Seasonal flu vaccination - recommendations 2023/2024

Due to the high variability of influenza viruses, the antigen composition of the vaccines must be reviewed annually and adjusted if necessary. The composition of the three quadrivalent split vaccines available in Switzerland is based on the recommendation for the northern hemisphere published by the World Health Organization (WHO). In addition to two products with a standard vaccine dose, a high-dose vaccine is also available for ≥65-year-olds.

According to the WHO (World Health Organization ), an estimated 1 in 5 unvaccinated children and 1 in 10 unvaccinated adults are infected with seasonal influenza every year [1]. In Switzerland, a flu epidemic usually occurs in January and February, the course of which can vary greatly. A look at the data from the previous flu season shows that in the winter of 2022/2023, a first, averagely strong flu wave caused by influenza A viruses occurred in Germany in December 2022 and January 2023. This was followed by a second, smaller wave caused by influenza B in February and March 2023. According to the Federal Office of Public Health (FOPH), it is not possible to predict the start or severity of the 2023/2024 flu epidemic in Switzerland [1]. In order to keep track of the epidemiological development of influenza, Covid-19 infections and other communicable diseases in Switzerland, the FOPH is launching a new information portal in the last quarter of 2023, which will replace the previous influenza situation reports [1]. The previous reports remain accessible on the website www.grippe.admin.ch.

Poultry farmers have recently also been included in a risk group

The current Swiss influenza vaccination recommendations largely correspond to those of previous years (Table 1) . Vaccination is recommended for all people with an increased risk of complications and their regular contacts (including all healthcare professionals). Since fall 2023, poultry farmers and people who have regular contact with wild birds have also been considered a risk group. As protection against influenza usually only lasts for just under six months, people who were vaccinated the previous year also need to be vaccinated again in the fall/winter. The clinical course of an influenza infection can vary greatly and can range from an asymptomatic infection to a mild course with few symptoms to a severe course with fatal consequences [1].

The following flu-related complications are the most common:

Sinusitis
Middle ear infections
Bronchitis
Pneumonia
Pseudocroup

However, pleuritis, myositis, myocarditis and pericarditis with subsequent dilated cardiomyopathy, myocardial infarction and toxic shock can also occur and be life-threatening. Meningitis, encephalitis, myelitis or Guillain-Barré syndrome are also possible [2].

Vaccines protect against these viruses

At the end of February 2023, the WHO published the definitive recommendations for the composition of the 2023/2024 influenza vaccines for the northern hemisphere [3]. It replaced the vaccine strain of influenza A subtype H1N1 of the 2022/2023 season with a different strain in order to optimally cover the circulating viruses [4]. According to these WHO recommendations, the vaccines for 2023/24 contain the surface antigens of two influenza A strains (H1N1, H3N2) and two influenza B strains (Yamagata, Victoria) [3].

The products** available in Switzerland for the 2023/2024 season are all influenza split vaccines with inactivated virus fragments, i.e. they do not contain infectious viruses, but only the antigens of the influenza virus strains [2].

** In Switzerland, a fourth influenza vaccine, Fluenz ^Tetra®, has been approved but is not available; it is a vaccine for children and adolescents aged 2 to 17 (before their 18th birthday) [1,5].

As before, two of the products contain a standard dosage of antigens:

Fluarix ^Tetra® is approved for adults and children aged 36 months and over [5]. The costs are covered by basic insurance for all persons aged 65 and over, as well as for all adults and children with at least one risk factor according to the flu vaccination recommendation.
Vaxigrip ^Tetra® is approved for adults and children from 6 months of age [5]. Costs are covered by basic insurance for all persons aged 65 and over, as well as for all adults and children with at least one risk factor according to the flu vaccination recommendation.

A fourth vaccine for older people aged 65 and over contains a higher amount of antigen (high dose):

^Efluelda® is approved for adults aged 65 and over [5]. Costs are covered for all persons aged 75 and over and for persons aged 65 and over with at least one other risk factor according to the flu vaccination recommendation.

Many studies indicate that the flu vaccination reduces the severity of the course of the disease, the risk of complications, hospitalizations and flu-related mortality. Co-infections with influenza and SARS-CoV-2 viruses are rare (<5%), but tend to be more severe than influenza or SARS-CoV-2 infections alone.

High-dose vaccine with increased efficacy for ≥65-year-olds
With ^Efluelda®, a high-dose vaccine that can increase efficacy for older people has also been available in Switzerland since 2022. A meta-analysis with 34 million ≥65-year-old participants over an application period of more than 10 influenza seasons shows a higher efficacy of high-dose influenza vaccines (60 μg instead of the standard dose of 15 μg antigen per vaccine strain) against influenza complications in older people. As a result, compared to a standard dose, high-dose vaccination reduced influenza-related hospitalizations by 11.7% (95% CI: 7.0-16.1%), pneumonia-related hospitalizations by 27.3% (95% CI: 15.3-37.6%), combined pneumonia/influenza-related hospitalizations by 13.4% (95% CI: 7.3-19.2%) and hospitalizations due to cardiorespiratory events by 17.9% (95% CI: 15.0-20.8%).

to [1,5,7,8]

Flu vaccination is considered very safe

The effectiveness of the vaccines is reported to be 20-80%, depending on the age and immunocompetence of the vaccinated person and the viral coverage of the flu vaccine [1,6]. In older people and people with certain chronic diseases or an immune deficiency, a high-dose vaccine can increase efficacy (box) [1]. The flu vaccine is considered to be very safe, and several billion doses have been administered worldwide since 1945. The most common side effect is a slight local reaction at the injection site. It occurs in 10-40% of vaccinated individuals and resolves after a few hours or a maximum of two days. General mild symptoms such as fever, nausea, muscle aches, joint pain, headache, and other flu symptoms occur in 5-10% of vaccinated individuals. Severe allergic reactions such as angioedema, asthma or anaphylaxis are very rare (less than 1 in 10,000 people vaccinated). They can generally be explained by hypersensitivity to the proteins of the chicken egg. Adverse neurological phenomena, such as Guillain-Barré syndrome, are also very rare.

Literature:

BAG-Bulletin 41, 9.10.2023, www.infovac.ch, (last accessed 30.10.2023)
INFOVAC: «Grippe (influenza)», Stand der Daten, 26.10.2023, www.infovac.ch, (last accessed 30.10.2023)
WHO: Recommended composition of influenza virus vaccines for use in the 2023-2024 northern hemisphere influenza season, www.who.int, (last accessed 30.10.2023)
BAG-Bulletin 38, 18.09.2023.
Swissmedic, Medicinal product information, www.swissmedicinfo.ch/accept.aspx,(last accessed 30.10.2023)
WHO/OMS – World Health Organization/Organisation mondiale de la santé. Vaccines against influenza: WHO position paper – May 2022. Weekly epidemiological record 2022,19(97): 185–208.
Bricout H, et al.: The relative effectiveness of a high-dose quadrivalent influenza vaccine vs standard-dose quadrivalent influenza vaccines in older adults in France: a retrospective cohort study during the 2021–22 influenza season, June 16, 2023, www.medrxiv.org, (last accessed 30.10.2023).
Lee JKH, et al.: Vaccine 2021; 39 Suppl 1: A24–A35.