The top bar chart draws on weekly data which Public Health Canada updates on Friday evenings. Unfortunately, the data collection time renders it 2-to-3 weeks out of date, which is forever in a rapidly-evolving pandemic. To provide a more useful, up-to-date context, we now accompany the Canadian chart with the US equivalent, also published on Fridays but the latter provides a nearly immediate picture of the week just passed whereas its Canadian counterpart is two weeks out of date. With borders are fully open and both Canada and the US having essentially given up on preventative measures, it is safe to assume that the US picture provides a reasonable representation of what is currently happening in Canada or will emerge within the ensuing two-week period.

The previously-dominant Delta family of variants has been entirely replaced by Omicron, numerous strains of which have for the past year been responsible for essentially all new cases in Canada and across the globe.  The above Canada and US bar charts are colour-coded by Omicron lineage, with the different XBB subvariants depicted in shades of red, BQ subvariants in shades of blue, CH subvariants in yellow and others in black.

The lineage charts below were compiled to put the literally hundreds of different COVID strains which have developed since the original Hunan strain began spreading at the beginning of this decade into a more readily understandable context. Most rapidly disappeared due to being outcompeted (i.e., less transmissible) than others whose combination of mutations enabled them to infect more people. Exponential growth in the spread of the currently-most-transmissible strains is, unfortunately, a fundamental characteristic of pandemics.  Because XBB alone now includes more than a hundred different stains, it has of neccesity been moved to a separate chart. In the same was that Omicron became synonymous with COVID in 2022, XBB is assuming that role for 2023.

The Alpha variant was 60% more transmissible than original Wuhan strain. Delta likewise had a 60% advantage over Alpha. BA.1 (the first Omicron subvariant) was a whopping 3.2 times more infectious than Delta; BA.5 was 50% better than BA.1. The currently-dominant XBB.1.5 is 40% more transmissible than BA.5, but is already being outcompeted by XBB.1.91. Others were briefly detected but, lacking sufficient competitive advantage, failed to spread widely and essentially disappeared. 

XBB evolved unusually rapidly from a different Omicron lineage than BA.5. It is a recombination of two BA.2 strains, BA.2.10.1 (a.k.a BJ.1) and BM.1.1.1, which had accumulated sufficient mutations to merit separate nomenclature. Recombination is in some ways analous to sexual reproduction in animals in that genes from two different parents are reshuffled to create genetically unique offspring. It is a more random process for viruses, occuring in the relatively rare instances when a cell is simultaneously infected by two or more viruses. The cellular machinery coopted by the viruses then assembles new virus particles somewhat randomly from the circulating viral genes. This process spawned XBB, which then mutated first into XBB, then XBB.1. Very shortly thereafter, another mutation resulted in XBB.1.5. which is the only variant known to have developed in North America (first detected in New York State).

Each of these subvariants succeeded because its unique combination of mutations enabled it to infect more people more quickly than the then-dominant strain. That advantage is based on a combination of three factors: inherent ability to penetrate human cells by binding to the human ACE2  cell membrane receptor; average time after infection before their victims can spread the virus; and ability to evade existing immunity (whether derived from vaccination, prior infection or both). For example, Omicron’s preference for infecting our upper respiratory tracts rendered its victims infectious in two days or less, compared to Delta’s slower but more deadly preference for the deep lungs. Most new infections were therefore Omicron. Because such advantages enable a subvariant to rapidly achieve dominance, the millions of people around the world carrying them at any point in time provide the ideal conditions for breeding new strains, some of which will convey sufficient competitive advantage to rise to dominance. Regardless of what politicians say and what people want, we will remain on this roller coaster until and unless both of the following happen: science develops a very different vaccine that fully protects against all coronaviruses and the vast majority of all humans around the planet are given access to and choose to take that new vaccine. Sadly, in today’s dumbed-down world, the former is far more likely than the latter.

The normal progression thus far in the pandemic has been for one variant to rise to dominance in the form of a new wave in which it comprises 90% or more of all new cases. In the absence of widespread social distancing and other public health measures, it quickly infects the majority of vulnerable individuals, after which point its wave begins to recede due to the remaining population being immune due to some combination of vaccination and prior infection. Whichever the source of immunity, that wanes over time but, by then, natural selections favours a newer variant which can evade the remaining immunity of enough people to enable it to serve as the driving force for yet another wave.  Over time, individuals’ and therefore their politicians’ patience with preventative measures likewise wanes, further fuelling this destructive cycle.

The combination of BA.5’s long period of dominance through the second half of 2022 and its relative symptomatic mildness in all but the most elderly led to a sense that the virus was tolerable at least to the extent that everyone could go back to living their lives as before the pandemic, including ditching those anoying masks.  By the end of the year, the majority of Canadians had experienced at least one bout of Omicron, which added to the somewhat illusory sense of being largely immune to further infection. 

In the first two months of 2023, BA.5 was gradually eclipsed by a mix of newer, more infectious variants, including BQ.1, BQ.1.1 (which was briefly dominant in the US and perhaps Canada), BN.1, BF.7, CH.1.1, BA.5.2.6 and BA.2.75. By late March, all were overtaken in North America by XBB.1.5 which, after reaching 90% dominance, is now itself on the verge of possible decline due to competition from its XBB.1.9 cousin.  

This pandemic has a history of surprising us. It makes sense that patterns which emerged at a time when the overwhelming majority of new victims were encountering the COVID virus for the first time no longer apply now that most current victims have been previously infected, vaccinated or both. Apart from China which long adhered to a strict infection-limiting regime and where vaccination lagged, most humans now have some immunity to COVID infection.  Lab testing indicates that prior COVID infection coupled with recent vaccination including the bivalent booster gives the best available protection against recent variants like BQ.1 and XBB.1.5.

With tens of millions of mostly unreported COVID infections occurring around the world and between one and a hundred billion copies of the virus replicating in our bodies every day that we are infected, humans are giving the virus everything it needs to keep mutating into ever more infectious strains.  Ominously, despite XBB only having emerged in September, it has already spawned at least 115 descendent lineages! By this point, it is insane that reality-denying politicians can continue getting away with making patently false statements to the effect that the currently-dominant variant is the last and the pandemic is therefore over.

The real question is not whether the next dominant variants will appear but when, and what combination of virulence and vaccine-resistance they will possess. We got lucky with Omicron in that it typically lodges higher in the airways than its Delta processor, resulting in fewer life-threatening cases. But future variants could just as easily be as deadly as Delta was, if not more. That risk is currently being highlighted by the rapid rise of XBB.1.9.1 in the UK, where as of March 7 it already accounted for 12% of all new cases. As reported in the UK Health Security Agency’s March 10 “Technical Briefing 51”, 6,364 XBB.1.5 sequenced cases had been identified in England. Among those cases, there were 39 deaths (3.0%). As of that same date, 1,168 XBB.1.9.1 sequenced cases had been identified, with 39 deaths (6.7%). The latter was extraordinarily high for that point in the pandemic and comparable to the death toll for Delta in late 2021.  Given the relatively small numbers at the time, it is possible that those 39 deaths could be a statistical fluke, but any continuance of that ratio as XBB.1.9.1 continues to spread could be catastrophic from a public health and safety perspective.  It certainly bears watching.  Even should the death rate drop, it highlights our society’s continuing vulnerability to new variants.

Earlier in pandemic, the time interval between each successive dominant variant was nearly a year. Now it’s a few months, a trend which continues to accelerate. After more than 50,000 mostly avoidable Canadian deaths, is it at all unreasonable to expect our leaders to prepare for the coming waves? Why are they not taking such obvious steps as urgent expansion of ICU capacity, more hospital beds, funding universities to train way more nurses to replace those who have been burning out on our behalf, offering better pay to attract and retain them, creating shelters for those without the means to self-isolate, dramatically improving air filtration in schools and other high-risk public places? Why do public health officials no longer warn Ontarians of looming waves when more infectious variants are obviously heading our way so that we can take common-sense precautions. The list of how poorly we are served by our elected representatives continues to grow.