Red to Yellow and back again: Doña Ana County case averages and test positivity, explained
Algernon D'AmmassaLAS CRUCES – On Feb. 10, Doña Ana County learned it had moved up to Yellow on New Mexico's COVID-19 Red to Green framework.
Restaurants could resume some indoor dining. Places of lodging that had completed the state's free COVID-19 safety certification program could welcome more guests. Retail stores could let more customers visit at a time. Permitted mass gatherings doubled from 5 to 10.
Yet when the map was updated two weeks later on Feb. 24, Doña Ana was back at Red, while other Yellow counties maintained their status or upgraded to Green. Restaurants in Doña Ana County had to close their indoor dining rooms again after ordering food supplies and arranging for staffing, and other businesses were required once more to adjust to different public health restrictions.
As the Las Cruces Sun-News reviewed daily reports, trying to analyze why the county shifted twice in one month, we realized the extent to which daily case counts (which are reported daily in our print edition) do not provide the most accurate picture of how the SARS-CoV-2 coronavirus is moving through our county and around the state.
With the help of the state Department of Health, let's unpack some of the math behind rolling averages, test positivity and how they measure community spread of COVID-19 in each county.
How DOH counts new COVID-19 cases
Every day, the state health department issues a report, typically close to 4 p.m., with that day's new cases statewide, listed by county; new deaths attributed to the disease; the number of COVID-19 patients currently hospitalized; and more.
Those reports are all available on the state's COVID-19 portal, http://cv.NMHealth.org.
Those daily case counts, however, are from raw data and not what health officials use to decide whether a county moves up or down on the Red to Green framework.
Instead, they calculate 14-day rolling averages to provide a more accurate picture of the disease's prevalence in a given county.
That's because the daily counts are adjusted frequently to account for discrepancies in reporting — for instance, when a case is counted twice or a COVID-19 patient is found to reside in another county or state. Cases are ultimately grouped by the patient's residence, not where the test was taken.
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Daily reported case totals and later adjusted figures can vary widely. For instance, in the period of Feb. 9 to 22, daily case counts tallied 867, but in the Red to Green framework report the total was 797.
An even bigger jump occurred in the Jan. 26 to Feb. 8 period, which prompted Doña Ana County's upgrade from Red to Yellow.
Going by the daily case count, 1,017 new cases were identified in the county during those two weeks; if you subtracted all duplicate reports and other fluctuations included in daily reports, it was 1,001.
And yet the number of cases produced by the rolling average for that period was 640 — a difference of 361.
While a fluctuation of that size is rare, the health department's Jim Walton said, "The counts are in constant flux" as multiple labs report, and later correct, "mountains of data on a variety of subjects."
"Some laboratories that report by fax or email are not necessarily reporting all their results in a timely manner," Walton continued. "When the Department of Health's Epidemiology and Response Division receives these reports, they may be receiving older records in the system that were not previously reported."
As an example, Walton said the ERD received a report of 851 positive results from Artesia, of which 750 had been reported previously, leaving 101 new cases. Out of those 101, only one was recent enough to be included in Eddy County's next Red to Green calculation.
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Thus, the data presented every two weeks in the state's Red to Green county-level analysis, posted online at the COVID-19 portal, presents the most accurate assessment of the county's average daily cases for a two-week period.
How the 14-day average is calculated
The health department's formula for calculating the 14-day average consists of four steps:
- Gathering the county's total cases for the 14-day period
- Dividing the total by the county's population
- Multiplying by 100,000 to produce a whole number
- Dividing by 14 to produce a daily rate
As a mathematical equation, that looks like this:
For instance, in Doña Ana County for the Jan. 26 – Feb. 8 period:
- 640 cases totaled
- Divided by a population of 218,836 = 0.00292456
- Multiplied by 100,000 = 292.456
- Divided by 14 = 20.9 per 100,000 population
Why Doña Ana County went Yellow
There are two criteria on which a county's status in the Red to Green framework is based:
- No more than 8 average daily cases per 100,000 residents
- Test positivity rate no more than 5 percent
If a county meets both those metrics, it is rated Green. If it maintains them for two reporting periods in a row, it gets to Turquoise — with the least stringent set of restrictions on gatherings and business activity.
Yellow means the county met one of the two criteria. Red means it missed both.
After weeks in the Red, the map update announced on Feb. 10 showed Doña Ana County with an average daily case rate of 20.9 — far exceeding the gating criterion of 8 per 100,000.
However, the number of residents in Doña Ana County testing positive for COVID-19 had dipped below 5 percent, bringing it up to Yellow. In addition to the 640 positive cases during that period, there had been 14,153 negative tests: A positivity rate of 4.33 percent.
In the next two-week period, however, positive cases went up and fewer county residents got tested. There were 797 new positive cases and 10,981 negative tests, yielding a positivity rate of 6.77 percent and flipping the county back to Red.
One way to move positivity rates back down is for more residents to get tested. Believe it or not, that may include residents who have received COVID-19 vaccinations.
Vaccinations and testing
"There’s no doubt the more people we vaccinate the better chance we have of reducing positivity rates in Doña Ana County," health department spokesman David Morgan said.
Since December, New Mexico has been rolling out two vaccines approved for emergency use against COVID-19, with a third receiving authorization last weekend.
The health department emphasized that the vaccines cannot induce COVID-19 disease, but Morgan said anyone experiencing symptoms or who suspects they may have been exposed should get tested, vaccinated or not.
Vaccinated individuals are also advised to continue following New Mexico's mask mandate and take the same public health precautions against the disease's spread as everyone else.
"It’s important to keep in mind, whether you’ve received just your primary dose or your primary and booster it takes up to two weeks each for you to build up antibodies to resist the virus," Morgan continued, "so even in doubt, the safest thing you can do for you, your family and community is to get tested just in case."
Algernon D'Ammassa can be reached at 575-541-5451, adammassa@lcsun-news.com or @AlgernonWrites on Twitter.