What is the difference between a vaccine's effectiveness and its efficacy?
What is the difference between a vaccine's effectiveness and its efficacy?
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Though vaccine efficacy and vaccine effectiveness are similar terms and are often used interchangeably, the differences between the two are important. In this entry, we rely on the United States Centers for Disease Control and Prevention's (U.S. CDC) definitions of effectiveness and efficacy. When a new vaccine is being developed and studied in clinical trials, scientists report on vaccine efficacy. Efficacy is a term used to describe how well the vaccine protects clinical trial participants from getting sick or getting very sick. The term does not describe how well a vaccine works on the general public. The efficacy of a vaccine reflects ideal circumstances, like a research trial, which are different than real-world conditions. Once a vaccine is made available for large population groups, vaccine effectiveness can be measured. Effectiveness is the amount of protection given by a vaccine in a certain population when its used under field conditions (somewhat normal practices, less than perfectly controlled like in a research study). It considers other factors like population-level differences in health status, weight, age, and other factors across communities. Effectiveness is a more reliable and accurate term for how helpful a vaccine is at preventing disease in daily life when people are doing regular community-based activities like socializing, going to work or school, and grocery shopping.
Though vaccine efficacy and vaccine effectiveness are similar terms and are often used interchangeably, the differences between the two are important. In this entry, we rely on the United States Centers for Disease Control and Prevention's (U.S. CDC) definitions of effectiveness and efficacy. When a new vaccine is being developed and studied in clinical trials, scientists report on vaccine efficacy. Efficacy is a term used to describe how well the vaccine protects clinical trial participants from getting sick or getting very sick. The term does not describe how well a vaccine works on the general public. The efficacy of a vaccine reflects ideal circumstances, like a research trial, which are different than real-world conditions. Once a vaccine is made available for large population groups, vaccine effectiveness can be measured. Effectiveness is the amount of protection given by a vaccine in a certain population when its used under field conditions (somewhat normal practices, less than perfectly controlled like in a research study). It considers other factors like population-level differences in health status, weight, age, and other factors across communities. Effectiveness is a more reliable and accurate term for how helpful a vaccine is at preventing disease in daily life when people are doing regular community-based activities like socializing, going to work or school, and grocery shopping.
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What our experts say
Though vaccine efficacy and vaccine effectiveness are similar terms and are often used interchangeably, the differences between the two are important. In this entry, we rely on the United States Centers for Disease Control and Prevention's (U.S. CDC) definitions of effectiveness and efficacy.
When a new vaccine is being developed and studied in clinical trials, scientists report on vaccine efficacy. Efficacy is a term used to describe how well the vaccine protects clinical trial participants from getting sick or getting very sick. The term does not describe how well a vaccine works on the general public. The efficacy of a vaccine reflects ideal circumstances, like a research trial, which are different than real-world conditions.
Once a vaccine is made available for large population groups, vaccine effectiveness can be measured. Effectiveness is the amount of protection given by a vaccine in a certain population when its used under field conditions (somewhat normal practices, less than perfectly controlled like in a research study). It considers other factors like population-level differences in health status, weight, age, and other factors across communities. Effectiveness is a more reliable and accurate term for how helpful a vaccine is at preventing disease in daily life when people are doing regular community-based activities like socializing, going to work or school, and grocery shopping.
Though vaccine efficacy and vaccine effectiveness are similar terms and are often used interchangeably, the differences between the two are important. In this entry, we rely on the United States Centers for Disease Control and Prevention's (U.S. CDC) definitions of effectiveness and efficacy.
When a new vaccine is being developed and studied in clinical trials, scientists report on vaccine efficacy. Efficacy is a term used to describe how well the vaccine protects clinical trial participants from getting sick or getting very sick. The term does not describe how well a vaccine works on the general public. The efficacy of a vaccine reflects ideal circumstances, like a research trial, which are different than real-world conditions.
Once a vaccine is made available for large population groups, vaccine effectiveness can be measured. Effectiveness is the amount of protection given by a vaccine in a certain population when its used under field conditions (somewhat normal practices, less than perfectly controlled like in a research study). It considers other factors like population-level differences in health status, weight, age, and other factors across communities. Effectiveness is a more reliable and accurate term for how helpful a vaccine is at preventing disease in daily life when people are doing regular community-based activities like socializing, going to work or school, and grocery shopping.
Context and background
Due to the number of definitions of the term "vaccine effectiveness" and the way different people can interpret it, there is a widespread confusion about what this number actually means. Additionally, factors like age, preexisting health conditions, health care access, geographic distribution and more can contribute to how protective a vaccine is considered to be.
There are also several points to consider about the direct effect and overall effects of vaccines. The direct effect of vaccines considers people who are protected because they have been vaccinated on an individual level. The indirect effect shows the effect of a vaccination program on an entire population, including those who never received vaccinations. This is important because in most populations, the vaccinated people help protect the unvaccinated people. These are measured in the terms 'vaccine effectiveness' and 'vaccine efficacy.' This is why using the measures of vaccine effectiveness that includes more real world circumstances and looks at entire populations is more accurate and realistic.
Due to the number of definitions of the term "vaccine effectiveness" and the way different people can interpret it, there is a widespread confusion about what this number actually means. Additionally, factors like age, preexisting health conditions, health care access, geographic distribution and more can contribute to how protective a vaccine is considered to be.
There are also several points to consider about the direct effect and overall effects of vaccines. The direct effect of vaccines considers people who are protected because they have been vaccinated on an individual level. The indirect effect shows the effect of a vaccination program on an entire population, including those who never received vaccinations. This is important because in most populations, the vaccinated people help protect the unvaccinated people. These are measured in the terms 'vaccine effectiveness' and 'vaccine efficacy.' This is why using the measures of vaccine effectiveness that includes more real world circumstances and looks at entire populations is more accurate and realistic.
