Study: Flu vaccines prevent hospitalizations in seniors, even when vaccine effectiveness is low

ATLANTA — A new study from the Centers for Disease Control and Prevention found that flu vaccines prevent flu-associated hospitalizations in people 65 years and older, even during seasons when vaccine effectiveness is low, the agency stated Friday. The study reinforces CDC’s existing recommendation for annual vaccination of adults 65 years and older who are at high risk for serious flu-related complications and often most-impacted by serious flu disease each year, resulting in hospitalization or death.

The study, published in Clinical Infectious Diseases online on May 6, used statistical modeling to estimate flu-vaccine-prevented hospitalizations in adults aged 65 years and older for estimates of vaccine effectiveness against medically attended influenza illness, ranging from 10% to 70%. Researchers used CDC flu surveillance data collected during the 2011-12 and 2012-13 seasons. The 2011-12 season was considered to be a mild flu season, whereas the 2012-13 season was characterized as moderate to severe. Using data from these two seasons, researchers were able to determine the varying impact that flu vaccination had in terms of hospitalizations prevented. 

Findings showed that during the more severe 2012-13 flu season, a flu vaccine with 10% effectiveness (and 66% coverage) would avert about 13,000 hospitalizations, whereas a vaccine with 40% effectiveness would avert about 60,000 hospitalizations. In contrast, during the more mild 2011-12 season, a flu vaccine with the same two effectiveness estimates would avert about 2,000 and 11,000 hospitalizations, respectively.

CDC estimates for overall vaccine effectiveness against medically attended influenza illness (across all age groups) for 2011-12 are 47% and for 2012-13 are 52%. The estimates for people 65 and older are 43% for 2011-12 and 32% for 2012-13. Vaccine coverage among people 65 years and older during 2011-12 is estimated to be 65% and 66% during 2012-13.

In addition to estimating hospitalizations prevented by vaccination during these two seasons, the study's authors also analyzed how many people would need to be vaccinated to prevent one hospitalization. For the 2012-13 season, 476 people would need to be vaccinated to prevent one hospitalization, assuming vaccine effectiveness was 40%; whereas, for the 2011-12 season, 2,457 people would need to be vaccinated to prevent one hospitalization.

Therefore, the study showed that flu vaccination provides a greater benefit against hospitalization during moderate to severe seasons compared with milder seasons, and the number of people who need to be vaccinated to prevent hospitalizations is lower during moderate to severe seasons than during milder seasons. The reason for this is that more people are hospitalized during more severe seasons.

Researchers have known for some time that flu vaccine effectiveness generally is lower in the elderly than in younger, healthy adults. However, this research is reassuring that even when flu vaccination is associated with lower vaccine effectiveness, it can still have a measurable and significant impact on preventing hospitalizations in adults 65 years and older, the CDC stated.