New CDC Vaccine Effectiveness Study Uses Innovative Approach to Measure Vaccine Benefits

A new report released in the Journal of Infectious Diseases (JID) describes one of the first vaccine effectiveness (VE) studies to combine patient VE data with laboratory testing data on the genetic properties of different H3N2 viruses that circulated during the 2014-2015 season. Researchers used this information to detect differences in protection provided by the 2014-2015 seasonal flu vaccine against the multiple genetic groups of H3N2 viruses that circulated that season. The study found that the 2014-2015 flu vaccine provided moderate protection (44%) against one genetic group of H3N2 viruses that were antigenically similar to the vaccine’s H3N2 component, but provided no measurable protection against the most commonly circulating group of H3N2 viruses that was substantially different antigenically from the flu vaccine’s H3N2 component. These findings underscore the possible protective benefits of flu vaccination even during seasons when the most common flu viruses in circulation are antigenically different from the flu vaccine. In addition, these findings highlight the potential benefits of using laboratory based methods to identify the genetic properties of circulating flu viruses to inform flu vaccine virus selection.

The 2014-2015 flu season was characterized by early and widespread flu activity accompanied by high levels of outpatient illness and flu-associated hospitalizations, particularly among adults 65 years of age and older. Circulating influenza A (H3N2) viruses were associated with the majority of flu illnesses during the 2014-2015 season.

In this study, laboratory data was collected on the genetic properties of different circulating H3N2 flu viruses. These genetic characterization data were obtained through advanced molecular diagnostics (AMD). The H3N2 viruses tested were isolated from respiratory specimens collected from patients enrolled at study sites in different geographic areas, including Michigan, Pennsylvania, Texas, Washington and Wisconsin.

Researchers identified multiple genetic groups of H3N2 viruses, including the predominant H3N2 virus group 3C.2a (which accounted for 81% of circulating H3N2 viruses). Vaccine effectiveness data collected from patients enrolled in the U.S. Flu VE network found that the 2014-2015 flu vaccine provided no measurable benefit against these group 3C.2a H3N2 viruses, consistent with U.S. virologic surveillance that showed these viruses were antigenically dissimilar to the vaccine.

The second most common genetic group of H3N2 viruses, 3C.3b, circulated primarily at the Pennsylvania site and accounted for 11% of circulating H3N2 viruses. Unlike the 3C.2a group, these 3C.3b viruses were antigenically more similar to the corresponding H3N2 component of the 2014-2015 seasonal flu vaccine. Researchers found that the 2014-2015 flu vaccine provided measurable protection against these 3C.3b viruses. VE against these viruses was 44% [95% CI, 16% to 63%]. These results showed how the protective benefits provided by flu vaccination can vary geographically depending on which genetic groups of H3N2 flu viruses are circulating locally. This new study approach also showed how laboratory testing of the genetic properties of circulating flu viruses can identify different genetic groups of viruses. This in turn can help researchers evaluate how well the flu vaccine protects against the different genetic groups of viruses that may circulate in a single flu season.

The use of these laboratory methods to identify genetic characteristics of circulating flu viruses in this study may serve to advance the science of vaccine evaluation and vaccine virus selection in the future. Combining genetic characterization data with data from VE studies has the potential to provide more complete and timely information on emergent flu viruses and the flu vaccine’s ability to protect against them.

This study is available for online viewing from the JID website.