Mortality rates and case-fatality

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Mortality rates and case-fatality


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Mortality rates and case-fatality

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A public health researcher is investigating the outbreak of Shigella diarrhea at a restaurant in a small town. He finds that 500 individuals dined at the restaurant during the past week. The incidence of Shigella diarrhea was 25% and the total number of deaths recorded among the incident cases was 5 individuals. What is the case-fatality rate for this episode of Shigella outbreak?  


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Let’s say we want to figure out the risk of dying for people who live in a certain population, like the population of France in 2015, which was around 66.6 million people.

The first way we could do this is by calculating the all-cause mortality rate or death rate.

That’s the total number of deaths from all causes in 1 year divided by the total number of people at risk in the population at mid-year, and typically that’s the number of people at risk of death in the entire population.

For example, in 2015, there were approximately 620,000 deaths.

So, the all-cause mortality rate in France was 620,000 divided by 66.6 million, or 0.0093.

We can express the mortality rate in a few different ways.

First, we could say that the absolute risk of dying from all causes in France in 2015 was 0.0093.

Alternatively, we might express it as a percentage, by multiplying it by 100 - so 0.0093 times 100 equals 0.93%.

Most often though, we express mortality rate in terms of number of deaths per 100,000 people.

So, we multiply the mortality rate - 0.0093 - by 100,000, which is 930.

This means that, in France, there were 930 deaths per 100,000 people in 2015.

Mortality rates can be calculated for any time period, like 1 month, 1 year, or 10 years, but it’s important to specify which time period is used in the calculation, since the rate might be different for different time periods.

For example, a natural disaster - like an earthquake or forest fire - might increase the number of deaths in a certain time period.

Sometimes we’re interested in the mortality rate in a certain subpopulation, like only women or only people who are older than 65 years.

In those situations, we use a specific rate - like an age-specific rate or a gender-specific rate - which is calculated by dividing the number of deaths from all causes in one year by the number of people at risk of death in the subpopulation at mid-year.

For example, there were around 33.9 million women in France in 2015, and 298,000 women who died in that year, so the female-specific mortality rate was 0.0088, or 880 deaths from all causes per 100,000 women in France in 2015.

We can also calculate a cause-specific or disease-specific mortality rate for people with a specific disease, like cancer.


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