What concerns should we have in the U.S. in light of worldwide experience?
How does BA.2 compare to BA.1: what are we (potentially) up against?
The omicron variant includes four lineages: the parental B.1.1.529 and descendant lineages BA.1, BA.2 and BA.3. CDC reports for Region 9 (CA, NV, AZ) do not distinguish between B.1.1.529, BA.1 and BA.3 but do now report BA.2 separately.
Currently, the proportion of BA.2 for our region is 41%, surpassed only by its proportion in the Northeast (55%); the Pacific Northwest (39%), Midwest (31%) and Mid-Atlantic (30%) regions show slightly lower proportions, but BA.2 is rising in all areas of the country. By comparison, many areas of Europe have reported BA.2 proportions near or above 50% and this shift is likely a factor in the observed increase in cases in this part of the world. Without a doubt, however, other factors are also at play: vaccine status of the various populations, level of post-infection natural immunity, waning protection from either prior vaccination or illness, and state of current risk mitigation practices (masking, physical distancing, societal “re-opening”).
There is evidence supporting an increased growth rate for BA.2 compared to BA.1 and it appears to be somewhat more infectious with higher viral loads and a longer infectious period. Modeling suggests a BA.1 reproductive number of 1.99 compared to 2.51 for BA.2 and secondary attack rates of 10.3% and 13.4% respectively.
Clinically, however, the two appear to behave in a similar fashion with no significant differences noted so far in hospitalization rates. No difference in vaccine effectiveness against symptomatic disease has been seen either. And although an infection with BA.2 in the wake of a BA.1 infection can rarely occur, early studies indicate protection against such reinfection to be around 95%.
What can previous experience and current data tell us about the likelihood of a significant BA.2 impact?
We have seen how a rise in the proportion of a new variant can fuel a rapid rise in COVID cases and hospitalizations in the geographic area involved. This has occurred locally with the epsilon (California or West Coast) variant over the 2020-2021 year-end holidays and again with the delta variant (impact onset: July 2021) and the omicron variant (impact onset: December 2021). When an existing variant reasserts predominance (as with the alpha variant supplanting epsilon early last year), an increase in cases may not materialize [see figure 1].
When a new variant is in the process of taking over, the impact seems to become evident when it has risen to around 30% to 60% of total variants sequenced. This was true for the recent emergence at around 30% [see figure 2] as well as for the prior emergence of delta at around 60% and epsilon at around 30%. If BA.2 follows the same rule (and no guarantee it will since it is a sub-lineage of the same variant as BA.1), then the absence of any change in the case curve so far could be encouraging as the latest data for the week ending 3/19 shows >40% BA.2. Observations over the next several weeks will allow us to determine if the BA.1 to BA.2 sub-lineage transition will follow the prior variant to variant experience or not.
Bottom line: we don’t yet know if BA.2 will fuel an omicron BA.1 or delta-like surge in new COVID cases (although there is some indication it may not); but even if it does, this would be predicted to follow the same milder clinical course observed with omicron BA.1 thus far with a commensurately minimal impact on hospital capacity and resources.