Credit WHO
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Although COVID cases in the United States have plummeted in recent weeks following the first Omicron wave (BA.1), we are closely watching the progress of another Omicron subvariant - BA.2 - which is thought may overtake BA.1 in the weeks and months to come.
This is a question that Denmark - because of their relatively small population (5.8 million), a well-monitored universal healthcare system, a high degree of genomic testing, and their early transition from BA.1 to BA.2 (see Denmark SSI: Omicron Variant BA.2 Rising - Expected To Soon Comprise 100% Of Cases) is well positioned to answer.The $64 question now is whether exposure to BA.1 conveys immunity against BA.2. And if so, how long does that immunity last?
At least partially.
The caveat being that due to its recent arrival, reinfection data is only available over a period of roughly 60 days. For now, reinfection with a second Omicron variant - while possible - is described as rare, and usually mild.
First this summary from Denmark's SSI, followed by a link and the abstract from the preprint.
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.Updated 22 February 2022
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.
Occurrence and significance of Omicron BA.1 infection followed by BA.2 reinfectionMarc Stegger, Sofie Marie Edslev, Raphael Niklaus Sieber, Anna Cacilia Ingham, Kim Lee Ng, Man-Hung Eric Tang, Soren Alexandersen, Jannik Fonager, Rebecca Legarth, Magdalena Utko, Bartlomiej Wilkowski, Vithiagaran Gunalan, Marc Bennedbaek, Jonas Byberg-Grauholm, Camilla Holten Moeller, Lasse Engbo Christiansen, Christina Wiid Svarrer, Kirsten Ellegaard, Sharmin Baig, V Thor Bech Johannesen, Laura Espenhain, Robert Skov, Arieh Sierra Cohen, Nicolai Balle Larsen, Karina Meden Soerensen, Emeily Dibba White, Troels Lillebaek, Henrik Ullum, Tyra Grove Krause, Anders Fomsgaard, Steen Ethelberg, Morten Rasmussendoi: https://doi.org/10.1101/2022.02.19.22271112
Abstract
The newly found Omicron SARS-CoV-2 variant of concern has rapidly spread worldwide. Omicron carries numerous mutations in key regions and is associated with increased transmissibility and immune escape. The variant has recently been divided into four subvariants with substantial genomic differences, in particular between Omicron BA.1 and BA.2.
With the surge of Omicron subvariants BA.1 and BA.2, a large number of reinfections from earlier cases has been observed, raising the question of whether BA.2 specifically can escape the natural immunity acquired shortly after a BA.1 infection. To investigate this, we selected a subset of samples from more than 1,8 million cases of infections in the period from November 22, 2021, until February 11, 2022.
Here, individuals with two positive samples, more than 20 and less than 60 days apart, were selected. From a total of 187 reinfection cases, we identified 47 instances of BA.2 reinfections shortly after a BA.1 infection, mostly in young unvaccinated individuals with mild disease not resulting in hospitalization or death. In conclusion, we provide evidence that Omicron BA.2 reinfections do occur shortly after BA.1 infections but are rare.
(SNIP)
Discussion
The present study confirms the occurrence of Omicron BA.2 reinfection shortly after a previous BA.1 infection. This is to our knowledge the first study that reports aggregated Omicron BA.2 reinfection cases and document a time interval as short as 20 days after initial infection. Among the 1,848,466 million infected individuals in the study period, we identified 1,739 cases that fulfilled the criteria of two positive samples with more than 20 and less than 60 days apart.
(SNIP)
In conclusion, we provide evidence that Omicron BA.2 reinfections are rare but can occur relatively shortly after a BA.1 infection, causing mostly mild disease in unvaccinated young individuals. The reinfections were identified among SARS-CoV-2 cases testing positive for more than one time in a country with a high PCR test capacity and extensive community transmission.
