This is where the virus is least deadly

Patterson Town Hall, (Wikicommons - Anthony22). Putnam County NY has the lowest IFR for COVID-19 in the United States.



SARS-CoV-2 is more virulent in southern Europe than in northern Europe. The reason, I’ve argued, is that the Mediterranean Basin is one of several regions where humans have coevolved for a longer time with crowded social environments. By "crowded" I mean not only proximity to other people but also proximity to domesticated animals. In such environments, which are prone to deadly pulmonary diseases like tuberculosis and pneumonia, natural selection may have favored susceptibility to infection by coronaviruses, which are normally mild in their effects, as a means to maintain a strong immune response to respiratory infections (Frost 2020). 

If we look at case fatality rates, Italy and Spain have been hit much worse than Germany, Switzerland, Austria, and Iceland. The United Kingdom falls between the two extremes, although a confounding factor is its large population of non-native origin (Singh 2020).

This pattern also shows up in a meta-study of infection fatality rates. Meyerowitz-Katz and Merone (2020) examined thirteen estimates of IFR from a wide range of countries. They came to two main conclusions:

- Mean IFR is 0.75% but varies considerably between countries;

- IFR has increased over time, being lower in February and March than in April and May.

Earlier estimates were based on the assumption that the average time lag between infection and death is two weeks on average. Actually, it's probably longer, perhaps a month. Later deaths may have thus been missed by estimates made in February and March.

If we look only at IFRs from April and May, the meta-study shows a north-south cline in the virulence of SARS-CoV-2:

Germany - 0.36%
France - 0.70%, 0.80%
Italy - 0.95%, 1.29%, 1.60%

For the same time period, the meta-study also presented three estimates from the United States:

New York City - 0.93%
California - 0.20%
United States - 1.30%

The last study provides estimates ranging from a low of 0.5% in Putnam County NY to a high of 3.6% in King County WA (Basu 2020). These numbers are so high because IFR is calculated only in relation to symptomatic cases. In my opinion, this study is not comparable to the others and should not have been included in the metastudy. It is nonetheless useful for charting the virulence of SARS-CoV-2 within the United States.

So why would the virus be less virulent in Putnam County NY than in King County WA? Let's consider the demographics in each case. The first county is 80% non-Hispanic White, 14% Hispanic, 3% Black, and 2% Asian. The second county is 65% non-Hispanic White, 15% Asian, 9% Hispanic, and 6% Black. Putnam County is whiter and probably less cosmopolitan than King County, which encompasses the Seattle area. This impression is strengthened by the voting pattern in Putnam County, which trends much more Republican than Democrat (Wikipedia 2020). The virus thus seems to be least virulent among "old stock" Euro Americans. I would also predict low virulence in Amerindian communities.

Virulence may also differ between west coast Hispanics and east coast Hispanics, as suggested by the difference between California and New York City. East Coast Hispanics are less often Mexican and more often Puerto Rican. They may thus be more vulnerable because they are more Mediterranean and less Amerindian by ancestry.


References

Basu, A. (2020). Estimating The Infection Fatality Rate Among Symptomatic COVID-19 Cases In The United States. Health affairs (Project Hope). 2020:101377hlthaff202000455.
https://www.healthaffairs.org/doi/full/10.1377/hlthaff.2020.00455

Frost, P. (2020). Does a commensal relationship exist between coronaviruses and some human populations? Journal of Molecular Genetics 3(2): 1-2.
https://researchopenworld.com/does-a-commensal-relationship-exist-between-coronaviruses-and-some-human-populations/

Meyerowitz-Katz, G. and L. Merone. (2020).  A systematic review and meta-analysis of published research data on COVID-19 infection-fatality rates.  medRxiv, May 18, 2020
https://www.medrxiv.org/content/10.1101/2020.05.03.20089854v2 

Singh, S. (2020). BCG vaccines may not reduce COVID-19 mortality rates. medRxiv April 11, 2020 
https://www.medrxiv.org/content/10.1101/2020.04.11.20062232v1

Wikipedia (2020). Putnam County, New York.
https://en.wikipedia.org/wiki/Putnam_County,_New_York 

Does a commensal relationship exist between coronaviruses and some human populations?

Nanjing Road, Shanghai (Wikicommons - Stephen Codrington). Populations with a long history of social crowding may have become more susceptible to coronavirus infection.


I've published a paper on coevolution between coronaviruses and "crowded" social environments. Comments are welcome. Here is the abstract:


Coronaviruses enter lung tissue via the ACE2 receptor, which varies structurally among human populations. In particular, the Chinese population has fewer variants that bind weakly to the coronavirus S-protein. This global variation suggests that the ACE2 receptor has coevolved with different environments, some of which have favored susceptibility to infection of lung tissue by coronaviruses. 

It has been argued that respiratory viruses boost the immune response of lung tissue and thereby prevent more serious pulmonary diseases, like tuberculosis, pneumonia, and pneumonic plague. This preventive effect has been shown with other viral pathogens, notably γherpesvirus 68 and cytomegalovirus. Some human populations may have therefore gained protection from severe respiratory infections by becoming more susceptible to mild respiratory infections, such as those normally caused by coronaviruses. 

This commensal virus-host relationship would have been especially adaptive wherever respiratory pathogens could easily propagate, i.e., in crowded environments, where many people live in proximity not only to each other but also to animal sources of infection. In regions that have long had crowded environments, natural selection may have favored susceptibility to infection by coronaviruses, which are normally mild in their effects, as a means to maintain a strong immune response to deadly pulmonary diseases.


Reference

Frost, P. (2020c). Does a commensal relationship exist between coronaviruses and some human populations? Journal of Molecular Genetics 3(2): 1-2.
https://researchopenworld.com/does-a-commensal-relationship-exist-between-coronaviruses-and-some-human-populations/