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November 16

Herd immunity thresholds

When calculating herd immunity for a given disease, what geographical level is typically considered - a country, a city, etc.? Particularly, does it make sense to calculate herd immunity for city only? Say, if per Herd_immunity#Theoretical_basis, 82% of people are immune to pertussis, but only within a city, would it be considered safe even if on a country level the value is much lower? 212.180.235.46 (talk) 18:37, 16 November 2021 (UTC)[reply]

The idea behind herd immunity is that even as the virus or bacteria is still being transmitted, it is not being transmitted in a self-sustaining way. So if a person is infected, they will infect, on average, fewer than one other person. So the disease will die out. So then the question is, how to you define a herd? By the degree of isolation from other herds, one supposes. Is the population of a city a herd? I suppose it really depends on how much members of the population travel outside the city, and how many travelers visit the city. Abductive (reasoning) 04:28, 17 November 2021 (UTC)[reply]

Many experts now believe that Delta is so infectious (think measles), even with the best vaccines we will not achieve herd immunity, which I think of as human sacrifice. With lesser diseases, it's possible. Imagine Reason (talk) 16:58, 17 November 2021 (UTC)[reply]

^{[citation needed]} on that. Also, vaccines are not just about "never being infected". They also lessen the severity of disease, which appears to be the greatest value in the current pandemic; people are having many more asymptomatic or mild disease because of the vaccines. Regarding vaccines and the Delta variant, per the CDC: [1] Vaccines continue to reduce a person’s risk of contracting the virus that cause COVID-19, including this variant. Vaccines continue to be highly effective at preventing hospitalization and death, including against this variant. Fully vaccinated people with breakthrough infections from this variant appear to be infectious for a shorter period. --Jayron 32 17:19, 17 November 2021 (UTC)[reply]

I have read everywhere in the news some variation of the claim that vaccinated people, if infected, have less severe forms of the disease than non-vaccinated people; whether for the Delta variant specifically or for all strains of sars-cov-2. That claim seems plausible both from a biological point of view (the vaccine gives your immunity system a "head start" even if it failed to entirely prevent infection) and from a cursory look at the statistics (the ratio of hospitalization to cases, or deaths to hospitalizations, is lower for vaccinated people). Nonetheless, many plausible things are false, so I would very much like to see a proper statistical study (which the CDC page does not link to). Tigraan^{Click here for my talk page ("private" contact)} 14:11, 18 November 2021 (UTC)[reply]

Here is a synopsis from Johns Hopkins. Here is a short article published in JAMA which links to a NEJM published paper. Here is an article from Yale University citing a Lancet published paper. Here is a NEJM paper that comes to the conclusion "Most breakthrough infections were mild or asymptomatic, although persistent symptoms did occur." Here is an article from the University of Minnesota citing three more studies that show "COVID vaccines protect very well against the Delta (B1617.2) variant, with unvaccinated people having 5 times the risk of infection and more than 10 times the risk of hospitalization or death compared with vaccinated people." There's many more out there, as a google search will show you. --Jayron 32 12:06, 19 November 2021 (UTC)[reply]

Thanks. This paper one click away from your second link was the sort of thing I wanted, but could not find by an online search, because I did not know the keyword "breakthrough infection". It does shows lower viral load and lower symptoms, and I see no obvious selection bias in the methodology. Tigraan^{Click here for my talk page ("private" contact)} 16:47, 19 November 2021 (UTC)[reply]