The influenza vaccine for the 2022-2023 season was mildly effective in reducing influenza-A-associated emergency and urgent care encounters and hospitalizations in the US, according to study findings published in the Journal of Infectious Diseases.
Although influenza activity in the US during the first year of the COVID-19 pandemic (ie, the 2020-2021 influenza season) was historically low, increased and prolonged activity was noted in the subsequent season. Therefore, investigators sought to assess VE among adults with acute respiratory illness (ARI) against influenza-A-associated emergency department and urgent care (ED/UC) visits and hospitalizations during the 2022-2023 influenza season.
The investigators used data from the VISION Network to conduct a test-negative case-control study to estimate influenza VE among adults at least 18 years of age with an ARI-associated ED or UC encounter or hospitalization. Eligible patients underwent reverse transcription-polymerase chain reaction (RT-PCR) testing for influenza during the 2022-2023 flu season (mid-October through March). All testing for influenza was clinician-initiated in the VISION Network health systems (Kaiser Permanente Northern California [KPNC], Intermountain [IM] Healthcare in Utah, and HealthPartners [HP] in Minnesota and Wisconsin).
VE was estimated by comparing odds of current-season influenza vaccination among those who tested positive for influenza A (case-patients) with those who tested negative for influenza A and B (control group patients).
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During the 2022-2023 United States influenza season, we observed moderate VE against influenza-A-associated ED/UC encounters and hospitalizations, ranging from 35%–44% across care settings.
Eligible patients received testing up to 72 hours after or within 10 days before an ED/UC encounter or hospital admission, and received SARS-CoV-2 molecular testing (to exclude control group participants with COVID-19). Median time between most recent influenza test and the ED/UC encounter was 0 days. Those who tested positive for influenza were more likely to be younger than 65 years of age, and less likely to have a respiratory or nonrespiratory chronic condition.
The investigators found 85,389 ED/UC ARI encounters; of those, 17.0% involved individuals who were influenza-A positive and 37.8% involved patients who were vaccinated (24.1% of case patients; 40.6% of control patients).
A total of 19,751 hospitalizations were identified; of those, 9.5% involved individuals who were influenza-A positive and 52.8% involved individuals who were vaccinated (39.1% of case patients; 54.3% of control patients).
The investigators noted that overall VE was 44.0% against influenza-A-associated ED/UC encounters (95% CI, 40.0-47.0%); VE among adults 18 to 64 years of age was 45.0% and VE among those at least 65 years of age was 41.0%. Adults with vs without likely immunocompromising conditions experienced similar VE (44.0% vs 38.0%, respectively).
Overall VE against influenza-A-associated hospitalizations was 35.0% (95% CI, 27.0%-43.0%); VE among adults 18 to 64 years of age was 23.0% and VE among those at least 65 years of age was 41.0%. Adults with and without likely immunocompromising conditions experienced similar VE. With respect to comorbidities, VE was 39.0% for those with less than 3 underlying comorbidities and 36.0% for those with at least 3 comorbidities.
Study limitations include inability to compare VE by age group. Additionally, the 3 included health care systems are not representative of all encounters among adults in the US.
“During the 2022-2023 United States influenza season, we observed moderate VE against influenza-A-associated ED/UC encounters and hospitalizations, ranging from 35%–44% across care settings,” the study authors concluded. “Similar VE was also observed among groups at higher risk for severe complications from influenza, including older adults, those with immunocompromising conditions, and those with multiple underlying chronic medical conditions,” the researchers added.
Disclosure: Some study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures.
