A new study by researchers at Statens Serum Institute, Denmark's largest infectious disease institute, shows that a BA.2 subtype infection can follow the BA.1 subtype infection very quickly, but that this is a rare occurrence.

These two subtypes differ in 40 mutations.

While BA.2 currently accounts for more than 88 per cent of cases in Denmark, the number of newly infected people with this subtype has started to rise in the UK, South Africa and Norway.

Reinfections hit young, unvaccinated people hardest and caused a mild form of the disease, but none resulted in hospitalization or death, Danish scientists said.

The study, conducted between November and February, found 1.739 cases showing that the same person was first infected with subtype BA.1 and then, 20 to 60 days later, with subtype BA.2.

There were 1,8 million infected people in Denmark at that time.

In a smaller group of samples, the study found 47 cases of BA.2 reinfection shortly after BA.1 subtype infection.

The researchers also found less viral material in the second infection, indicating that the person had developed immunity after the first infection.