures may be introduced in order to protect the most vulnerable members of the community.

We remain vigilant but cautiously optimistic.

To learn more about protecting yourself during the Wuhan Virus, please click here.

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