R.t — the effective infection rate. Update; changes in social distancing have yet to worsen infectivity.

I am reposting this Blog because I have an update with data from May 26, 2020 at the end. I think it is important given the reductions in stay at home orders and violations of social distancing observed over Memorial Day weekend.

“R.t” is a commonly used measure to describe the expansion of infections during the COVID-19 pandemic. It measures the average number of people who become infected from a single, infected person. The higher the R.t is the quicker the virus is spreading.

If R.t is above 1.0, the virus is spreading faster than the rate needed just to sustain the pandemic. If R.t falls below 1.0, the pandemic actually shrinks because less than one person is infected by each infected person. R.t therefore provides an easy measure to see if the pandemic is expanding or shrinking.

Instagram founders Kevin Systrom and Mike Krieger have launched a new website called Rt.Live. It provides daily information on the R.t numbers for COVID-19 broken down by state. On April 1, the pandemic was spreading quickly, with almost all states having R.t greater than 1.0.

Rt April 1. Most states had rates greater than 1.0 (Red)

Of interest, on April 21 the country was split with about half the states below and half above 1.0.

Rt April 22; Green starts are below or at 1.0, Red states are above.

Most interesting is that some of the hardest hit areas, such as New York Connecticut and Michigan, now have some of the lowest Rt values. In contrast, a few states that were doing the best (Washington State) now have some of the highest R.t. Also, states without shelter in place orders were evenly split between sub 1.0 (Arkansas and South Dakota) and higher (Nebraska 1.8 and North Dakota 2.2) R.t numbers.

While these findings suggest there has been an overall decline in the expansion of the pandemic, the reasons are not clear. Shelter in place would seem to have an effect, but not consistently. It is possible that in NYC and Detroit so many people are infected that the R.t had to decrease. Let’s hope that the positive trends continue. At least this website gives a snapshot of how we are doing.

Rt values for every state on May 26, 2020. This still shows marked improvement from April and is improved or at the worst unchanged from the prior two weeks.

An update. The most recent Rt values show continued improvement despite reductions in stay at home orders and social distancing. It may be that the effects of those changes are not yet evident, but it may also be that the change in conditions after the initial wave of infection has made for smaller outbreaks when they occur. I will discuss the “K” factor that may be responsible for this tomorrow.

Published by jbakerjrblog

Immunologist, former Army MD, former head of allergy and clinical immunology at University of Michigan, vaccine developer and opinionated guy.

3 thoughts on “R.t — the effective infection rate. Update; changes in social distancing have yet to worsen infectivity.

  1. Excellent. Thanks for the news about Rt.Live! To the extent it is accurate, helpful information to get a sense of trends.

    In a tweet by @rosnovsky, apparently the death count had deviated significantly from CDC official numbers (https://washingtonpost.com/graphics/2020/national/coronavirus-us-cases-deaths/) and what other outlets report (https://washingtonpost.com/graphics/2020/national/coronavirus-us-cases-deaths/, https://bnonews.com/index.php/2020/01/tracking-coronavirus-u-s-data/ for instance). Perhaps, only reporting what states report is not very reliable.

    The COVID Tracking Project @COVID19Tracking responded:
    The main discrepancy is in how New York City and New York State are reporting presumed COVID deaths. Both methods are defensible, just different. It’s a tough call, as the numbers have become highly political, but our preference would be that states report them both. States are where public health happens, so impt to see what they report -@alexismadrigal

    Another tweet by @tatertothead noted:
    Data grades need to be adjusted down for States that have not tested all of their citizens.

    Liked by 1 person

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