How quickly is the coronavirus spreading in Ada County? This number offers a clue
Ada County reverted to Stage 3 of Idaho’s reopening plan Wednesday as a consequence of a consistent spike in positive coronavirus tests in the area.
Central District Health, which made the decision, estimated that one infected person could lead to seven more cases in the county. But what does this number mean and where did it come from?
The R0 factor
The number is known by epidemiologists as the reproduction number – or R0 – and it contains information about the rate of transmission of a disease. It describes the number of people that can acquire the disease, on average, for each infected person.
Many factors can be taken into account when estimating this number, but it mainly depends on three parameters: how long a sick person remains infectious, how likely it is that a person exposed to the disease will become infected and how much social interaction is happening in a community.
R0 can provide information about the fate of a disease, and how quickly that fate will be met. When R0 is less than 1 the disease is expected to eventually disappear, but a disease with an R0 of 0.5 will disappear a lot faster than one with an R0 of 0.9. When R0 is larger than 1, the amount of people infected is expected to keep growing at a pace that is proportional with how large the number is.
According to Dr. David Peterman, pediatrician and CEO of Primary Health Medical Group in Boise, the R0 of 7 given by CDH on Monday was estimated by local physicians, who based their decision on the observed surge in the positive percentage of tests conducted in the area in recent days.
Primary Health has administered about 40% of all COVID-19 tests in the Treasure Valley since March.
Positivity rate uptick
According to the Centers for Disease Control and Prevention, the nationwide test positivity rate was 5% to 6% by June 13. With more tests being done in a community, the positivity rate should be similar to the nationwide estimate.
Before June 10, Primary Health was doing around 100 tests per day and the positivity rate was 1% to 2%.
“Now, we’re testing close to 300 a day … and for the last six days, the positivity rate of everyone we’re testing is over 20%,” Peterman said Wednesday. With a 10-fold increase in test positivity rate, “we have an incredible surge of infection,” he said.
Even when most of the positive cases are in the 18-to-40-year-old group, Peterman said that Primary Health is already starting to see some positives in older members of the community.
Tip of the iceberg
Considering that around 40% of infected patients don’t show symptoms, Peterman is worried that the actual number of infections in the community is much higher than observed. Another tell-tale sign of high rates of community spread is that the numbers in Canyon County are also rising, although not as much as in Ada.
Ada County has added 665 confirmed coronavirus cases over the past 11 days. Canyon County has added 150 over the past four days, with Wednesday and Thursday having its two largest case counts yet.
“I believe it is only a matter of five to six days, when some of these patients will begin to get sicker. And we will begin to see admissions to the hospital,” Peterman said. “If you’re a health care worker, that is very scary.”
Peterman urges the community to follow the guidelines established by the CDH, especially wearing masks in closed areas, hand washing and social distancing.
This story was originally published June 26, 2020 at 4:00 AM.
