COVID-19 Numbers and Hope

Summary First: There is hope, hope with many “ifs”, but hope. Good daily data is hard to come by, but new “cases” reported in China are very low or zero now. There is hope in that fact.

Worldwide, as reported on Saturday, March 21st, nearly 90,000 “known cases” have recovered. “Recovered” suggests there are 90,000 people out there with good antibodies against this disease–and those are only the folks we know about, people with diagnosed illness who were tracked.

IF, as suspected, there are many in the general population of the world who have been exposed to SARS-CoV-2, people who never had symptoms or only mild symptoms that didn’t bring them to medical attention, and IF those folks developed good protective antibodies, then the future is not nearly as bleak as many of us currently fear.

Why? Epidemiologists current best suggestion is that each infected individual is passing the disease on to 2.2 others. That is the Basic reproduction number, the R0, for Covid-19. (Click that link if you like numbers.) Postulating random transmission with that Rof 2.2 suggests that once 55% of the population is immune, the disease will cease to spread.  That’s an ideal world number but it offers this hope: If immunity (which we are not equipped to broadly measure for this or any other virus because it’s expensive and time-consuming) is growing among asymptomatic individuals in the world population, then this scourge has an end point–herd immunity–when 55% of the populace is immune. The other 45% remain vulnerable–but are still relatively protected by the immunity of the herd.

This hope in no way suggests we should abandon our efforts to “flatten the curve.” Flattening the curve still buys time. Flattening the curve will decrease the number of people who will be diagnosed, suffer, and die of this disease on our way to herd immunity while that immunity quietly builds.But maybe, just maybe there is light at the end of this tunnel–as the Chinese experience may now be demonstrating. We still have quite a knothole to get through to get there.

Here’s the much less condensed version of that same argument, along with a lot of other thoughts and speculation:

In an imaginary world in which all human interaction were totally curtailed for fourteen days there is evidence to suggest the Covid-19 might stop dead in its tracks. Going through that thought experiment and its assumptions might be useful.

Some number of virus particles enter the nasal passages of a host human. Some of the virus particles gain access to the cytoplasm of some number of the host’s cells (cytoplasm is the innards of the cell, innards bounded by the cell’s outer membrane). In the cytoplasm the virus hijacks the cell machinery to the virus’s own use: making thousands or millions of copies of itself until the cell bursts and releases new virus particles that infect adjacent cells. Unchecked, it is easy to see how this process might quickly overwhelm the host. (Notice that the numbers of virus particles to which the host is exposed might be an important variable in this ramp up.)

The immune system of nearly every host will start to make antibodies that destroy viral particles. It is a race. If the host’s immune system is up to the task, the antibodies will win. The host human will get better or may not even have had any symptoms while this is all happening. In this ideal world the antibodies will remain available and on high alert to kill any new arrivals of SARS-CoV-2, i.e. this patient is immune, he(she) is no longer susceptible to a new presentation of virus (his antibodies kill any virus that manages to replicate in his cells). This person no longer sheds any intact viral particles that can infect someone else. In a population of host humans eventually there are so many hosts who are immune that the virus cannot propagate–that’s called herd immunity. 

We think we know the following about SARS-CoV-2:

1) The incubation period (the time between exposure and the first symptoms) is around 3-6 days.

2) In a very small, but very detailed, study I read within the first six days after the onset of even very mild symptoms these patients were making effective antibodies to the virus, antibodies that damaged viral particles badly enough so they are not infective.

3) In the same study, shedding of large numbers of virus particles occurred before there were notable symptoms–that’s why the disease spreads so fast–we don’t know who is harboring and spreading the virus. But viral shedding of infective particles ceased as the antibody response took over.

KEY POINT: Testing for immunity (presence of effective antibodies) can be done but it is very expensive and time-consuming. Such a test cannot possibly be used broadly to tell if a given individual human host has made antibodies and is immune. It would be nice, but that is a long way off, much further off than a vaccine–and a vaccine available for widespread use is likely a year away at the very least.

In this imaginary and ideal world everyone would be strictly quarantined for fourteen days, during which each and every exposed person would incubate the virus, shed virus (to which no one would be exposed), and mount an effective antibody response that would destroy the infective power of any virus that remained. (Some other hosts would, in roughly the same period, die and their remnant virus would be incinerated with their corpse.) Finally, all those immune folks with great antibodies, some of whom weren’t even aware they had been ill, would be released into population and contribute to herd immunity, just in case there were a few bits of virus still harbored somewhere.

In this imaginary and ideal world the whole epidemic would be over and life would magically return to normal.

Too much to hope for? Probably. Nonetheless, we have the Chinese example, something the rest of the world should watch very carefully. With 17,836 cases as of last Friday evening we in the U.S. were roughly comparable (on a per capita basis counting the entire population of the U.S. as the basis) in “case” numbers (with all the caveats that go into counting a “case”), we were roughly comparable to the point at which Wuhan shut down–and Wuhan shut down with the sort of clang and determination that probably only an authoritarian society can muster.

Regardless, Wuhan sharply flattened its curve and, insofar as the Chinese are vigilant about detecting and pursuing contacts when cases reappear, their effort offers some hope–but only if we work together, recognize the gravity of the situation and the importance of isolating ourselves for several weeks. In the absence of martial law we are going to not only have to work at social distancing–we’ll need to engage in a group effort to socially shame those who flaunt their independence from the group effort.

Keep to the high ground,

P.S. I fear we, the United States, are the Wuhan of the following quote from epidemiologists Dr. Helen Jenkins of Boston University, and Dr. Bill Hanage of Harvard:

It’s really instructive to compare what happened in Wuhan and what happened in Guangzhou. In Wuhan, they shut down when they had 495 cases in a city that is roughly the size of New York. In neighboring Guangzhou, by contrast, they took action when there were seven cases. The epidemic curve in Guangzhou was completely manageable. As we all know, in Wuhan it was absolutely catastrophic. 

P.P.S. There are many things that could mess up (or enhance) my imaginary and ideal world, factors beyond our societal skepticism and unwillingness to conform. There are many unknowns: How long will the observed antibodies offer immunity? Does the virus mutate like the influenza virus does and so might it return in another wave another year? Are there people who will continue to shed infective virus while remaining entirely asymptomatic like “Typhoid Mary” spread the salmonella bacterium that causes typhoid fever? On the flip side, what portion of the population will make antibodies and become immune to SARS-CoV-2 without ever knowing they’ve been ill? There are abundant opportunities for research–if proper funding is restored to these efforts.