Currently, there is a very small number of Americans that have gained immunity from vaccination trials.  The FDA would probably approve the Pfizer vaccine by Dec 10th and the Moderna one by Dec. 20th.  By the end of this year, Pfizer and Moderna would have supplied enough vaccines to permit 20M Americans to have had their first shot.  By early next year, 19M Americans would have gained immunity from these two shot vaccines (taking into account the current estimated 95% effectiveness of these vaccines).  Many people estimate that enough vaccine would be available to vaccinate 25M-30M Americans every month in 2021 subject to logistical and psychological constraints (deep blue area in the graph below).  Thus vaccine “herd immunity threshold = 230M” could be reached in the USA by the end of summer. 

Herd immunity could actually be reached sooner than that.  Currently over 14M Americans have gained immunity from being infected by SARS-Cov-2.  The immunity may not be perfect but seems to be at least 99% effective for 6 months.  We project that by yearend 19.5M Americans will have tested positive for COVID-19 (red area).  Due to testing bottlenecks and scarcity, there is a hidden population of asymptomatic and mildly symptomatic cases that we estimate to be twice as big as confirmed cases for a total of 42M currently immune and 55M immune by yearend (red+green areas).  The total infected has been estimated to be as high as 8x by the CDC or 111M currently (red+green+purple areas).  We consider this to be highly unlikely for the USA as a whole even though antibody surveys for small sections of the USA have ranged from 2x to 20x the confirmed population.  If we adopt our best guess estimate for total immunity, we project that it will exceed the threshold for herd immunity by mid-year and stop exponential growth in the USA.     

The CDC has established guidelines for prioritizing vaccinations among different groups of people (healthcare workers, and residents of long-term care facilities). But they omitted to spell out who in those groups should be vaccinated first.  In fact, we believe that everyone should be tested for antibodies before they are vaccinated.  This would allow scientists to:

1.    optimize the distribution of an initially scarce vaccine to those who really need it,

2.    provide a baseline and periodic retests to measure how effectively these new vaccines are producing antibodies

3.   provide us with detailed measures of the true population of infectees.