What does Santa Cruz County’s sky-high COVID positivity rate mean?

Santa Cruz County was spared much of the first wave of the COVID-19 pandemic, but that is no longer the case; on average nearly 200 residents are now testing positive every day.

One of the metrics illustrating the dire situation here is the positivity rate, the 14-day average of how many local tests are returning positive results. Over the past four days, that value has exceeded 20%.

Positivity rate is not always the best way to measure the level of virus in a community, because by and large the people who get tested are those with symptoms or suspected exposure — especially in scenarios where testing is limited or hard to access. But the significant change in this metric locally is notable.

A 20% positivity rate means that, on average, one in five COVID-19 tests is coming back with a confirmed infection. This is much higher than the state average rate of 13.3%.

In Santa Cruz County, the rise in the “14-day-rolling-average positivity rate” to 20.7% surprised even some county officials, who have been trying to confirm this number’s accuracy with the California Department of Public Health ever since Lookout presented them with the figure on Monday.

The CDPH also reports a second figure, the “7-day-average positivity rate” in the Blueprint for a Safer Economy dataset released every week. The most recent number from that dataset is 12.3%, with the rate exceeding 20% in some South County areas — but that rate is for the week of December 27 to January 1, and does not cover the time period in which the positivity rate’s rolling average has breached 20%.

County spokesperson Jason Hoppin said “Nothing in our other metrics indicates that our overall positivity rate is over 20%,” but neither he nor county health officials could explain the county-state discrepancy.

But the bottom line is either metric, Hoppin said, “demonstrate(s) that coronavirus transmission is as high as it’s ever been in our community.” The absolute number of positive tests has also increased significantly, by about 25% since before Christmas.

Lookout caught up with Dr. David Ghilarducci, the county director of emergency medical services, for a brief conversation on where we are at now, and what to take away from the rapid rise in positive tests.

Here’s more from Ghilarducci, edited for clarity and conciseness.

Lookout: The positivity rate has really just been going up like crazy, it’s now 20.7%, up from just 12.4% on Christmas Eve. What are you taking away from that increase?

Ghilarducci: Positivity rate tells you two pieces of information. It tells you most directly how many people that get tested are positive. It’s also a function of the prevalence of disease in the community and the amount of testing that’s available. So sometimes positivity rates can be high because there’s just not enough testing capacity, and so only those who are really motivated to get tested, actually get tested. That tends to be a group that is concerned about illness. So, I can tell you,down at Salud Para la Gente, their positivity rate is over 40%, and it routinely has been that high for quite a while. That group is a different group than the county as a whole. So that’s going to drive up the rates.

The other thing I think that will drive up rates is [that] elective surgeries have been really reduced recently, because of the health care surge. A lot of those kinds of screening tests are not happening. So that was part of the denominator that’s no longer in there.

But overall, the bottom line message is that we have a lot of disease in the community and that’s reflected by the positivity rate. We’re very worried about that, and we have been worried about that for a long time

We are starting to see some slight relaxation in our hospitalization rates lately, which is encouraging.

But it seems like you can expect that would not continue, right? That the increase in positivity rate will mean more people are going to be hospitalized soon.

Yeah. I think that if we follow the lessons from the Thanksgiving gatherings and gatherings before Memorial Day, 4th of July and so forth, we are not seeing the full effects — or we may be seeing the lull before the effects — of the Christmas and New Year’s holiday show up in the hospitals. So we continue to be concerned about January, and probably early February because of that.

The other factor that would drive up the positivity rate is people were using testing inappropriately in order to sort of get a free pass, to travel and go visit family for holidays. Those numbers probably also made the denominator bigger and therefore sort of obscured the more positive cases.

What would your answer be to people who are wondering if the new variant is behind this sort of surge?

One of our hospitals has been trying to find a way to get some of their samples sequenced for genomic sequencing. And so I’m reaching out to UCSC [UC Santa Cruz] right now to see if they can do it. Otherwise, we would use the state lab in Richmond to do the sequencing. There’s been a high demand, and we haven’t been able to get the sample sent up there at this point to do it. So, to be honest with you, there’s very little sequencing going on in the county. So that variant may very well be present and we just don’t know it yet.