Flu Vaccine Provided Substantial Protection This Season

February 22, 2023—This year’s flu vaccines reduced the risk of influenza A-related hospitalization among children by nearly three quarters and among adults by nearly half according to CDC. Vaccination also provided significant protection against flu-related illness and flu-related emergency department visits, with people who were vaccinated about half as likely to have those outcomes as people who had not been vaccinated. Benefit from vaccination was observed across all age groups. These interim vaccine effectiveness (VE) estimates were presented today during an Advisory Committee on Immunization Practices (ACIP) meeting in Atlanta. This is the first time that estimates of flu vaccine effectiveness against more severe outcomes have been available this early and from three different VE networks, which is a result of the early flu activity seen this season. These data underscore that flu vaccination can offer substantial benefit against flu and its potentially serious complications. While flu activity has returned to low levels at this time, CDC continues to recommend annual vaccination as long as flu is spreading in the community.

According to data from the New Vaccine Surveillance Network (NVSN), VE against the predominant H3N2 viruses was 45% among children, which is higher than seen previously for this virus. To compare, during previous seasons, VE against H3N2 has been around 30%. The higher VE this season is likely because flu vaccination elicited good immunity against the variety of viruses circulating. Flu vaccine effectiveness against circulating influenza A(H1N1) viruses was 56%.

Consistent estimates were calculated though different networks:

The New Vaccine Surveillance Network (NVSN)

For 2022-2023 interim flu VE estimates, NVSN includes seven study sites that evaluate how well flu vaccines protect against flu-related hospitalizations and emergency department visits among children.

From September 13, 2022, to January 25, 2023, children who were vaccinated against flu were 68% less likely to be hospitalized because of flu illness or related complications, and 42% less likely to visit an emergency department because of flu-related illness. The overall VE against laboratory-confirmed influenza A in hospital and emergency department settings was 49%. This is encouraging news about vaccine effectiveness during a season that had a lot of early, severe illness among children.

 Investigating Respiratory Viruses in the Acutely Ill (IVY) Network

For 2022-2023 interim flu VE estimates, IVY includes 22 medical centers in 19 U.S. states that evaluate how well flu vaccines protect against flu-related hospitalization among adults.

 Similar to data in children, from October 1, 2022, to January 31, 2023, flu vaccination significantly reduced flu-related hospitalization among adults. Overall, adults who were vaccinated against flu were 43% less likely to be hospitalized because of flu illness or related complications. Adults 65 years and older were 35% less likely to have a flu-related hospitalization, and adults 18-64 years were 51% less likely to have a flu-related hospitalization.

Importantly, IVY can also evaluate flu VE among adults who are immunocompromised. Flu vaccination provided important protection among adults, including among older adults and adults with a documented immunocompromising condition. Adults with a documented immunocompromising condition were 44% less likely to be hospitalized with flu-related complications. Older adults and adults with a documented immunocompromising condition are at higher risk of serious flu complications and less likely to have an immune response to vaccination.

VISION VE Network

For 2022-2023 interim flu VE estimates, VISION includes 367 health facilities that evaluate how well flu vaccines protect people against flu in urgent care, emergency department, and hospital settings.

 Results from VISION VE Network were consistent with the other VE networks. From October 15, 2022, to January 24, 2023, flu vaccination significantly reduced emergency department and urgent care visits as well as hospitalizations among adults. Overall, adults who were vaccinated against flu were 44% less likely to visit an emergency department or urgent care center and 39% less likely to be hospitalized because of flu illness or related complications. These estimates are higher than VE estimates from the 2021-2022 season against emergency department or urgent care visits (25%) and hospitalization (25%) when H3N2 viruses were predominantly circulating at the same VISION Network sites.

Data from VISION VE Network also show protection among older adults and adults who are immunocompromised. Adults 65 years and older were 39% less likely to visit an emergency department or urgent care and 42% less likely to be hospitalized because of flu illness or related complications. Adults with a documented immunocompromising condition were 30% less likely to have a flu-related emergency department or urgent care visit and 31% less likely to be hospitalized with flu-related complications.

Data on VE by vaccine type are not available. However, according to data from the VISION VE Network, more than 90% of adults 65 years and older with a known vaccine type received high dose or adjuvanted flu vaccine. This is encouraging news after ACIP voted in June 2022 to preferentially recommend the use of specific flu vaccines for adults 65 years and older, including higher dose and adjuvanted flu vaccines.

Vaccine Effectiveness Against Medically Attended Illness

Interim VE estimates against medically attended illness were provided by Marshfield Clinic Health System (WI), in collaboration with CDC. According to data presented to ACIP, the study found that children and adults younger than 65 years who received a flu vaccine were half as likely to visit a health care provider because of flu illness, and flu vaccination reduced risk of getting sick with flu by more than two thirds among children. Data from the Marshfield Clinic are being published in an MMWR on February 24, 2023. Estimates from CDC’s outpatient VE network are not yet available this season.