A study published in the Journal of Infectious Diseases led by CIDER researcher Dr. Andreas Handel and his colleagues aimed to elucidate the vaccine efficacy (VE) of high-dose (HD) Fluzone vaccine in older adults (> 65 years) compared to the standard-dose (SD) in older and younger adults (< 65 years). Older adults are at a particular risk for severe influenza infections and death due to the decline of the immune system function as we age. To combat this, older adults are given HD vaccines, which are associated with improved immune responses and clinical outcomes in those > 65 years, compared to SD vaccines.
Fluzone HD is one of the vaccines the Centers for Disease Control and Prevention recommends for older adults. Studies show that Fluzone HD is associated with reduced risk of influenza-like illness, increased hemagglutination inhibition (HAI) titers, and protection against laboratory confirmed influenza. Although these and other studies have shown improvements in outcomes for those who have received Fluzone HD, HD vaccine formulations are often limited to two to three influenza seasons. Recurring changes to vaccine components minimizes the overall understanding of VE – a key metric for vaccine quality.
VE can be measured directly using traditional endpoints of observational studies and clinical trials, but these types of studies are not only costly, but time consuming. As such, influenza researchers – such as those at CIDER – are leveraging immunogenicity data, which are easier to measure and more readily available. HAI titer, used in this study, is an already established correlate of protection that can be used to estimate VE more efficiently.
In this study, CIDER researchers assessed HAI titers from a human vaccine cohort that received either SD or HD Fluzone vaccines across nine influenza seasons from 2013-2022 using a previously developed statistical model based on a meta-analysis of observational studies, clinical trials, and challenge studies. Participants were defined as being part of one of three groups: < 65 years of age who received SD vaccines, > 65 years of age who received the SD vaccine, > 65 years of age who received the HD vaccine.
As expected, the HD vaccine generally improved the estimated VE in older adults compared to the SD. However, HD recipients often demonstrated a lower estimated VE compared to younger adults who received the SD vaccine. These results indicate that while HD vaccines are more protective than SD vaccines in older adults, the impact is not large, and older adults still rarely reach the level of protection seen in younger adults.
While age is a non-modifiable factor, conducting this study was important to better understand vaccine responses between older adults receiving HD vaccines and younger adults receiving SD vaccines as this comparison has not been previously established. These analyses highlight opportunities for further improvement of vaccines in older adults, including increasing dosage or addition of adjuvants, and other formulation changes that may offer additional protection.
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