With the decline of COVID-19 cases in some areas and the resurgence in other areas, what is the correct scientific position on “immunity” to COVID-19? To begin the analysis, let’s look at the concept of “herd immunity” on a societal level. Herd immunity occurs when enough people of a population obtained immunity against an infectious disease such that the transmission is decreased significantly or eliminated.
There is a mathematical formula that calculates the percentage to achieve herd immunity: herd immunity threshold = 1-(1/Ro) where Ro is the average number of new people a single infected person infects. For historical reference, the 1918 “Spanish Flu” Pandemic infected 500 million people worldwide, roughly one-third of the global population. It is very challenging to obtain Ro even with nine months+ of COVID-19 pandemic experience “under our belt”. In the scientific literature, a conservative range for Ro is 2-3 that translates to 50-67% infection to achieve herd immunity. At the same time, we have two illustrative examples from the infections that occurred on cruise ships. Because the people on the cruise ships were not allowed to leave for a certain period of time, these cruise ships became a human “petri dish” for COVID-19 infection to spread. The first example is the famous Diamond Princess (docked in Japan). Out of 3,618 people tested on-board, 712 people were infected (data from Wikipedia). If we assumed that only herd immunity stopped further COVID-19 transmission on board the cruise ship, then the herd immunity threshold is 20%. The second example is Celebrity Apex (docked in France). Out of 1,444 people tested, 224 people were infected (data from Wikipedia). The herd immunity threshold is 16%. Though these two examples are limited in “actual real world demographics”, they do give us a real working range for expected herd immunity against COVID-19.
Let’s turn next to assess qualitatively the transmission of COVID-19 in geographical locations globally. As discussed in previous post, https://sciencebetold.com/science-leads-to-victory/, there is a seasonal element to the spread of COVID-19 based on the Hope-Simpson model. In the “N. temperate” areas (areas north of 30°+ latitude) comprising the majority of EU countries (UK, France, Spain, Italy, Germany, Sweden, etc.) and Northeast states (New York, New Jersey, Massachusetts, Michigan, etc.), the COVID-19 infection curve has hit its end for season 2019! Based on antibody testing, the level of COVID-19 infection in those areas has not been reported higher than 20%.
Let’s turn lastly to scientific findings recently published on the presence of T cell immunity found in both COVID-19 patients and unexposed people. T cells play a vital role in our body’s immune response to infectious disease. In the study by J. Braun et al.,[i] they detected SARS-CoV-2 S-reactive (spike protein-reactive) CD4+ T cells in 35% of unexposed people that were found in COVID-19 patients. These T cells responded similarly with human endemic coronaviruses (common cold), demonstrating S-cross-reactivity probably generated from past exposure to endemic coronaviruses. These results are corroborated by the studies of R. D. de Vries et al.[ii] and M. Buggert et al.[iii] Furthermore, in a broad study looking for T-cell generated responses, J. S. Walz et al.[iv] detected cross-reactive SARS-CoV-2 T-cell epitopes in 81% of unexposed people. This evidence provides further support to a functional basis for “borrowed” immunity in SARS-CoV-2 from prior exposure to common cold coronaviruses.
Combining the recent scientific findings on “borrowed” immunity for COVID-19 and the human infection data collected, we have a clearer understanding on the following key points:
- “Herd immunity” of much less than 50%, as low as 15-20%, could slow down significantly or halt the spread of COVID-19.
- Because of the inherent “common cold” nature of COVID-19 virus, we expect a seasonal transmission of COVID similar to the common cold and the yearly flu.
- As a corollary to Point 2), a significant number of the population possess “borrowed” immunity for COVID-19 ALREADY from prior exposure to common cold.
- As a corollary to Point 3), we are not as dependent on effective vaccines and therapies to return to normalcy!
[i] doi: 10.1038/s41586-020-2598-9
[ii] doi: 10.1126/sciimmunol.abd2071
[iii] doi: 10.1016/j.cell.2020.08.017
[iv] doi 10.21203/rs.3.rs-35331/v1