Study: Inactivated Vaccines Prevent Influenza for Children, Remaining Effective After COVID-19 Pandemic

The current inactivated vaccines for preventing influenza A and B in children remain effective after the relaxation of strict COVID-19 measures during the 2023-2024 season. Investigators state that health care providers should consider the use of 2 doses to increase the effectiveness against influenza B.

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After the COVID-19 pandemic, investigators reported intermittent suppression of the circulation of seasonal influenza virus. However, they also reported that the virus circulation has returned to normal levels, demonstrating the need for annual vaccination against influenza. In other studies, the effectiveness has been evaluated with test-negative case-controls due to varying levels of effectiveness each year. In the current study, investigators aimed to study the vaccine efficacy of inactive influenza vaccines for preventing infection in the 2023–2024 season.

Children aged 6 months to 15 years were enrolled if they had a fever of 38ºC or higher and underwent testing prior to hospitalization at one of the study hospitals in Japan. Investigators included data from November 2023 to March 2024, according to the study authors.

There were a total of 17 hospitals that included inpatient children and 2 hospitals for outpatient children, which included a total of 1832 children. Investigators included 1005 children in the influenza A cohort, with 153 testing positive and 852 testing negative. In the outpatient setting, investigators included 591 children, with 172 testing positive and 419 testing negative. For influenza B, there were a total of 918 children in the hospital setting, including 66 who were positive and 852 who were negative; and 579 in the outpatient setting, with 160 positive and 419 negative.

Among those in the hospital and influenza A cohort, the vaccine coverage was 22% for those with the virus and 40% for the controls. The adjusted vaccine efficacy for preventing influenza A hospitalization was approximately 51%, according to the study results. Further, investigators found no difference between 1 and 2 doses for hospitalized children aged 6 months to 12 years, which is what is recommended for the patient population. For outpatients, vaccine coverage was 23% and 37%, respectively, with efficacy of preventing influenza A in the outpatient setting at approximately 54%. There was also no difference for 1 and 2 doses.

For influenza B, vaccine coverage was approximately 21% in the influenza B group and 40% for the control group. The adjusted vaccine efficacy for preventing influenza B hospitalization was 60%, according to the results. The vaccine efficacy for 2 doses compared with no dose was significant, but 1 dose compared with no dose was not. For the outpatient setting, vaccine coverage was 21% and 37%, respectively, with vaccine efficacy for preventing influenza B being 56%. The 2-dose regimen was found to be superior for children with influenza B in the outpatient setting, according to the investigators.

The authors noted limitations of the study, which included the lack of data to differentiate between the 2 subtypes of influenza A: A(H1N1)pdm09 and A(H3N2). Additionally, the effects of any co-infections with other viruses or bacteria were not analyzed as well as the date of vaccine administration and the intervals between the first and second doses.

However, the results were still significant in showing the effectiveness of inactive influenza vaccinations among children in both inpatient and outpatient settings.

REFERENCE

Shinjoh M, Yaginuma M, Yamaguchi Y, et al. Effectiveness of inactivated influenza vaccine in children during the 2023/24 season: The first season after relaxation of intensive COVID-19 measures. Vaccine. 2024;42(23):126241. doi:10.1016/j.vaccine.2024.126241