The defintion of outbreak is clear for me : it is the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season. outbreaks of single cases is also clear for me . What is vague for me is as follows... eg : if the regular vaiation for a disease is about 100 -200 per year when to say that it is an outbreak ? if it exceeds 250? or 300 or 400? how to confirm that this is an outbreak ? at what level ? and on what basis? is there any reference or general rule?
I see you are aware of the meaning of outbreak so will not go into that. In order to know how much more than the normal occurrence we need to label it as an outbreak, we need statistics.
Whenever the increase is more than 2 SD (standard deviations), we call it as epidemic. Hope, this helps you
A disease can be considered as an outbreak if the mortality acceeds the 5%...
It is better to define outbreak (epidemic) first> Epidemic is
"The occurrence of an illness, in a given population, in clear excess of normal expectancy and the cases are related to each other and derived from a common or propagated source".
Or
"A substantial increase in the incidence rate of a disease in a specified population or region over a specified period of time".
In both these definitions, the epidemic is:
1. Excess incidence rate
2. Related cases by the incubation period
3. One or propagated source
4. The creation of a public health emergency
The latter is important criterion (public health emergency)
The number of cases is not always very important. For example a high number of common colds is not substantive. but A single case of small pox is very significant at any place in the world.
Eva
Despite all the points which you mentioned, it remains true that the excess in incident cases was reported and a public health problem was created across many countries.
I do remember a statement by a wise man saying" The reality of......is beyond any one to measure or understand. What we try to measure is an approximate value of reality"
The true incidence rate of many conditions remains unknown particularly in diseases with a wide range of presentation from sub-clinical to mild to moderate to severe to very severe to immediately fatal. Thus the iceberg phenomenon is true for almost all diseases. Regardless we need to take actions when threats are imposed. Actions are triggered by significant changes in the level of risk. When the risk is high (Say epidemic), action is a must.
Normally, we do not need to set a definite number(cut-off) to declare the occurrence of epidemic (outbreak). Any number which represents a threat to the public is sufficient even if it is a single case in some situations.
Dear Colleges:
Based on the previous answers ¿How many cases do we need in order to declare an outbreak in smallpox?
One case is sufficient to alert the WHO and the World that a public health emergency is imenent because:
1. Smallpox is serious disease
2. The normal expectancy is zero since 1980
That is right!!
Therefore the endemic level is a particular measure for each disease, there is not a general rule.
The better way to define an epidemiological situation as an outbreak is to use the international consensual definition.
1. take mean of three past years data of the disease under study in the same days and same place . If the calculated mean is less than cases happening presently , then it is an outbreak otherwise not.
2. MEAN+ 2 standard deviations , if your cases are more than it , then outbreak will be labelled otherwise not.
Just remember that the mean is affected by the extreme values, and also when estimating the epidemiological chart or canal, you have to take off the epidemic years
The recommendation is to use between 5 or 7 years, but that will depend on the data you have on hand.
Obviously this should be applied to the same population, place and unit of time.
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