Metric for predicting COVID-19 steps into spotlight
PARKERSBURG, W.Va. – The R0, or reproductive value, is a calculation of the average number of people infected by a single case of a disease.
Recently coming into the national conversation as a metric for predicting the spread of the novel coronavirus, the R0 (pronounced “R-naught”), and variations of it, are based on known factors about the disease as well as variables in the environment and the behavior of those infected.
Though the numbers are determined through complex calculations, the overall meaning is pretty clear.
“Anything over one means the virus is spreading, and anything under one means the virus is reducing itself,” Dr. Clay Marsh, who is heading up the state of West Virginia’s response to the COVID-19 pandemic, said of the R0 value.
While West Virginia hasn’t had nearly the number of cases or deaths as other states, at one point last week, Marsh said, its R0 value was the highest in the nation.
That’s one of the numbers state officials monitor to determine the effectiveness of actions taken to reduce the spread of the virus.
After West Virginia allowed many businesses and activities to reopen and resume, the number of positive cases has climbed throughout the last two weeks. This prompted Gov. Jim Justice to issue an order requiring masks in indoor locations, lower the limit on public gatherings and close bars in hard-hit Monongalia County for a 10-day period.
Initial estimates for the coronavirus R0 number reflected what might be without any precautions in place, said Angela Consentino, epidemiologist for the Division of Public Health and Community Services for Nashua, N.H.
“Based off the beginning of this pandemic, the reproductive number looked to be between 2.5 and 3, although some research estimated that the R0 could be as high as 6,” she said. “Essentially, this means that every one case will infect on average up to three people without any precautions in place.”
Precautions including social distancing and wearing masks or face coverings are factored into calculations as time goes on. Nashua health officials use a similar measure to track potential spread by looking at the average number of close contacts for a person who tests positive for COVID-19.
“As of right now in Greater Nashua, each case has on average about 1.7 contacts,” Consentino said. “This number had been less than one during the stay-at-home order, but it has increased with reopening activities.”
Close contact is defined as being within 6 feet of someone who has tested positive for 15 minutes or more. Most of these individuals live in the same home as a person with a confirmed case and are at high risk of developing the virus, so they’re monitored daily by public health, Consentino said.
“This number is important for us to keep track of because it is a good indication of the spread of the virus in the community,” she said. “For us here in Nashua, the change in the number of contacts per each case over time has shown us that social distancing and other prevention efforts work.”
Marsh said West Virginia uses a similar calculation, R-t, to determine the rate of transmission in the state and individual counties. At this point, those numbers are not publicly available through the state’s online COVID-19 dashboard because they’re still a work in progress, he said.
“We’re continuing to try and work to make sure that that’s very accurate,” he said.