Much Ado About BA.2
Goodness, you’d think that BA.2 was fast upon us based on press reports. Granted, moving forward cautiously does make sense – particularly since the pandemic to date has consisted of a series of spikes and lulls. The epidemic curve for Los Angeles County suggests that we have had four surges to date (Figure 1 below).
On some occasions these surges have been preceded by rising case rates in Europe – most notably the initial seeding of the US epidemic by travel from Europe to New York and again prior to the Delta wave. Popular press reports will also mention that Europe had an Omicron spike before we did in the US, but that actually is not the case. Two of the most commonly cited countries are Germany and the Netherlands, but neither had an Omicron spike that preceded that of the US. Figure 2 below shows that the US spike clearly preceded that in each of those countries. Both Germany and the Netherlands have now entered a sharp downslope as well.
At this point in time, about 35% of COVID-19 cases sequenced in the US for the week ending March 19th were caused by BA.2.
Wastewater surveillance (link: https://covid.cdc.gov/covid-data-tracker/…) is one promising way to monitor for potential outbreaks – remembering of course that COVID-19 transmission historically in the US has been highly regional with a hopscotch pattern criss-crossing the country. Both wastewater surveillance sites in Los Angeles County (one covering 100,000 individuals and the other 3.5 million) are in the “-10% to -99%” category over the last 15 days. This fits with the continued decline in cases we are seeing as well – both in incident (new) and prevalent (existing) cases (Figures 3 and 4 below)
One point of modest caution, however, does exist when looking at Santa Monica / Malibu (SMMUSD) school-based data which has become an unintended passive surveillance cohort (I say unintended because I don’t think the school districts ever considered their weekly PCR testing having more broad use than simply isolation / quarantine). Also, because they do not delineate whether or not these are new cases reported week over week we are forced to assume that the number represents prevalence rather than incidence rates. Nevertheless, these rates do seem to share some predictive relationship with case rates of the County at large. For instance, for the week ending January 7th, the prevalence in the school district was at its highest rate of 9%. The week following, cases peaked in Los Angeles County. Understanding the massive number of limitations that exist in generalizing a single school district to a County of 10 million individuals – nevertheless, a small uptick in cases is observed for the week ending March 26th.
But, all in all, I do not expect BA.2 to remotely approach the massive surge we experienced with Omicron. Both the Santa Monica Malibu and Los Angeles Unified School Districts will be sending students home with rapid antigen tests to be performed prior to returning back to class after the upcoming Spring Break. It is at once encouraging and deflating to see schools embrace the kind of test, trace and isolate mechanisms that could have been employed in September of 2022 to keep schools open during the pandemic.
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