The fourth dose will offer markedly better protection than if a person has only received a “third” dose or single booster jab, SSI said in a press statement.

SSI researchers, working with colleagues from the other Nordic countries, have analysed the effect of the additional booster jab with the vaccine, which has been updated in line with newer dominant subvariants of the coronavirus.

When the Danish population was first vaccinated against Covid-19, the vaccines were designed to offer protection against the original form of the virus, SSI writes.

But newer variants have made the original vaccines less effective. The updated vaccines are designed to have the best possible effect against both the original variant as well as the Omicron variant.

There are currently two versions of the updated vaccine. One is adapted towards the BA.1 Omicron subvariant, with another adapted to the BA.4-5 subvariant.

In the Nordic countries, the updated vaccines were offered during autumn 2022 to all persons over the age of 50 in Denmark and Sweden, over 60 in Finland and over 65 in Norway.

A fourth dose with the BA.1-updated version reduced the risk of hospitalisation with Covid-19 by 74 percent and the risk of death by 80 percent compared to the third dose, SSI found.

The BA.4-5 updated version reduced the risk of hospitalisation by 81 percent and the risk of death by 78 percent.

The latter of the two updated versions (BA.4-5) was found to reduce the risk of hospitalisation with Covid-19 by 32 percent compared to the BA-1 version.

“This is maybe not so surprising because BA.4-5 subvariants were dominant in autumn 2022,” SSI head of department Anders Hviid said in the statement.

“But I think we are among the first [countries] to be able to measure this based on the large quantities of data we have available from working across four countries,” he said.

The research was supported by the EU’s European Medicines Agency (EMA).

SSI notes that the frequency of hospitalisation and particularly death due to Covid-19 was very low after both the third and fourth doses of the vaccine.

The academic paper resulting from the study can be read in English here.

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