Difference Between Index Case and Primary Case

Although the medical sector has advanced massively, communicable and non-communicable outbreaks are still common, such as the coronavirus pandemic, which has now spread worldwide. There are terms used to define outbreaks based on the onset of the outbreak and how it is spread. The terms index case and primary case are used in this context. Though sometimes used synonymously, these two differ. 

What is Index Case?

This is the first patient identified by health authorities with a disease that makes the team aware of a possible outbreak. This applies to all diseases, whether spread from human to another or not.  Although an index case may remain unknown in the course of an outbreak, there will always be one. 

An index case may not indicate the source of the disease, the reservoir which holds the disease or how the disease may be spread. However, it always brings awareness of a possible outbreak. 

Examples of primary cases include Mary Mallon, who infected 47 people with typhoid in early the 1900s, Mabalo Lokela, a 44-year-old teacher who was the first victim of Ebola and a baby in the Soho neighbourhood of London in the 1854 cholera outbreak. 

What is Primary Case?

This is a person who brings disease to a community, school, another person, school or even country. It is used concerning infections or diseases that are only spread from human to human. For most cases, however, it is difficult to identify the primary case leaving most unknown. 

For instance, the primary case for the HIV epidemic remains unknown. An example of a known primary case is that for Ebola in 2014 whereby a child who is suspected to have had contact with an infected animal.  

Similarities between Index case and Primary case

• Both are used in the context of disease outbreaks

Differences between Index case and Primary case

Definition

The index case refers to the first patient identified by health authorities with a disease which makes the team aware of a possible outbreak. On the other hand, the primary case refers to a person who brings disease to a community, school, another person, school or even country. 

Types of diseases

While the index case applies to all diseases whether spread from human to another or not, the primary case applies to infections or diseases that are only spread from one human being to another. 

Examples

Examples of an index case include a 44-year-old teacher who was the first victim of Ebola and a baby in the Soho neighbourhood of London in the 1854 cholera outbreak. On the other hand, an example of a primary case is that for Ebola in 2014 whereby a child who is suspected to have had contact with an infected animal.  

Index case vs. Primary case: Comparison Table

Summary of Index case and Primary case

Index case refers to the first patient identified by health authorities with a disease which makes the team aware of a possible outbreak. On the other hand, primary case refers to a person who brings disease to a community, school, another person, school or even country. Despite the differences, both are used in the context of disease outbreaks. 

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