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New CDC Study Shows Flu Vaccine Reduces Severe Outcomes in Hospitalized Patients

May 25, 2017 – A new study in the journal Clinical Infectious Diseases (CID)  showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized flu patients. This study is an important first step in better understanding whether flu vaccines can reduce severe flu outcomes even if they fail to protect against infection.

The study looked at hospitalized flu patients during 2013-2014 and compared patients who had been vaccinated to those who had not. The observed benefits were greatest among people 65 years of age and older, which is notable because people in this age group are at increased risk of serious flu complications and have the highest hospitalization rate among all age groups.

The study found that vaccinated adults were 52-79% less likely to die than unvaccinated flu-hospitalized patients. In other words, an unvaccinated hospitalized flu patient was 2 to 5 times more likely to die than someone who had been vaccinated.

Additional benefits were observed in terms of reducing ICU admissions for several age groups. For example, vaccinated adults 18-49 years of age as well as adults 65 years of age and older hospitalized from flu were 37% less likely to be admitted to the ICU than those who were not vaccinated. In terms of length of ICU stay, vaccinated adults 50-64 years of age as well as adults 65 years of age and older admitted to an ICU with flu illness were more likely to have a shorter stay in the ICU than those who were not vaccinated. Furthermore, vaccinated adults 50-64 years of age and adults 65 years of age and older were more likely to be discharged from the hospital earlier than those hospitalized with the flu who had not been vaccinated.

Researchers did not observe a difference between length of hospital stay or length of ICU stay among people 18 to 49 years of age. The authors discussed whether a larger sample size may have been needed to measure this outcome in this age group.

In the past 10 years, CDC and its public health partners have published many vaccine effectiveness studies that assess how well the seasonal flu vaccine protects against flu illness. Less research has been done to determine whether vaccination might reduce severity of illness among people who are vaccinated but still get sick. There is some data available that supports this conclusion. This study adds to that data, but more research on this topic is warranted. CDC plans to continue to monitor and research the benefits of flu vaccination against severe disease outcomes in future studies to explore this important public health issue. This study is available online from the CID website at