Reinfection with different subtypes of Omicron is possible
The Omicron variant can be divided into various subtypes. Denmark has mainly been affected by subtypes BA.1 and BA.2. A novel study from Statens Serum Institut (SSI) aims to establish if reinfection with BA.2 can occur shortly after initial BA.1 infection. The answer was yes – but generally, reinfection is estimated to occur rarely.
Since the Omicron variant reached Denmark in late November 2021, a considerable share of the Danish population has become infected – approx. 2 million Danes have tested positive by PCR. Several subtypes of the Omicron variant exist that vary significantly, raising the question of whether it is possible to become infected by several of these subtypes successively.
The BA.1 subtype of Omicron is more prevalent than the other subtypes, but another variant, BA.2, is gaining ground in many places worldwide.
BA.2 has the upper hand in Denmark
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. Therefore, it was essential to establish if it is possible to become infected by both subtypes, one after the other, and if so, what was the disease severity?
67 people have had Omicron twice
Our knowledge about which Omicron subvariant infects Danish people is mainly derived from sequencing of virus from Danes who test positive by PCR. At SSI, researchers have studied how many individuals have had two positive tests and used genome sequencing to investigate the virus variants with which they had become infected.
Researchers found 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.
Mild disease courses
In 47 of the cases, the affected individual first became infected by BA.1 and then by BA.2. The majority of the infected were young and unvaccinated, and 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.
In summary, the study shows that infection with two different Omicron subtypes is possible. This seems to occur relatively rarely in Denmark, and reinfections have mainly affected younger unvaccinated individuals.
The study has been submitted for publication and is available as a preprint on MedRxiv.