Reinfections from Omicron subvariant BA.2 seem rare, study finds, making 'new wave' unlikely

Getting infected more than once by subvariants in the Omicron family does seem possible, but appears rare, scientists in Denmark found in a recent real-world study — offering reassurance that countries won't experience another sudden surge of infections.

The early research, which was published online as a preprint on Tuesday and has not yet been peer-reviewed, involved an analysis of recent SARS-CoV-2 infections by a team from the Statens Serum Institut (SSI), a top Danish public health institute.

Many of those were the highly-contagious BA.2 subvariant, which is now dominant in that country — and on the rise elsewhere — after squeezing out other Omicron subvariants featuring different mutations, including the original lineage, as well as BA.1, BA.1.1 and BA.3.

Out of nearly two million infections logged in Denmark between mid-November and mid-February, the researchers zeroed in on those who tested positive twice between 20 and 60 days apart, and whose infections had gone through previous genomic surveillance and were labelled as a specific subvariant.

Fewer than 1,800 people hit those criteria, and a subset of close to 1,000 samples were randomly selected for sequencing.

The team ended up finding 187 cases of reinfection, including 47 instances where BA.2 reinfections happened shortly after a BA.1 infection, "mostly in young unvaccinated individuals with mild disease not resulting in hospitalization or death," the team wrote. 

One of the researchers, Dr. Troels Lillebæk, chair of Denmark's SARS-CoV-2 Variant Assessment Committee, told CBC News this offers the first evidence of reinfections among members of the Omicron family, but it appears this is a "quite rare phenomenon."

"If it was a major problem that you could catch BA.2 after BA.1, you could imagine a new wave," he said.

"This does not really point in that direction."

After weeks of uncertainty, it's a welcome finding, following the upward spike in BA.2 cases in multiple countries that sent scientists scrambling to understand whether the heavily-mutated subvariant could prolong this year's Omicron wave — or even spark a new one.

In Denmark, BA.2 infections now make up roughly nine in 10 cases, with cases also rising in countries including Norway, South Africa and the U.K.

Here in Canada, the subvariant was barely a blip in federal data by early January, but the latest-available data by month's end suggest it makes up roughly one in 10 cases, with recent data still accumulating. 

Its rise comes as the overall Omicron wave is subsiding, and while much of the country is further loosening or outright dropping COVID-related restrictions, granting Canadians the opportunity to socialize indoors more freely than at many points during this more than two-year pandemic.

The virus will still be circulating, said Dr. Zain Chagla, an infectious diseases specialist and professor at McMaster University in Hamilton, Ont., but based on the emerging evidence about how BA.2 operates, it's unlikely Canada will experience another wave of infections tied to this subvariant.