The spread of the novel coronavirus in Colorado has, for the past several days, trended away from the accelerating growth that drove case counts up in the state in early and mid-March, encouraging researchers who have been waiting for signs of social distancing impacts.

As of Friday afternoon, the state counted 6,510 positive COVID-19 cases, and 1,312 related hospitalizations. While the numbers continue to rise, the growth has slowed.

Dr. Jennifer Nuzzo, an epidemiologist and professor at Johns Hopkins University in Baltimore, said Friday that although interpreting new data is challenging because reporting lags, “in many parts of the U.S., we are hearing stories about the rate of growth slowing, which is good news for sure.”

Members of a local team of experts monitoring the pandemic said Friday the same trend appears to be emerging in Colorado. Dr. Jonathan Samet, the dean of the University of Colorado School of Public Health, who is leading the effort to model the virus’s spread in the state, said the latest numbers could be the indication they’re looking for that social interactions have been effectively cut down enough to reduce the immediate number of cases and lessen the burden on the health care system.

An April 6 report from Dr. Samet and the researchers he’s working with concluded that social distancing measures put in place March 17 had the effect of reducing social interactions by around 45%, which they estimated cut the number of known cases in the state by 1,200 during the last week of March.

“What we’re looking for now is the effect of March 26 measures,” Dr. Samet said, “and what we might see now is another bending of the curve.”

Prior to March 28, the growth of total known positive cases increasingly correlated with an exponential curve, meaning a growing rate of growth. But since then, the correlation has steadily weakened and its slope has declined, indicating a flattening curve. 

Researchers agree that the number of known positive cases is a drastic undercount and lacks precision. But since April 2, the same pattern has shown up for COVID-19 deaths, which are agreed to be a better indication of the virus’s prevalence.

Dr. David Bortz, an applied mathematics professor at CU-Boulder who worked on the local model being used by state officials, said the latest data show social distancing measures appear to have had the intended effects: “The numbers are headed in the right direction, and we expect this trend to continue as long as the stay-at-home order is respected.”

Nuzzo emphasized that signs of slowing growth might be coming from areas that are further along the cycle of an epidemic viral spread, or from areas that had rapid growth but put social distancing measures into place.

“There are other parts of the country,” Nuzzo pointed out, “that are experiencing a much faster increase in cases.”

And the cases tracked by the state are still far below what the local experts believe is the true number of cases. The local researchers believe that the 6,510 tracked cases is close to about one-quarter of the actual number of symptomatic cases, which are now estimated to be above 20,000 in Colorado. And their research estimates another 30,000 asymptomatic cases, meaning around 1% of the total population of Colorado may have already been infected.

Dr. Daniel Larremore, a CU-Boulder computer science professor, cautioned that it’s possible the testing capacity could be creating the appearance of a decelerating growth. If there aren’t enough tests, he said, it’s possible greater numbers of cases aren’t being tracked. 

Dr. Drew Harris, a public health professor at Thomas Jefferson University in Philadelphia, said it would take a more widespread testing program to make concrete conclusions now.

“A more robust testing program that included random community members,” Harris said, “would give a better sense of what’s to come rather than this rear view mirror view.”

Scott Bookman, the state’s coronavirus critical incident commander, said Thursday that the state has effectively slowed the spread of the virus.

“Social distancing has worked, and we have pushed that curve to the right as we’ve hoped to,” Bookman said.

If the virus’s spread in Colorado has been slowed by social distancing, Samet said, that’s good news for access to health care: “Maybe we’ve avoided a peak that we can’t handle with our health care system.”

If social distancing measures reduce interactions by 70% or more, the models developed by the team led by Samet predict a steady number of cases for many months without a dramatic peak. If 80% reduction is achieved, as few as 1,400 people might die from the disease by the end of the year, Samet’s model predicts, but, if only 60% is achieved, that number could be as high as 43,000, according to the model.

Tough policy choices await, because the virus will continue to spread, he said, and there is still a large susceptible population. So the degree to which social distancing measures can be relaxed isn’t clear.

In some areas where the spread of the virus was initially effectively slowed, like in South Korea, Taiwan and Singapore, Samet said, there’s evidence of second waves of spread.