Colorado’s backlogged COVID data plays havoc on national case trackers

Colorado looks like a COVID-19 hotspot on some national pandemic trackers, but those numbers can be misleading because many of the cases being recorded as new are actually months old.

On Monday morning, The New York Times’ tracker showed Colorado averaged an eye-catching 1,323 new COVID-19 cases per day over the previous week. But the Colorado Department of Public Health and Environment showed a more-modest average of 446 cases per day.

The problem is that the Centers for Disease Control and Prevention and national trackers, like the one published by the Times, have been getting their case numbers by subtracting the cumulative number of cases on any given day from the cumulative number a day earlier.

For Colorado, the most recent daily number was 1,831 — the difference between 1,363,400 on Thursday and 1,365,231 on Friday. (The state no longer updates its dashboard on weekends.)

Yet that method doesn’t distinguish how many of those confirmed COVID-19 cases are truly new and how many have been backlogged for months. It was never a perfect measure, since delayed reports have been a fact of life throughout the pandemic, but usually it was close.

During the omicron surge, however, Colorado developed a significant case backlog. It’s not unusual, halfway through April, to see cases that happened months ago added to the state’s spreadsheet on any given day.

For example, in the state health department’s Friday afternoon data release, 1,084 of the 1,831 newly added COVID-19 cases came from the first weeks of January.

Colorado has recorded a modest uptick in confirmed new COVID-19 cases in recent weeks, though hospitalizations of people with the virus have held steady at a very low level. The growing reliance on at-home tests, the results of which aren’t always conveyed to the state, means that’s not a full picture, either.

A spokesman for the Colorado Department of Public Health and Environment who did not identify himself said the agency is working with the CDC and major sites, including The New York Times, to make the public-facing numbers more clear. The CDC’s public-facing dashboard now displays numbers that are close to the state’s, but not all of the trackers that pull numbers automatically from the state’s website have changed their methods.

“We realize this continues to cause confusion and will continue to reach out to these outlets to streamline the process,” the state health department spokesman said.

Massachusetts also had significantly different numbers, with a spread of 535 cases between the numbers on the state’s website and on the Times’ tracker as of Friday morning. Some states had narrower gaps, however: Rhode Island’s state-reported cases were only 20 below the national numbers, and in Alaska, the difference was only three cases. It’s not clear why some states appeared to have much larger backlogs than others.

Brian Spencer, a state health department spokesman, said Colorado’s backlog is due to a high number of COVID-19 test results that needed human review in January. Colorado’s data system can usually verify cases by matching the testing information received with individuals, which allows the state to keep track of the demographics of people getting sick and to see how common reinfections are, he said.

Sometimes the system can’t match the information, however, and a team has to look at the case, Spencer said. With 20,000 cases coming in each day during the high point of the omicron surge, that added up to 200,000 cases that needed human review, he said.

“CDPHE is continually checking data for quality and completeness and strives to be transparent in any necessary adjustments,” he said.

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