According to a report published by national infectious disease agency State Serum Institute (SSI) on Monday, infection with one subvariant of the Omicron variant of Covid-19 does not give full immunity against reinfection, since a different subvariant can escape immunity.

Around 2 million people in Denmark have tested positive for Covid-19 – almost a third of the population – since Omicron emerged in the country in November 2021, according to SSI.

The BA.1 subtype of Omicron is the most prevalent but another variant, BA.2, is gaining ground in many places worldwide, the agency writes.

“This pattern is reflected very clearly in Denmark. The majority of those infected with Omicron in December were affected by BA.1. However, in January, BA.2 gradually became ever more prevalent. This variant currently accounts for most coronavirus infections in Denmark, whereas BA.1 is now seen much more rarely,” SSI writes.

Gene sequencing of positive PCR tests by SSI detected “67 cases in which the same individual had become infected twice at a 20-60-day interval and where both infections were due to Omicron subtypes”.

In 47 of the 67 cases, the first infection was with BA.1 and the second with BA.2.

The majority of the reinfected individuals were “young and unvaccinated” according to SSI.

“Most experienced mild symptoms during their infections. The difference between the severity during their first and second infection was negligible. None of the infected individuals had become seriously ill, and none required admission to hospital,” the agency writes.

The SSI study shows that infection with two different Omicron subtypes “is possible” but “seems to occur relatively rarely in Denmark”, the agency writes.

Reinfections “have mainly affected younger unvaccinated individuals”, it also notes.