Hazard ratio

Hazard ratio ("HR") is a quantity often used in medical studies to show a decrease in risk of mortality or harm from using the studied course of treatment. A hazard ratio of 1 signifies no change, while a hazard ratio of 0.5 signifies a 50% reduction in harm.

In other words, the hazard ratio is the risk of a hazard relative to a control group risk of 1.

The hazard ratio is defined formally in two different ways:

• The hazard ratio is the ratio of (chance of an event occurring in the treated group)/(chance of an event occurring in the control group).

• The hazard ratio is the ratio of (risk of outcome in one group)/(risk of outcome in the control group), occurring in a certain interval of time

Example

In a study showing the effectiveness of hydroxychloroquine to treat COVID-19, French researchers led by Professor Raoult considered the outcomes of 3,737 COVID-19 patients and found that:

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Treatment with HCQ-AZ was associated with a decreased risk of transfer to ICU or death (Hazard ratio (HR) 0.18 0.11–0.27), decreased risk of hospitalization ≥10 days and shorter duration of viral shedding.

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