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AMES, Iowa—A team of researchers in the Department of Statistics at Iowa State University has developed a mathematical model that reveals critical characteristics about COVID-19—such as how contagious the virus is and how rapidly it spreads through populations. Applying the model, which calculates COVID-19’s effective reproduction number (R), reveals key insights into how Iowa cases have grown from a few to more than 1,700.

“We believe that our team has developed an accurate and reliable way for modeling the epidemic spread of COVID-19,” said Yumou Qiu, assistant professor of statistics at Iowa State, and member of the research group. “Our very detailed methods calculate those important reproduction numbers, offering rich insight into how COVID-19 behaves and impacts populations.”

How it works
The R value estimates the average number of people one person with COVID-19 will infect, within a given population. In short, R quantifies how contagious a disease is and the speed at which it spreads.

The mathematical models developed by Iowa State researchers calculate effective R values. These numbers measure how contagious COVID-19 is inside a population which has unique demographics, socioeconomic attributes and policies for handling the outbreak.

An R above 1.0 indicates that a disease is spreading; an R below 1.0 suggests that spread is decreasing and under control.

“The R in Iowa is different from the R in New York City,” Qiu said. “Our models produce analysis that is pertinent and specific to each location.”

What the models reveal about Iowa
Similar to how meteorologists use models to analyze and predict weather patterns—Iowa State researchers use their models to explain and predict the spread of COVID-19 in Iowa. The models also reveal COVID-19 transmission rates and shed light on how Iowa’s current government policies may be impacting the spread of COVID-19

When the first cases of COVID-19 were discovered in Iowa, in Johnson County, the estimated R0 value was 3.45. The group finds that Iowa’s effective R value peaked at 3.91 on March 16.

By delving into the data, these statisticians extracted the story of how COVID-19 spread during the time when Iowa’s public schools, colleges and universities released students for spring break.

“That peak, or spike, in Iowa’s R value signifies a time when there was a lot of movement and activity around the state,” Qiu said. “Iowans were traveling, college students were returning home and many people were headed to and from vacation locations across the globe.”

The data also reveal that Iowa’s current R is 1.65; a 50% drop from the March 16 peak of 3.91.

That’s good news, with a major caveat.

Although a 50% drop is encouraging, Qiu notes that an R value greater than 1 signals a compelling reality: that COVID-19 continues to spread across Iowa, increasing new cases.

“These dynamics indicate that Iowa has not fully controlled the spread yet,” he said. “We estimate that there are 741 undiagnosed COVID-19 cases in Iowa, and that this week’s confirmed infections will range from 680-1397.”

Iowa Gov. Kim Reynolds declared a statewide public health disaster emergency on March 17. Since this proclamation, Iowa’s R has dropped from the 3.91 peak, to an all-time low of 1.54 on April 3. Iowa’s R has ticked slightly upward to its current value of 1.65.

Iowa’s average R value during the last 10 days was 1.66, a further indication that infected Iowans continue to spread COVID-19 to others.

“We don’t know when an infection will end. Those realities are sensitive to average recovery days and future infection rates, which can change rapidly. This is a very dynamic situation,” Qiu said. “Social distancing has made a significant impact in Iowa to slow the spread, but Iowa’s battle with COVID-19 continues to develop and change rapidly.”

National Insights
According to the group’s models, COVID-19’s R value in the United States peaked at 6.1 on March 11 and currently sits at 1.1. Although social-distancing measures have been very effective, the team’s models predict a worsening and prolonged outbreak for the United States.

The team estimates 165,000 COVID-19 cases remain undiagnosed within our borders, and that 217,000–233,000 new COVID-19 infections will be confirmed this week. Based on the data, Qiu predicts final U.S. case numbers will reach 1.2 to 1.5 million.

“The R numbers continue to be very important in gauging the statewide and national spread of COVID-19,” Qiu said. “Our models can help policy makers determine what strategies are working and when more needs to be done to mitigate this global pandemic.”