Childhood Influenza in Norway: Burden of Disease and Vaccine Effectiveness

Influenza or flu is a highly contagious infection caused by a virus that gives fever, respiratory and other symptoms. Flu affects all age groups, though children tend to get it more often than adults. Some children can develop severe influenza which requires hospital treatment. There are effective vaccines that can be used to prevent influenza in children. These vaccines should be given each year before annual influenza epidemics begin. In Norway, influenza vaccination is currently recommended only for children with chronic diseases or other conditions that can increase their risk of developing severe flu. However, healthy children can also develope severe influenza which can be prevented by vaccination. In order to review the national vaccine recommendations, health authorities need more information about the extent of severe influenza in Norwegian children. Currently, little is known about this. Therefore, this project aimed to understand how often children under 18 years of age in Norway develop severe influenza that leads to hospital visits and hospitalizations. We also examined how many children were vaccinated against influenza in order to help choosing effective strategies to prevent this infection. Since winter 2015 we screened all children under 18 years of age which came to hospital with fever and/or respiratory symptoms. Five large Norwegian hospitals participated in the study. We collected a swab sample from the nose of each child enrolled in the study and tested it for the presence of influenza virus in the laboratory. This screening was implemented from 2015 to 2018 during three consecutive influenza seasons from October to May each season. In addition to a swab sample, we collected information about the child via hospital records and relevant registries. More information about the study is available at the website of the Norwegian Institute of Public Health: https://www.fhi.no/en/studies/luftveisinfeksjoner-hos-barn-og-unge-i-norge/about-respiratory-infections-study/ A total of 2 619 children below 18 years of age were enrolled in our study. During the first study season of 2015-2016, 13.5% of screened children had influenza detected in the nose sample, 6.1% in the season 2016-2017 and 10.9% in the season of 2017-2018. Most children in our study were young below 5 years of age, but influenza was detected across all age with a higher proportion among those aged 6-17 years. We did not find a higher prevalence of influenza in children with chronic diseases or other risk conditions compared with previously healthy children. Very few children (1.3%) among those screened in our study were vaccinated against influenza before they became ill. Our study suggests that severe influenza affects many children in Norway who are admitted for hospital treatment. Vaccination should be considered to prevent pediatric influenza infection.

Prosjektresultater indikerer viser at influensa rammer mange barn i Norge som behandles på sykehus. Prosjektfunn belyser omfanget av alvorlig influensa hos norske barn, kartlegger spesifikke pasientgrupper som er mer utsatte for å utvikle alvorlig influensa samt estimerer graden av bruk av årlig influensavaksinasjon hos norske barn. Slik kunnskap er nødvendig for å hjelpe helsemyndigheter for å vurdere behov for endring av nasjonale vaksinasjonsanbefalinger. Prosjektet etablerte et unik sykehusbasert forskningsnettverk som kan brukes for å overvåke forekomst av influensa og andre vaksineforebyggbare infeksjoner. Aktiv sykehusovervåkning for vaksineforebyggbare sykdommer er viktig for å estimere virkninger av vaksinasjon på befolkningsnivå og oppdage utbrudd og epidemier i befolkningen som kan forebygges med vaksinasjon. Prosjektet er relevant for den norske folkehelsepolitikken og har forbedret evnen til å evaluere effekten av vaksinasjonsprogrammer i Norge og Europa.

Influenza is an important cause of acute respiratory illness in children. Paediatric influenza vaccination policies, however, vary within Europe with the majority of countries recommending vaccination for specific risk groups only, usually persons with underlying medical conditions. A few European countries recommend influenza vaccination of healthy children and have introduced universal influenza vaccination in the national immunization programs. There are several reasons behind the existing controversy within paediatric influenza vaccination policy in Europe, including the unrecognized influenza disease burden in children, the effectiveness of influenza vaccines, and the cost-effectiveness of universal influenza vaccination. This project aims to measure influenza-associated disease and economic burden and assess influenza vaccine effectiveness among children 0-17 years of age in Norway. Further, we aim to evaluate the cost-effectiveness of different influenza vaccination strategies in Norway such as vaccination of specific groups or universal vaccination of healthy children. This project is a collaborative initiative between the Norwegian Institute of Public Health, the five Norwegian hospitals, the University of Oslo, and two European networks represented by the coordinating hub EpiConcept, France: the InNHOVE (Influenza Network Hospital-based Vaccine Effectiveness) and the I-MOVE (Influenza Monitoring Vaccine Effectiveness in Europe). The InNHOVE is a hospital network including hospitals in Spain, Italy, and France that previously conducted a pilot study to measure influenza vaccine effectiveness against hospitalised laboratory confirmed influenza in adults. The I-MOVE network monitors IVE against medically attended influenza-like illness (ILI) and currently includes 19 partners from 12 European countries. The project has a strong potential to contribute to a scientific revision of paediatric influenza vaccination policy in Norway and Europe.