“The transmission of SARS-CoV-2 could resemble
that of pandemic influenza by circulating seasonally
after causing an initial global wave of infection.”
A recent mathematical simulation study from the Harvard Chan School of Public Health suggests that our current pandemic could follow the path of the 1918 flu pandemic, i.e. recurrent surges of infections after quarantine measures are relaxed.
FACT: SARS-CoV-2 was, and is, NOT containable. Each infected person transmits the virus to 3 other people on average. Most people infected with this virus will have mild cold-like or flu-like symptoms. In fact, two other known coronaviruses are the second most frequent cause of colds. Unfortunately this coronavirus of SARS-CoV-2 can cause severe illness and even death in some per cent of the infected.
FACT: “Flattening the curve” through social distancing and isolation does not decrease the number of infected people. It’s sole purpose is to change the timeline of illness to reduce the peak demand for services for the severely ill (estimated 3% hospitalized). It is hoped that spreading that demand over time for ICU services (estimated about 1%) will reduce the number of deaths due to “lack of available resources”.
FACT: There are really only two ways to stop an pandemic, a vaccine or herd immunity. Immunity develops when individuals get infected, respond by making antibodies, get better, and end up protected from getting the illness again. Herd immunity exists when so many people have developed antibody protection that the few people who don’t have such immunity are surrounded by others who cannot have the disease again, and so can not transmit it to them. (There is a third way an epidemic stops, of course, where most people die before they can pass it on, like Ebola, but this virus is not like that.)
You have heard about herd immunity for children unvaccinated against measles (another very highly contagious virus) when they are surrounded by children who have been vaccinated against measles. Herd immunity works by lowering the risk of anyone being exposed to a person with the infection. I can remember the “last of the chicken pox parties”, another method to expedite herd immunity in a timely fashion, in my old neighborhood in the 80’s.(The chicken pox vaccine became available in 1984). An epidemiological term for herd immunity may be more intuitive, though it sounds a whole lot colder; “depletion of susceptible individuals.” The estimated herd immunity threshold for measles is 95% of the population vaccinated. The ideal is often stated as 98% vaccinated against measles.
SPECULATION: A team of scientists from Harvard using mathematical model simulations have diagramed how herd immunity might develop for SARS-CoV-2 depending on different durations of social distancing while we wait the 12-18 months necessary for vaccine development and testing. Their study of other coronaviruses (the common cold ones) indicates that a herd immunity threshold of 60% would reverse the epidemic, BUT that different durations of social distancing could result in a HIGHER number of total cases.
I, as a pediatrician, don’t do math very well, so I’ll let the conclusions of their mathematical model simulations speak for themselves. These estimates assume rigorous social distancing and that about 80% of us will eventually be infected.
No social distancing – 60% herd immunity reached in October, no peak delay, no “flattening of the curve”.
4 weeks of social distancing (to mid May) – peak is delayed, 60% herd immunity threshold is reached in October, total number of cases are 10% higher than “no social distancing”.
8 weeks of social distancing (to mid June) – peak is delayed, 60% herd immunity is reached in November with 15% higher number of total cases than “no social distancing”.
12 weeks of social isolation (to mid July) – markedly reduced new cases during the summer, 60% herd immunity in January, and 20% higher number of total cases than “no social distancing”.
20 weeks of social isolation (to late August) – markedly reduced new cases through November, 60% herd immunity in February, and 20% higher number total of cases than “no social distancing.” (1)
The mathematical model predictions represent infections only, not amount of severe illness or death rates.
I know just enough mathematics to appreciate that there are lots of assumptions incorporated into mathematical modeling, just like CFO reports to Boards of Trustees, you know , as in “smoke and mirrors”, but these predictions are sobering. The researchers suggest that periodic, intermittent relaxation of social distancing might give the best results in the long run, i.e. allow and deal with recurrent surges of infections from relaxation of restrictions until most of the population becomes immune.
“Intermittent social distancing might maintain critical care demand within current thresholds, but widespread surveillance will be required to time the distancing measures correctly and avoid overshooting critical care capacity.”
Thankfully for us mathematically impaired the study stated some conclusions in English :
SARS-CoV-2 can proliferate at any time of year.
The duration of immunity to SARS-CoV-2 is not known. If immunity to SARS-CoV-2 is not permanent, it will likely enter into regular circulation.
High seasonal variation in transmission leads to smaller peak incidence during the initial pandemic wave but larger recurrent wintertime outbreaks.
New therapeutics, vaccines, or other interventions such as aggressive contact tracing and quarantine – impractical now in many places but more practical once case numbers have been reduced and testing scaled up – could alleviate the need for stringent social distancing to maintain control of the epidemic.
Then they blur that bit of hopeful glance into the future with, of course, “virus mutation might give us a new SARS-COV every winter season just like the flu . . . until 2024”!
Enough of these not-so-fun medical facts. Let’s finish with some good news.
The recovery rates from this virus as of today are 78% in the U.S.
For even more good news (on a variety of subjects) tune into SGN – SOME GOOD NEWS , a brand new weekly YouTube news program by John Krasinski.
1. Boston Globe, April 12, 2020, Ideas Section K; includes excellent graphs of the different social distancing duration scenarios.