For months, Utah has been looking at the percentage of coronavirus tests that are positive as one way to tell whether the virus was spreading through communities uncontrolled and undetected.
A “percent positivity” of 5%, along with low case counts and ample hospital capacity, would show “we’re doing good,” state epidemiologist Dr. Angela Dunn said in a news conference six weeks ago.
Utah’s current measure of percent positivity — calculated by dividing the number of positive tests in a week by the number of tests taken in that week — has met that threshold for more than three weeks, according to data from the Utah Department of Health.
But during that time, more than 100 Utahns had died from the coronavirus. Hundreds more have been hospitalized. Only six of Utah’s 29 counties were deemed “low transmission” last week by state calculations. By the more stringent benchmarks set by the U.S. Centers for Disease Control and Prevention, 22 of Utah’s counties were at “high” or “substantial” transmission, only seven reaching “moderate” or “low.”
Here’s what we know about the metrics that will tell us whether coronavirus is under control in Utah.
Is there a certain number of cases, hospital admissions or a percent positivity that will let us know we’re in the clear?
No. “I think we have to be really clear: There is no consensus on metrics by which to pull back public health policies,” said Dr. Eddie Stenehjem, an infectious diseases specialist for Intermountain Healthcare. “We don’t have definitive metrics to say, ‘OK, we can lift the mask mandate. OK, we can safely congregate.’”
In June, when cases were in the 300s and Salt Lake County ICU beds were just beginning to fill, Dunn warned state lawmakers that Utah’s test positivity had “shot up to 8% to 10%, and that indicates there’s more spread in the community.”
She added: “I would love to see that drop down to 3%. That would give me confidence that we really have this under control.”
But Tom Hudachko, spokesman for the Utah Department of Health., said overall, state officials “do not have metrics nor a definition of what ‘under control’ looks like. And there certainly isn’t one single metric we would use to make such a determination.”
The state has changed how it calculates the test positivity percentage since then, and now chiefly relies on a metric that takes the number of positive tests in a day, or a week, and divides it by the number of tests conducted in that same time period. That results in a lower percentage than the earlier method, which excludes results from people who get tested multiple times in three months.
The new method shows the state’s increased testing capacity and makes it more comparable to other states, Dunn said at the time of the switch. Under the method the state now uses, the 3% to 5% range still reflects having the virus more under control, she said.
Both measures have been dropping for weeks.
Can’t experts use what’s known about the virus by now to estimate how many people will probably catch it at certain case levels, adjust for vaccinations, and make a decision about when it’ll be safe to lift certain protections?
We don’t actually know enough about the virus to make those kinds of predictions, Stenehjem said.
“This is still a novel pandemic,” he said. “This is the first time where we’re actually in this position where we have vaccine delivery and we’re having decreasing case counts.”
For example, because the virus had only begun to pop up in the United States last spring, we don’t have much evidence of how seasonal change will affect case numbers now that it’s widespread in the community.
And, Stenehjem said, we are only now learning about the impact of more-infectious variants.
If we don’t really know when the virus is under control, why does the state have color codes and transmission levels? Aren’t those tied to regulations that imply certain behaviors are safe at certain case numbers?
The state in October introduced a three-level transmission index that attached various rules for businesses and gatherings to each county, based on case counts, percent positivity and statewide hospital crowding levels.
“We rely on the metrics outlined in the transmission index to define areas of high, moderate, and low transmission,” Hudachko said. “And, of course, public health guidance is connected to these transmission levels.”
But not everyone agrees on which numbers should define levels, or which restrictions should be triggered by which numbers. Different states, different agencies and different experts urge varying levels of caution.
What are some of the rules and benchmarks that lack consensus?
For example, Utah’s restaurants, bars and gyms have spacing requirements at the highest two transmission levels, but they can stay open at all transmission levels. Arizona, by contrast, requires all such businesses to close at the highest transmission level — and the case rates that trigger the highest restriction level are lower than the case rates that trigger the highest level in Utah.
And in Utah, there are no restrictions for those businesses in counties that reach the lowest level of transmission. In Arizona, those businesses may operate only at 50% capacity at the lowest transmission level — and the “low” level there requires counties to report fewer than 10 new cases per 100,000 people per week. That’s about one fifth of the case rate required for a county to be deemed “low transmission” in Utah.
Arizona’s three transmission levels generally use the same definitions as the CDC. Case numbers that indicate “high transmission” under CDC guidelines only count as “moderate transmission” in Utah.
Meanwhile, infectious disease experts in Utah have criticized the Legislature’s decision to end the statewide mask mandate on April 10.
“It’s scary to be putting dates on timelines that take things away when we don’t have definitive metrics,” Stenehjem said. “I think a mask mandate should be one of the last things to go. A mask mandate doesn’t cost us anything, it allows our business to be open, it allows economies to flourish without societal cost.”
Gov. Spencer Cox said he agreed to the end date based on expected vaccination levels.
With vaccines rolling out, can we start looking to herd immunity to know that things are “under control?”
Yes — but not for a while.
“One thing we can look at, which I think we should be moving more toward, is: ‘What is the percent of the population that is protected, and what percent of the population is still vulnerable?’” Stenehjem said.
Statewide, immunity estimates are anywhere between 30% and 45%, he said. “That’s not anywhere near where we have to be for herd immunity.”
In Salt Lake County, for example, about 22% of the population had received one shot of a two-shot vaccine course by the end of last week, and another 12% was fully vaccinated, said Eileen Risk, manager of Salt Lake County’s epidemiology bureau.
Meanwhile, researchers from the University of Utah used case numbers and antibody testing results to estimate that about 25% of residents have had the coronavirus — but some of those had the virus up to a year ago and may have reduced immunity over time; others are included in the number of people vaccinated.
In all, county officials estimate that about 35% of Salt Lake City area residents now have immunity to the virus, Risk said. “We’re maybe, at best, 50% of where we need to be,” she said.
And that’s assuming 70% immunity is enough to suppress the spread of the virus, even as increasingly infectious variants take hold.
“Our target for herd immunity in Utah is somewhere between 75 and 90%,” Dr. Andrew Pavia, a pediatric infectious diseases specialist, said in a webinar hosted last month by the U. “That’s a level at which we’ll really have control of this virus.”
But that number could rise quickly, with the state announcing Thursday that everyone age 16 and up will be eligible for vaccines starting this week.
So should we stop paying attention to percent positivity since we reached the goal, the virus is still spreading, and herd immunity might be in sight?
No — but we should pay attention to other metrics, too.
If enough Utahns are being tested, the percent positivity can be low, even if new cases and hospitalizations are high. But the low percent positivity would still indicate that testing is identifying more of the people who have the virus, and it’s less likely the virus is spreading undetected in Utah communities.
“It’s not one metric; it’s how they all work together,” Dunn said at a Feb. 5 news conference.
Cases have indeed declined from the winter peak — but testing still is pretty widespread, with the number of people being tested falling more slowly than the number of people testing positive.
Even by the CDC’s tougher standards, 25 of Utah’s counties had “low” or “moderate” percent positivity.
If Utah’s percent positivity is so low, why is the CDC calling transmission rates “high” or “substantial” in most counties? What factor is bringing our threat level up?
The CDC uses only two numbers to determine transmission rates: percent positivity and the number of new cases each week per 100,000 people. Each of those figures receives a rating, and the higher of the two is the state’s overall transmission level.
Salt Lake County, for example, reported that 3.97% of its tests were positive in the past week — defined as “low transmission” by the CDC.
But it also reported almost 103 new cases per 100,000 people in the past week, which the CDC defines as “high transmission.”
The county is getting close to being deemed “moderate transmission,” with fewer than 100 new cases per 100,000 residents, Risk noted.
But to be considered at low transmission, that rate has to drop from 100 to fewer than 10. “We’re a long shot from that,” Risk said.
And while new cases declined sharply from January to early March, Risk said, during the past two weeks “we’re seeing a bit of a plateau.”
What about hospital capacity? Are we at least done worrying about that?
As of Thursday, the state estimated that Utah’s intensive care units were 72% full, right at the “warning” level. But that’s a far cry from December and January, when ICU beds were well over 85% full — which is considered over capacity due to staffing fluctuations.
Meanwhile, only 12% of ICU patients were being treated for COVID-19, far below the peak of 45% in mid-January. That means hospitals no longer are stretched so thin when it comes to specialists and equipment that most coronavirus patients need.
And for health care workers, a little reprieve has gone a long way, Stenehjem said.
“I was just rounding the hospital last week, and the feeling in the hospital is dramatically different than it was back in November, December, January,” Stenehjem said. “The mood of the caregivers is distinctly different, and the amount of patients we’re dealing with, with COVID-19 right now is completely manageable. Right now our health care networks are in a safe spot.
“We’d like to see fewer cases; there are still people suffering from this. But we can manage cases right now.”
Like Risk, Stenehjem was wary of cases leveling out rather than declining.
“It’s just whether or not we stabilize at this rate we’re at, or we continue to go down,” he said. “We really don’t want to see it go in the other direction. But I don’t expect that.”