New CDC Study Finds Strong Vaccine Effectiveness Against Severe Flu Outcomes

CDC update

December 5, 2023 — A new CDC-coauthored study published in Clinical Infectious Diseases found that flu vaccination last season substantially reduced the risk of serious flu outcomes. The study looked at how well flu vaccination protected against flu-related hospitalization, organ failure and death among adults. The study findings underscore the importance of flu vaccination this fall and winter respiratory virus season. CDC recommends everyone 6 months and older get an annual flu vaccine.

The 2022–2023 flu season in the U.S. was characterized by early flu activity driven largely by flu A(H3N2) viruses and flu A(H1N1)pdm09 viruses that were well-matched to the vaccine viruses. Vaccine effectiveness varied across outcomes, age groups, immunocompromised status, and virus type.

Flu vaccines reduced the risk of hospitalization by 37% overall, with the vaccine effectiveness estimates varying by age group and influenza virus A subtype as follows:

• People 18–64 years who were vaccinated were 47% less likely to be hospitalized with flu.
• People ≥ 65 years who were vaccinated were 28% less likely to be hospitalized with flu.
• Overall, vaccination reduced the risk of hospitalization caused by influenza A(H3N2) by 29% and against A(H1N1) by 47%.

Flu vaccines also provided significant protection against other severe-flu related outcomes. People who were vaccinated were:

• 41% less likely to experience hypoxemia treated with supplemental oxygen.
• 65% less likely to have respiratory, cardiovascular, or renal failure treated with acute. organ support.
• 66% less likely to have respiratory failure treated with invasive mechanical ventilation
• 69% less likely to be admitted to the ICU.

The study looked at data from the Investigating Respiratory Viruses in the Acutely Ill (IVY) Network, which is a multistate network of hospitals in the United States that enrolls adults hospitalized with acute respiratory illness (ARI) to evaluate the effectiveness of respiratory virus vaccines. Between October 1, 2022, and February 28, 2023, 24 hospitals from 19 states within the IVY Network prospectively enrolled 7,129 patients into this surveillance program. Among 714 flu-case patients, flu type was determined by RT-PCR laboratory testing for 431 patients, including 286 (67%) with flu A(H3N2) and 145 (33%) with flu A(H1N1).

For all adults, vaccination protected substantially against some of the most severe outcomes that can occur among very ill patients. Flu vaccination provided better protection for younger adults compared to people 65 years and older and better protection against influenza A(H1N1) virus-related hospitalizations than against influenza A(H3N2)-related hospitalizations. More information on flu vaccine effectiveness is available.