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Baylor College of Medicine

Beware the Siren Song

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Sept. 30, 2020

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Dear Members of the Baylor College of Medicine Community,

Beware the Siren Song

"First you will come to the Sirens who enchant all who come near them. If anyone unwarily draws in too close and hears the singing of the Sirens, his wife and children will never welcome him home again, for they sit in a green field and warble him to death with the sweetness of their song." – Homer, The Odyssey

Ulysses was a reluctant but skillful warrior, condemned to command a sea voyage of uncertain outcome and duration. Among his challenges was an encounter with the Sirens. In calm summer seas, the sweet, irresistible song of the Sirens lured sailors causing them to dash their ships upon the rocks.

Fortunately for Ulysses, he had advanced warning of this danger, and took the sensible precaution of packing the ears of his crew with wax and ordering them to lash him securely to the mast. Like Ulysses, we are reluctant warriors in this SARS-CoV-2 battle. We are on a journey of unknown duration. Who are our Sirens?

Our COVID-19 numbers look very good, directionally the same as I reviewed last week. Our effective reproduction rate has been less than 1.0 (virus is receding) for 20 days. New cases have dipped into the 200s and approaching levels we have not seen since last April. Test positivity rate (rough prevalence estimate) has been below 5% for three weeks. New hospitalizations are trending down, albeit more slowly than I would like. We saw no Labor Day surge. Good, good, good.

We appear to be sailing on calm seas. So why do we keep hearing from so many masking and distancing nags (myself included)? Because even though things appear to be going well now – and they are – we must remain wary of our Sirens:

  • Unawareness of the experience in the global community. We have recently seen a recurrent scenario play out across the world. Early in the pandemic, a city/region/country took decisive action, achieved excellent viral control, and received accolades as a model for other to follow. Recently, as those communities have started to reopen, they have seen the virus come surging back, and have been forced to re-impose substantial economic shutdowns. A common root cause of the resurgence: indoor aggregation of people in bars, and is some cases, social gatherings in homes. To replow old ground – the nature of this virus has not changed. If we let down our guard, fail to mask and distance, begin to return to crowded environments, we will see a COVID-19 resurgence. It is worth spending some time reading about the experience of others - QuebecIsraelLondonMarseilles to name a few. All the reporting highlights on how dispiriting this experience has been to people in those communities. Having lived through one major surge in Houston – having seen the health, societal and economic impact of our surge – I think most would join me in my desire to avoid another COVID-19 crisis.
  • Failure to appreciate the stealthy nature of exponential growth. Our new cases in Houston look good. Hospitalizations are flat. Why worry? Because in an immunologically naïve community – absent masking and distancing – disease will spread exponentially, and this growth is insidious early on. Here is an easy means of explaining exponential growth to others, with minimal math (you do need to multiply). Assume you have an effective R-value of 2.0. One individual will, on average, infect two others (1X2=2). Those two, in turn, will infect two others (2X2=4). It helps to get a piece of paper and write this down:
    -- 1st doubling 1X2=2
    -- 2nd doubling 2X2=4
    -- 3rd doubling 2X4=8
    etc., etc.

Continue this for a total of 10 cycles, and your answer should be 1024.

So, if you look at your numbers for each doubling, early on it does not look like a big deal. Two becomes 4, 4 becomes 8. But by the 10th doubling, 512 becomes 1024, and it feels like it is exploding. That is the nature of exponential growth – it starts slow, and accelerates. It is easy to find comfort in the early part of the growth curve; by the time the steep part is apparent, the infectious horse is out of the barn. It is too late to contain.

We should be encouraged by our current flat/declining numbers – as Quebec was – but not be lulled into complacency.

  • Hope we are at or approaching "herd immunity." Herd immunity is the point at which enough of the population is immune that it becomes difficult for the virus to spread. If an infected individual walks into a place of worship, school or restaurant, and no one is immune to the virus, it will spread easily to others.

On the other hand, if many or most already have immunity, the virus has difficulty finding someone to infect, and it fades for lack of new hosts (as an aside, this effect can be mimicked by effective masking and distancing). No one is sure what percentage of the population needs to have immunity to achieve this effect, but most estimates I have seen are between 50-70%. Part of the ultimate goal of vaccination is to produce herd immunity, without a lot of people actually developing the disease.

Again, let us do some simple math.

For the purposes of this exercise, I am going to make very optimistic assumptions. Reality is probably worse.

There are approximately 325 million people in the U.S. Let us assume individuals under 18 (about 75 million) cannot be infected, nor spread the virus. This is not a valid assumption, but we are being optimistic. That leaves us with a "herd" of 250 million people. If we assume the lower end of the range to reach herd immunity – 50% – 125 million people would need to be infected. The mortality rate for COVID-19 is in the range of 0.5-1.0%. We will assume the more optimistic 0.5% rate. At that rate, infection of 125 million people would result in 625,000 deaths – about three times the number of all Americans that have died so far.

There is an active debate among thoughtful and intelligent people regarding at what level we will achieve herd immunity. For an example, refer to a recent article from The Atlantic. Theories abound regarding factors that may drive the herd immunity percentage down: differential susceptibility of sub-populations, chaos theory, differential importance of cellular immunity. Perhaps our trigger is lower.

Our Siren here is the word "perhaps." However valid the theories, they are still theories and belie some actual observations of spread that occurred early in the pandemic, such as on the Diamond Princess cruise ship. Personally, I am not willing to accept the risk of larger scale death or disability when the alternative is simple – mask and distance as a bridge to the vaccine. Availability and use of an effective and safe vaccine will ensure protection, and so poses the best long-term strategy. 

Houston should pause for a moment of self-congratulation. We have pulled together as a community and done a very effective job combating SARS-CoV-2. We are in a position to begin to relax restrictions, and to incrementally resume more of our prior activities. We must all maintain our attention to proven viral control practices – masking, distancing, avoiding aggregation in crowds. We need to be thoughtful about our community priorities (I would vote for reopening schools successfully before bars).

It is time to lash ourselves to our mast, resist the allure of the Siren's song, and sail on to the end of this pandemic.

James T. McDeavitt, M.D.
Senior Vice President and Dean of Clinical Affairs

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