Quoting Jonathan Del Arroz (jdelar...@gmail.com): > I’ll just host a party for the unvaccinated instead, since studies are > showing we spread the new variants less, anyway. Mine should be safer!
Nope. Your knowledge of epidemiology needs repair, Jon, as you've been misinformed. There _aren't_ "studies" showing that unvaxed spread Delta and successors less. That is wrong twice over: Unvaxed are massively more likely to get infected at all, and also massively more likely to remain infectious for longer periods, lengthening potential exposure. What actually came out wasn't a study, but rather a brief item in the Aug. 6 edition of CDC's _CDC Morbidity and Mortality Weekly Report_ (MMWR) covering a Delta outbreak at Provincetown, tip of Cape Cod, after Independence Day. Social media glommed onto _one_ figure: among 469 PCR-confirmed COVID-19 infections found among seaside party-goers, 346 (74%) occurred in vaccinated persons. Also, breakthrough (vaxed) infectees during their period of infectiousness carried about the same viral load as did the unvaxed infectees (the same "cycle threshold" aka Ct, during operational cycles of the machine that does PCR testing to detect and duplicate viral genetic material). So, whoa! Being vaxed means you spread Delta _more_, right? Nope. This falls haplessly into Base Rate Fallacy, where you flub estimating probability by ignoring context. To understand that, imagine a slightly inattentive researcher studying alcoholism in Dad's hometown of Kristiansund, population ~25,000. Say (not real numbers here, but credible ones) results note that out of 550 identified alcoholics, 542, were blond. Whoa, says he! If you're blond and resident of Kristiansund, you've 97% chance of alcoholism. We must search for genetic links. Does this researcher get a Nobel? Nope, he gets indulgent chuckles, as he forgot Kristiansund (north of Bergen) has wall-to-wall blonds, about as many blonds as herrings. Call that, in round figures, 22,000. So, more like a 542 / 22000 * 100 = 2% chance, not 97%. Getting back to Provincetown: That 74% is just wildly devoid of base rate context, in that the study of the outbreak made no estimate of how many persons got exposed, nor even what percentage of those present in total were vaxxed. The 469 infectees were essentially self-reported people, concerned enough to get PCR-tested. Nobody got told "We need to test you if you were in any Provincetown bar between July 10th and 18th": Thus, there's an automatic "detection bias". Second, as vaxed population has increased, especially among people like Cape Cod holidayers, you get the Kristiansund-blonds problem: Vaxed persons necessarily will then constitute a rising portion of cases. For all of the above reasons plus others I don't even get into, _MMWR_ warned the info was "insufficient to draw conclusions about the effectiveness of COVID-19 vaccines against SARS-CoV-2". But social media ignored that, and stumbled straight into the Base Rate Fallacy trap. https://www.snopes.com/fact-check/cdc-provincetown-covid-event/ https://www.snopes.com/ap/2021/07/30/study-vaccinated-people-can-carry-as-much-virus-as-others/ As a statistics guy and a Bayesian, that kind of thing makes me sad. Also, with Delta as with prior variants, vaxed infectees clear virus faster. https://www.cdc.gov/coronavirus/2019-ncov/variants/delta-variant.html https://www.technologyreview.com/2021/08/09/1031292/questions-delta-variant-contagious-tranmission-vaccination/ -- Cheers, "Orthodoxy is my doxy. Heterodoxy is someone else's doxy." Rick Moen -- William Warburton, Bishop of Gloucester (1698-1779) r...@linuxmafia.com McQ! (4x80) _______________________________________________ Basfa mailing list Basfa@lists.basfa.org http://lists.basfa.org/listinfo.cgi/basfa-basfa.org
