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Lags in reporting make Arizona's virus data unreliable for spotting certain trends, UA professor says

Lags in reporting make Arizona's virus data unreliable for spotting certain trends, UA professor says

State data shows a recent week-to-week decrease in the numbers of coronavirus cases, tests and hospitalizations, but the figures are very likely incomplete.

“Don’t believe it,” said Dr. Joe Gerald, an associate professor with the University of Arizona’s Zuckerman College of Public Health, referring to the observed decreases in weekly coronavirus stats.

Throughout the pandemic, there have been data-reporting lags that have made it difficult to interpret coronavirus trends. They occur across all entities, like hospitals and laboratories, that report data to the Arizona Department of Health Services.

“For the last two weeks, the reporting lag has grown longer and it just makes it more difficult to understand what’s going on because the data aren’t available soon enough,” Gerald said. “So I don’t think we know where we are right now. Testing has gotten so bad again in Arizona.”

Gerald said he typically accounts for these lags by waiting five days before collecting data from the week before. By collecting weekly totals, he smooths out coronavirus trends that have natural day-to-day dips and rises. The Arizona Daily Star aggregates data in the same way for its weekly coronavirus graphs, typically waiting seven days to collect data.

It takes about four to seven days for ADHS to publish new positive cases, according to the ADHS website. Each week, some cases are backfilled after seven days. But in recent weeks, the number of backfilled cases has grown. This creates the illusion of fewer cases in recent weeks.

Since May 31, Gerald saw 8% to 12% of total cases backfilled. On June 28, the number jumped to 28%.

“My suspicion is it may even be larger this week,” he said. “It could be 35%. I’m just guessing, but that’s what I’m thinking now.”

New confirmed cases in Arizona totaled 22,467 from June 28 to July 4, according to data published by ADHS on Saturday. This looks like a decrease of 4,786, or about 17%, from the week of June 21-27.

But if 35% of cases are backfilled in the coming days, as Gerald predicts, cases would total about 34,565 from June 28 to July 4. This would mean an increase of 7,312, or about 27%.

Gerald publishes a weekly analysis of novel coronavirus trends he sends to public-health officials and faculty, among others, across the state.

Last week, he reported a very mild week-to-week increase in cases, only 8%. But after more cases were backfilled over the last week, the increase was really closer to 30%, he said.

The increase in cases was still lower than it had been for several weeks, which he said suggests that local mask-wearing ordinances, such as Pima County’s, could be slowing the spread of coronavirus.

A statewide mask mandate is overdue, Gerald said. “It’s the right thing to do.”

“Still on road to hell”

He added that the local mask mandates have likely helped, but they haven’t saved us.

“Conditions are still worsening,” he said. “We’re still driving on the road to hell. We’re just not driving as fast.”

Deaths are still showing a clear upward trend in the state data, when accounting for a two-week reporting lag. The lags in reporting deaths to ADHS are even longer than for cases and tests.

“I don’t see anything but increases in the number of deaths for the next two or three weeks,” Gerald said.

On Thursday, Gov. Doug Ducey acknowledged that the pandemic is widespread in Arizona and announced a directive to limit dine-in facilities to 50% capacity. Pima County has had a 50% indoor capacity limit on restaurants since May 21.

Ducey’s statewide dine-in limit followed his June 29 mandate to once again close certain businesses, including gyms, bars and theaters.

Gerald called the governor’s action on Thursday a “baby step” in the right direction and one that’s not based on scientific evidence.

Some infections are likely happening at restaurants or gyms, he said, but coronavirus is also likely spreading at the post office, the dog groomer’s or anywhere people interact with one another.

“We do not have reliable data on this type of targeted approach that the governor’s taking,” he said. “We don’t have good data on where these infections are actually transpiring.”

Contact reporter Alex Devoid at or 573-4417.

On Twitter: @DevoidAlex

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Data/Investigative Reporter

Alex has been with the Star since June 2019. He previously wrote about the environment for the Arizona Republic and he's a graduate of the University of Arizona.

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