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Since large parts of Danish society were shut down on 12 March 2020 due to Covid-19, the incidence of influenza has been at a very low level until the beginning of 2022. Now the number of people infected with influenza A is doubling from week to week.

3D print of influenza virus. The virus surface (yellow) is covered with proteins called hemagglutinin (blue) and neuraminidase (red). NIH

In Denmark, the incidence of influenza tends to increase at the end of December. But this season, the flu has started very late, after being almost completely absent for the last two years during covid-19.

Currently, however, there is a steep increase in the number of flu cases in most of the country. In recent weeks, the number of infected has thus doubled from week to week and in week 10 reached 1,179 infected.

A total of 2,533 cases of influenza have been detected this season. The vast majority of cases – a total of 2,459 – are caused by the type of influenza virus called A / H3N2.

It is a well-known flu type, which is also circulating in other countries. H3N2 is included in the seasonal vaccine, but it is known to change and this may mean that the effect of the vaccine is reduced.

This season, changes have been seen in the H3N2 viruses in circulation, which may mean that the vaccine has a reduced effect.

Influenza has been detected in all age groups. At present, however, a predominance of children and the younger part of the population aged 15-44 as well as +85 year olds are affected by influenza.

It is now more than five months since seasonal flu vaccines became available. The late onset of influenza prevalence is therefore a challenge, as it is well known that the effect of vaccines decreases over time.

In the age group 2-6 years, the effect of the influenza vaccine, which is targeted at children, is calculated at 71% against influenza A. In the age group 7-44 years, the effect is calculated at 43%, while in the age group 45 years and over, no effect against the circulating influenza A type.

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“Despite the fact that the vaccine effectiveness against influenza infection is not measurable in the age group 45 and above, the vaccine will probably still have some effect against serious illness if you become infected with influenza,” says section leader Ramon Trebbien from Stans Serum Institut ( SSI).

It is not so surprising that the best effect of the flu vaccine is seen among the children, as children generally have a good ability to form antibodies after being vaccinated.

In addition, vaccinated children aged 2-6 years have received two doses of the live attenuated childhood vaccine, which was introduced in the Danish vaccination program for the influenza season 2021/22.

The other age groups have received a single dose of the inactivated influenza vaccine known from previous flu seasons.