Yesterday a friend asked me how this plays out and, as I tried to explain the endgame, I realized how much not understanding it was playing into everyone’s panic. Knowledge is our only weapon here. The problem is, no one alive today, has ever dealt with a serious, highly contagious disease with no cure and no vaccine. As I will explain below, right now, the disease is our vaccine. So, any anti-vaxxers out there should play close attention to what’s happening now and what unfolds over the next couple of months.

Our best reconstruction of events suggests that, on November 17, a virus that had never infected a human before, slipped into the cells of 55-year-old man in Hubei, took over the machinery of those cells, and began to replicate itself. From that point forward, each person that interacted with him or with the trail of droplets he would soon be leaving in his wake had a chance of becoming infected. The chance of new cases was driven by the number of people that entered his sphere and the likelihood that each of those people, because of the nature of their interaction with the man and his droplets or because of their response to the virus, would become infected. Each time one of them became infected, that process of spread began again, continuing until that person either recovered or died. The number of people each case infects depends on the virus, the person, and the circumstance.

Epidemiologists put a lot of effort in to calculating the average number of new infections generated by each case. They call it the basic reproduction number (R0). R0 for the Covid19 virus seems to be about 2, which means one case becomes two, becomes 4, etc. To stop the spread of a viral disease, R0 must be less than one. The goal of infection control is to make R0 as low as possible, ideally less than one.

There are only two ways to control spread. Limit the number of people who contact each new case or limit the likelihood that contact will become infected. Our first line strategy has been to try to isolate new cases completely. This only works if you identify cases quickly and/or the disease does not spread easily. Neither is true in this case. The other strategy, is to alter behavior (i.e., all of the handwashing and avoidance of touching) and keep people apart. It is, of course, neither possible nor desirable to shut down human interaction completely, so the disease will continue to spread, just more slowly. Epidemiologists and other experts are coming to accept that we will not get R0 below 1.0 in the near term.

Once a highly infectious disease is truly loose in the population, as Covid19 appears to be, the only endgame is herd immunity. As people become sick and eventually recover they become immune. That means that sometimes a person, who would otherwise have become infected by a new case, doesn’t. As that happens, R0 begins to drop. If it happens enough to make R0 less than one, the disease dies. (Note that this is why those who don’t vaccinate put everyone at risk by lowering herd immunity and increasing R0 for serious diseases.)

Herd Immunity

There are only two ways to get immunity: vaccination or disease. We can stop waiting for a vaccine to save us. (No Mr. President, not even with a really solid flu vaccine.) I expect we will eventually have a vaccine, but, for now, we are simply waiting until enough people have developed immunity that we can return to normal life without causing significant outbreaks. We actually do this all the time with diseases like the common cold that don’t have serious or deadly consequences. But Covid19 is clearly different. What we don’t want, is for that to happen so quickly that medical facilities are overwhelmed, as is happening in Italy right now. This is the so-called flattening of the curve and is the best we can hope for. 

There is a growing consensus that this will take a year or more to play out. This is actually a good thing. by spreading the impact out, we avoid overwhelming our hospitals and creating the devastating situation now faced by Italy. It will also create time for us to improve treatment and, ultimately, develop a vaccine. In the meantime, play it safe and play by the rules. Hopefully that will keep you from becoming vaccinated by the disease and becoming part of the herd.