The short reign of BA.2.12.1 as the dominant COVID variant in the United States hasn't quite ended, but the writing is on the wall, as this week's CDC Nowcast show it fell from 64.2% of cases last week to just 56% today. A drop of nearly 13%.
Meanwhile BA.2, the previously dominant strain, has plummeted to < 10% of cases.
Rapidly closing are BA.4 and BA.5 (estimated at 11.4% and 23.5% respectively), both significantly increasing their share week over week, but with BA.5 outperforming BA.4.
As we saw a week ago in ECDC: Implications of the Emergence and Spread of Omicron VOCs BA.4 and BA.5 for the EU/EEA, these emerging Omicron variants appear to have significant transmission advantages over older BA.2/BA.2.12.1 variants, and seem headed for global dominance.
While we've seen no evidence they cause more severe illness than their predecessors, last week Denmark's SSI reported Increased Cases & Hospitalizations With Rise of Omicron BA.5, and in the UK (where BA.5 is rising) their ONS (Office of National Statistics) reported a sharp rise in COVID cases over the previous week.
Their transmission advantage appears to be heavily linked to their ability to evade prior immunity, either from previous COVID infection, the COVID vaccine, or both (see Nature: Immune Escape Properties of BA.2.12.1 & BA.4/BA.5).
That said, evidence still suggests that those who are vaccinated and recently boosted are less likely to suffer severe disease, and are less likely to be hospitalized or die, than those who are under-or-unvaccinated.
Given the limits of surveillance, testing, and genomic sequencing around the country, all of the following numbers should be viewed as rough estimates, and we may see further revisions to the data in the weeks ahead.
During the first two years of the pandemic, we saw only 3 major variants (D614G, Alpha, Delta) rise to dominance in the United States, and each reigned for 6 months or more. Over the past 6 months, we've seen in rapid succession BA.1, BA.1.1, BA.2, BA.2.12 all rise to dominance, and BA.5 will likely hit that mark in the next couple of weeks.
How long BA.5 will reign, and what will replace it, are big unknowns.
But as long as this rapid replacement cycle continues, additional pandemic waves are all but guaranteed.
