, according to a modeling study published Aug. 26 online in Nature.
Jeffrey Shaman, PhD, professor in the department of environmental health sciences and director of the climate and health program at the Columbia University Mailman School of Public Health, New York City, and colleagues developed a model to simulate how SARS-CoV-2 was transmitted within and between all 3,142 counties in the United States.
In their model, the researchers considered migration data between counties, the observed case numbers, and estimates of infections based on the number of people who test positive for SARS-CoV-2 antibodies.
The United States had the highest number of confirmed COVID-19 cases and deaths in the world during 2020. More than 19.6 million cases were reported by the end of the year.
But the authors point out that “69% of the population remained susceptible to viral infection.”
The researchers also studied the ascertainment rate, or the ratio of detected cases to the number of confirmed cases. Nationally, that value increased from 11.3% in March 2020 to 24.5% in December 2020.
That’s one of the biggest pandemic lessons from the data, Dr. Shaman said: “It is vitally important when there is an outbreak and you’re counting cases that there are many more people infected in your community who are contagious than reported cases. Each individual is infectious for multiple days, and there are many more unreported cases.”
That applies now with the Delta variant, he said.
“Vaccinated people who get infected with the Delta variant are part of the transmission chain,” he said.
Fatality rates dropped
Some of the data were very positive, Dr. Shaman told this news organization. The infection fatality rate fell from 0.77% in April to 0.31% in December. The authors suggest that that may be because of improvements in diagnosis and treatment, patient care, and reduced disease severity.
Joe K. Gerald, MD, PhD, associate professor and program director of public health policy and management at the University of Arizona, Tucson, told this news organization that this article helps confirm that COVID is much deadlier than the flu and that the intensity of the response has been appropriate.
“We should be willing to invest a lot more in mitigating COVID-19 than seasonal influenza because it has much greater consequences,” he said.
The numbers emphasize that testing must improve.
“We didn’t have enough tests available, and they weren’t easily accessible. For much of the year we were flying in the dark,” Dr. Gerald said.
The number of tests has increased this year, he acknowledged, but testing still lags. “We just can’t miss this many infections or diagnoses and hope to gain control,” he said.
The study also points out the huge variation by state and by county in infections and deaths, and that variation continues. Gerald noted that the varied numbers make it difficult for some regions to accept broader mandates, because the threat from COVID-19 appears very different where they are.
“We have to think about regions, how many people are susceptible, and what the testing capacity is,” he said. “States and even counties should have some leeway to make some important public health decisions, because local conditions are going to differ at different points in time.”