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Where's the evidence for asymptomatic spread? Chinese publications are major outliers on this subject in the scientific literature and these papers have distorted the evidence
Scientific reviews from reputable institutions have taken these Chinese papers at face value. This has arguably resulted in the establishment of a dangerous assumption.
The assumption is that there is compelling evidence that people who *never* have symptoms are capable of spreading COVID to others. Let's take a look at some of these reviews (which weigh up all the anecdotal evidence in a meta-analysis).
Yanes-Lane et al, from McGill University, included 6 studies out of 6137 they considered. Despite the only evidence of transmission being from China and pre-symptomatic, they concluded that "18.8% of close contacts exposed to asymptomatic index patients were COVID-19 positive."
The two studies by Park et al were from South Korea and the remainder from China
Buitrago-Garcia et al, from the University of Bern, found 688 studies on asymptomatic and pre-symptomatic COVID and 5 which commented on transmission. One non-Chinese study from Brunei by Chaw et al was important here. The other studies demonstrating transmission were Chinese.
The publications originated from: Luo and Zhang - China; Park – South Korea; Cheng – Taiwan; Chaw – Brunei. The 1st column shows the fraction of contacts contracting COVID from an a-/ pre-symptomatic individual and the 2nd column from symptomatic individuals in the same study.
Wee Chian Koh et al also put great weight on the Brunei study but use different numbers for the same work. No other cases of asymptomatic transmission were included in this analysis. https://journals.plos.org/plosone/article/file?type=printable&id=10.1371/journal.pone.0240205
The columns labelled SAR shows the fraction of contacts that contracted COVID in each study.
The Lancet pre-published a meta-analysis by Byambasuren et al who chose 5 studies from 571, two of which reported asymptomatic transmission. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3586675
The first was a study of 4,950 contacts in Guangzhou who were quarantined with a single individual said to have caught it from a patient who was asymptomatic. https://www.medrxiv.org/content/10.1101/2020.03.24.20042606v1
The second study quoted was a Nature paper on an Italian study reports on numerous asymptomatic ‘cases’ and goes on to propose lines of ‘transmission’ concluding that two asymptomatic people were infected by two other asymptomatic people. https://www.nature.com/articles/s41586-020-2488-1
Every test has a false positive rate. A respectable rate for PCR testing is about 1%, especially for a newly rolled out test. (UK govn estimates between 0.8-4.0 (median 2.3%) https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/895843/S0519_Impact_of_false_positives_and_negatives.pdf
The estimates for the proportion of COVID 'cases' that are asymptomatic vary wildly because they depend, not on a feature of the disease, but on how many tests were done.
In the first round of testing they carried out 2,800 tests (so you'd expect 28 false positives). There were 29 asymptomatic 'cases' in this round of testing.
Deducing chains of transmission based on a high risk of material numbers of false positive results is bad science.
What about the Brunei study? Chaw et al reported on a large outbreak which started at a religious festival. There were apparently six people who had no symptoms initially (2 never had symptoms) but allegedly spread to COVID to others
The evidence is weakened by a poor case definition (any symptom of any severity was considered real symptomatic COVID) and a high probability of false positive results.
A further paper gave details. A 13 yr old girl with no symptoms was alleged to have spread COVID to her teacher who had "a mild cough on one day". The second was a father who remained asymptomatic but whose wife briefly had a runny nose and whose baby had a mild cough on one day.
No details were given of the testing protocols and criteria for calling a positive.
An important NEJM paper on pre-symptomatic spread was wrong. The patient was not interviewed and was symptomatic. https://twitter.com/ClareCraigPath/status/1336385958300749828?s=20
It is therefore at least arguable that the asymptomatic diagnoses in spring were all due to false positive test results. No testing system is perfect. Failure to acknowledge this and misinterpretation of positive results in patients with no symptoms has been hugely damaging.
The unavoidable fact is that if the Chinese papers are excluded, the evidence for genuine asymptomatic transmission of COVID is tissue thin.
There is no evidence, outside of China, that anyone has been hospitalised with COVID based on true asymptomatic spread, as opposed to pre-symptomatic spread.
The Chinese appear to want to back peddle now and are releasing more dubious evidence that now implies asymptomatic spread does not occur: https://twitter.com/ClareCraigPath/status/1331726336860229632?s=20
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