Coronavirus prevalence in San Diego County was relatively flat over the past week, according to the latest wastewater surveillance data, despite recent hints of a drop after the aggressive spread of the BA.5 variant throughout the region.
It’s a different pattern than was seen during the original Omicron wave last winter when the number of coronavirus copies per liter of wastewater peaked on Jan. 9 at 47 million per liter of wastewater, then fell sharply, dropping 16 percent in a week and falling below the 1 million mark on March 8, according to reports from San Diego’s SEARCH research coalition.
The shape on the epidemic curve is a classic spike — sharply up, then sharply down.
But the latest pattern is flatter.
Surging quickly up to 19.8 million copies per liter on July 26, counts fell to 18.1 million the following day but then reversed course, surpassing 19 million on July 31 and Aug. 1 then nearing 20.1 million on Aug. 2 before falling to 17.1 million on Aug. 3.
Nationwide, the pattern varies by location with the Pacific Northwest showing declines in the amount of virus detected while some Midwest states are still seeing significant increases, according to the latest wastewater COVID-19 surveillance data collated by the U.S. Centers for Disease Control and Prevention. It is difficult to get a current picture of nationwide activity, however, because some sites are significantly slower to post new updates than others.
What’s causing a different pattern for BA.5 than was seen for Omicron, its ancestor?
Kristian Andersen, an immunologist and molecular bioligist at Scripps Research, one of the groups collaborating on San Diego’s wastewater surveillance reports, said in an email that the plateau pattern now visible in the data is “probably a combination of still-changing human behavior, faster-waning immunity, and, of course, continued virus evolution that keeps chipping away at protection against infection (and) transmission.”
Andersen said that while wastewater numbers likely will fall a bit lower in the coming week, he does not “really think we’ll see the same steep decline as we did for BA.1,” the original Omicron variant.
Dr. David “Davey” Smith, a translational research virologist at UC San Diego, said that reinfection also appears to be playing a role. Those who got infected this spring, he noted, have little resistance to BA.5.
“I think that there are so many people still susceptible to BA.5 that this level will stay high for a while,” Smith said in an email.
Confirmed cases have shown a similar flat pattern, with 1,446 positive test results communicated to the county health department Wednesday, slightly fewer than the 1,678 listed one week earlier. COVID-related hospitalizations have declined slightly, with 435 in non-military hospitals Wednesday compared to 464 the previous Wednesday, July 27.