By Peter White
NASHVILLE, TN — Epidemiologists at Vanderbilt University released a new report last week with some good news. Tennessee has reduced its transmission number to about 1.0. It refers to the number of new infections caused by a person who carries the virus. A transmission number below 1.0 for several days running is necessary to slow an epidemic in a population.
Using data from the Tennessee Department of Health, the Vanderbilt team found 
“significantly” fewer new cases from April 6 to April 13. The research team is led by Dr. John Graves, (see Anatomy of an Epidemic, Tennessean Tribune 4/16-22/20, page 2A)
“Tennessee’s recent progress, while real and positive, is fragile,” the report states. Tennessee’s latest statewide numbers compare favorably with Sweden and Singapore for the same time period.
Graves said that while Tennessee’s overall rate was about 1.0 and better than it was two weeks ago (1.4), it might not stay at that level. “Our analysis of the recent data indicates that without further changes to the transmission number, Tennessee may be settling into a ‘simmer’.” Graves said.
He said if it stays at 1.0, hospitalizations would remain stable going forward. If the number increases, hospitalizations would rise. If fewer people get infected, hospitalizations would decrease.
Governor Bill Lee has been talking with business, government, and health leaders for a couple weeks about when to relax the social distancing orders, and if so, by how much. The cases of Singapore and Sweden are instructive.
“During the week that ended April 6, their transmission numbers were very similar to Tennessee’s. However, the experience of these three geographies diverged dramatically in just one week. Sweden remained relatively stable with a transmission number around 1.2 – meaning cases are still increasing. Tennessee, as noted above, continued to have a decline in new cases and now has a transmission number around 1.0. Singapore saw its transmission number increase to above 2.0 and took steps to tighten its social distancing policies as a result,” the report said.
Singapore instituted stricter social distancing rules March 26. Its numbers have not dropped yet. Sweden’s numbers are still above 1.0 and the country reported 1,203 deaths on April 15. By comparison, Tennessee has about 2/3 Sweden’s population and reported 135 deaths on April 15.
At a news conference earlier this week, Gov. Lee announced the state’s stay-at-home order would end April 30. He said the state would begin reopening the economy in stages. “It’ll be smart, it’ll be safe, it’ll be measured but we do not have a definitive decision on what those steps will be yet,” Lee said.
Doing the math about the rate of infection and determining the fatality rate of a disease is a pretty simple calculation. It’s expressed as a fraction or a certain number of cases or deaths per 100,000 people. The problem is that we don’t really know how many people are infected by COVID-19.
It shouldn’t have been that hard to figure out but the federal response to the coronavirus was so late and so bungled, it may be many months before we get an accurate count. In Tennessee, widespread testing did not begin until last weekend.
“You pick a sampling of the community and look to see what the frequency of positives is,” said Dr. James Hildreth, President of Meharry Medical College.
“What you want to know is the penetrance of the virus,” he said. “Obviously, if that’s 1% that’s less cause for alarm than if it’s 5% or 10%,” Hildreth said.
He said the contagiousness of a virus depends on how fast it can replicate and is expressed as “Ro”. “It is calculated as follows: Ro = Bt. (B is the average number of times contacts are made that result in transmission and t is the average infectious period—the time a person can spread the virus.)
“Having people stay at home or wear masks reduces B and t is reduced by eliminating, isolating, or treating the infection. Airborne viruses like measles have high Ro values (12 to 18), i.e. every person infected by measles virus could infect 12 to 18 people. Viruses that require exchange of bodily fluids like HIV have low Ro values (2 to 3). Viruses that require contact with animals have very low Ro values like MERS (0.5),” Hildreth said.
New York has 55 deaths per 100,000 people and California has two. There are lots of reasons why that disparity exists. Tennessee has been relatively lucky. It has 2.1 deaths per 100,000.
Over the next several months, the CDC and the National Institutes of Health (NIH) will be testing to identify antibodies in the blood of people who were exposed to the virus. That will allow for a better approximation of total infections in various places. It will also help scientists understand the virus better and how best to prepare to fight it in the future.
During a remote press conference this week, Hildreth said researchers in Los Angeles used an antibody test on 900 people. They extrapolated the results to conclude some 400,000 people in Los Angeles county were infected and had no idea they carried the virus.
That is a very important finding. It means that there are probably a lot of asymptomatic cases and a very low infection-fatality ratio. That means the virus is not as deadly as some researchers have feared.
Scientists say the best-case scenario is for the U.S. population to develop herd immunity so that the virus cannot easily find a host to infect. A vaccine is the best way to do that but having one is months away.
Hildreth said the worst way to develop immunity in a population is to do nothing and let the virus run its course. Those who survive will have acquired immunity from COVID-19. Some residents of New York, where the number of deaths exceed 1,000, have probably already acquired immunity.
