Intranasal quadrivalent live attenuated flu vaccine protects toddlers from hospital contacts

A quadrivalent influenza (flu) vaccine was particularly designed to protect against four different flu viruses, including two influenza A viruses and two influenza B viruses. Several years flu vaccines were designed to protect against three different flu viruses: an influenza A(H1N1) virus, an influenza A(H3N2) virus and one influenza B virus. Adding a B virus from the second lineage was done to give broader protection against circulating flu viruses.

A new study aimed to assess the real-world effectiveness of quadrivalent live attenuated influenza vaccines (LAIV-4) against influenza-related hospital contacts and admission and morbidity as there is scarce evidence on the real-world effectiveness of LAIV-4 in younger children.

The study published in The Lancet Child and Adolescent Health reports LAIV-4 For children aged 2-6 years, receipt of the quadrivalent live attenuated influenza vaccine was associated with a significantly lower rate of flu-related hospital contacts than being unvaccinated, but vaccination showed no associated with reduction in respiratory tract infection asthma, wheezing and antibiotic prescriptions.

Researchers searched nationwide Danish health-care registries, and designed a cohort study that emulates a target trial, comparing LAIV-4 to no vaccination in children aged 2–6 years. Eligible children vaccinated from Oct 1, 2021, to Jan 15, 2022, were matched to unvaccinated controls in a 1:1 ratio according to demographic characteristics and risk groups for influenza, and followed-up until May 31, 2022. Primary study outcomes any hospital contact for influenza and influenza-related hospital admissions more than 12 h in duration, while hospital admission for respiratory tract infections, or for wheezing or asthma, and antibiotic prescriptions were evaluated as secondary outcomes. They estimated incidence rate ratios (IRRs) and 95% CIs using Poisson regression for each outcome. Vaccine effectiveness was calculated as 1 – IRR.

The key findings of the study are

• A total of 308 520 Danish children aged 2–6 years, out of which 95 434 vaccinated children were matched with 95 434 unvaccinated children who acted as controls.

• Receipt of LAIV-4 compared with no vaccination was associated with a reduced IRR of 0•36 (95% CI 0•27 to 0•46) and estimated vaccine effectiveness of 64•3% (53•6 to 72•6) against influenza-related hospital contacts (76 vs 210 events).

• The corresponding IRR and vaccine effectiveness against influenza-related hospital admissions were 0•63 (0•38 to 1•05) and 36•9% (–5•2 to 62•1; 24 vs 38 events), respectively.

• LAIV-4 was not associated with reductions in admission rates for respiratory tract infections (IRR 1•14, 95% CI 0•94 to 1•38), wheezing or asthma (1•04, 0•83 to 1•31), or antibiotic prescriptions for respiratory tract infections (0•97, 0•93 to 1•00).

• Vaccine effectiveness assessed across risk groups for influenza showed similar effectiveness in children with and without coexisting risk factors for severe influenza.

Researchers concluded that “LAIV-4 offered moderate protection in younger children against influenza-related hospital contacts during a season dominated by influenza A(H3N2); however vaccination was not associated with reductions in secondary outcomes. This real-world study thereby supports trial evidence of moderate vaccine effectiveness of LAIV-4 against influenza-related outcomes when implementing broad vaccination schedules in younger children.”

Reference: Helene Kildegaard, MD, Lars Christian Lund, PhD, Anton Pottegård, DMSc, Lone Graff Stensballe, DMSc; Effectiveness of the quadrivalent live attenuated influenza vaccine against influenza-related hospitalisations and morbidity among children aged 2 to 6 years in Denmark: a nationwide cohort study emulating a target trial; October 25, 2023DOI:https://doi.org/10.1016/S2352-4642(23)00225-0.