News & Views: Making Sense of Immunogenicity and Efficacy Data in Vaccine Trials
Published on in Vaccine Update for Healthcare Providers
Published on in Vaccine Update for Healthcare Providers
Immunogenicity and efficacy may be used interchangeably in conversations about vaccination; however, they are not exactly the same thing. This is especially true when discussing or interpreting data from vaccine trials.
Typically, the first clue as to whether a vaccine works is provided by measuring the immune response initially in experimental animals. Immunogenicity can be defined by any of the aspects of the immune response that are measurable. Most often for vaccines on the market, the measures are of antibodies in the blood, but in the research lab, measures can include newly formed or memory B cells, antibody secreting cells, or killer T cells to name a few. For some diseases, scientists know from previous work with the disease that if a person has a high enough level of one of these immune system components, he or she will be protected; this is called a correlate of protection.
Example: Someone with 15 international units (IU) of rubella-specific antibodies in the serum is considered to be protected against rubella. This is known as a correlate of protection.
If a correlate of protection is known and the potential vaccine does not induce a potent enough immune response, scientists need to reconsider the vaccine composition, dosage, and number and timing of doses. These studies can take years and are most often done in animals.
Unfortunately, for many diseases a clearly defined correlate of protection is not known. This is particularly true with infections that occur at mucosal surfaces (nose, throat, lungs, intestines and anogenital areas). In these cases, the most useful information comes from comparing the levels of the immune response following vaccination to those in individuals with disease.
While immunogenicity is helpful in assessing whether a vaccine is likely to work, it does not completely answer the question about whether the vaccine actually does work. That is, will a person who got vaccinated be protected if he or she is exposed to the disease? Efficacy data provides this information. As with immunogenicity, when researchers measure efficacy, they can measure a few different things. Most often they look at mild, moderate and severe disease which identifies everyone who gets ill, but by classifying severity of disease, the findings provides additional information about how the vaccine works. In cases of mild or moderate disease, the vaccine worked by altering the course of disease, and likely prevented some deaths, so it could still be considered valuable.
So, now you might be wondering why we still hear about immunogenicity data if efficacy data better defines how well a vaccine works. The answer is for a few reasons:
Immunogenicity and efficacy data provide information on how well a vaccine works, but they are not the same. So, if you are reviewing or explaining data, it is important to determine how the study was designed and what it measured in order to appropriately interpret the data.
Contributed by: Charlotte A. Moser, MS, Paul A. Offit, MD
Categories: News and Views About Vaccines
Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.
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