Congratulations, fellow Kentuckians! Through hard work, listening to the Gov, and doing the right things, you got your R nought under the magical number of 1 – unlike all your neighboring states, whose R noughts are still over 1. Good for you!
Completely confused? Let me explain.
What is R nought, and why do we care?
When epidemiologists measure the impact of a disease, one of the measurements they use is the “basic reproduction number,” which is denoted in research as R0 and usually referred to as “R nought.” It is defined as “the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection.”
So, if the R nought of a particular disease in a particular area is right at 1, then one person having the disease will infect one other person. The disease is not growing, nor is it shrinking; it is static.
If, on the other hand, the R nought is above 1, then that one person will infect more than one person, and they will also, and so on. The disease is growing. If the R nought gets to 2, then the disease is doubling, and you could be in danger of a runaway disease.
What you want, of course, is for the R nought to be under 1. That means that one person will infect less than one other person; the disease is shrinking over time.
How do you get to an R nought under 1? Vaccines can do it; so can herd immunity. But if you don’t have either of those (and we don’t yet), then you have to lower the chance of infection through cleaning, social distancing, less movement of people so you have less contacts, and things like wearing masks. In other words, all the things we’ve been doing.
The Imperial College study of R noughts and mobility
The COVID-19 Response Team at the Imperial College in London did a study of the infection rates in the United States, and what effect increased mobility and loosening of restrictions would have on those rates. They broke their work out by states, but also did a summary view of what they expected for the U.S. over the next two months.
They note that their study doesn’t take into account any additional measures that may be taken even as states open up. They assume that the increase in mobility will increase contacts, and thus increase the rates of infection.
Their opening summary is fairly grim, even written in research-speak:
Our estimates suggest that the epidemic is not under control in much of the US: as of 17 May 2020, the reproduction number is above the critical threshold (1.0) in 24 [95% CI: 20-30] states. Higher reproduction numbers are geographically clustered in the South and Midwest, where epidemics are still developing, while we estimate lower reproduction numbers in states that have already suffered high COVID-19 mortality (such as the Northeast). These estimates suggest that caution must be taken in loosening current restrictions if effective additional measures are not put in place.
We predict that increased mobility following relaxation of social distancing will lead to resurgence of transmission, keeping all else constant. We predict that deaths over the next two-month period could exceed current cumulative deaths by greater than two-fold, if the relationship between mobility and transmission remains unchanged. Our results suggest that factors modulating transmission such as rapid testing, contact tracing and behavioural precautions are crucial to offset the rise of transmission associated with loosening of social distancing.
Overall, we show that while all US states have substantially reduced their reproduction numbers, we find no evidence that any state is approaching herd immunity or that its epidemic is close to over.
And in case you missed it, let me repeat that center point: if we increase mobility (get out more) and don’t do anything else differently, we could see twice as many people die in the next two months as we have had die up to this point. Since the current number of deaths in the U.S. is about 90,000, they are saying that if their model of R nought rates is correct, we could see 180,000 MORE deaths by the middle of July.
Kentucky’s numbers and our future
The thing that originally drew me to read this report was the breakout by states. We THINK we’re doing pretty good, but are we really? And how are we doing compared to the states around us?
As the following graph shows, not only have we pulled our R nought below 1, we are the only state in our area except West Virginia to do so. (For some reason they’ve got us in the “Southern Appalachia” group. I guess they didn’t get the memo that we are now in the Midwest.)
Initially, they estimate we were at about 1.75, but now have dropped below 1.00.
And here is a graph showing our work at lowering our infection rate over time:
The wide bars indicated the confidence levels and minimum/maximum levels, so as of their last estimate the worst case for Kentucky’s R nought is just above 1.0, and the best case is about 0.5.
This is Good Stuff, and shows that our work as a state to “beat the coronavirus,” as Governor Beshear says, is paying off.
But what about the future?
Of course, the study wasn’t just about the past. It was primarily put together to predict what happens across the United States if we loosen restrictions and mobility increases, thus increasing contacts and increasing the infection rates.
Here are three graphs that show what could happen in Kentucky:
If we keep everything as it is now (the current level of Healthy at Home), then we get the blue line. If 20% of us return to the level of mobility and the number of contacts pre-COVID, then we get the yellow range. And if 40% of us “go back to how it was,” we get the pink range of results.
Worst case in this model? We see 6,000 new cases in the state PER DAY by the second week of July, and almost 30 deaths PER DAY.
Of course, as the researchers note, these are based only on increased mobility. IF we do the other things, ESPECIALLY wear our masks, we may be able to increase our mobility without seeing these graphs become reality.
The bottom line
There are three basic conclusions we can take from this study:
- Our work to “beat” the coronavirus has worked so far, and in fact, is working better than most of our neighboring states. Good for us.
- If, however, we take our eyes off the ball and assume “it’s all over,” we could see that dreaded second spike, with our R nought surging past 1.0 and hot spots all over the state.
- The way to keep our infection rate low, even as we begin to loosen restrictions, is to test, trace, and especially follow all the guidance from our healthy professionals, so we lower the contacts and lower the possible spread of the infection.
So there you go, Kentucky – you got our R nought below 1, and you beat back the virus. Now, the message is simple:
Don’t blow it.
–30–