About that Asymptomatic Spread...
Asymptomatic and presymptomatic COVID spread is the entire rationale for lockdowns and masks. But results from contact tracing studies are mixed at best, with many pointing to very low transmission.
Buried far down in a well-publicized CDC report about low transmission rates in Wisconsin schools is a shocking admission: “recent research” shows an asymptomatic attack rate (infection rate) of 0.7% among household contacts. The authors cite a meta-analysis of household COVID-19 spread, including four studies of asymptomatic and presymptomatic index cases. The analysis itself notes that it’s not just asymptomatics, but presymptomatics, that are included in that attack rate of 0.7%.
Few journalists in mainstream media are going to share that with you, of course, just like they won’t share the substantial research that backs up this finding of low spread from people who test positive for COVID but don’t show symptoms. I’ve provided highlights from that research here.
Sure, I could write half a dozen posts poking Fauci-shaped holes in studies of mask filtration or spurious claims that a lockdown restriction in this or that county “flattened the curve.” I instead spent an absurd number of hours for a freelance writing mother of two (going on three) wading through contact tracing studies of asymptomatic/presymptomatic spread. Why? Because the supposedly rampant spread of COVID-19 by people who don’t have symptoms is the entire rationale for locking down economic and social life. If spread from people without symptoms is not what drives the spread of SARS-COV-2, then every COVID restriction besides staying home when you are sick is utterly pointless (unless the point is to keep people living in fear, but that’s another post altogether).
You’ve probably seen plenty of 2020 news articles touting computer models, based on viral loads peaking right before or at symptom onset, that suggest presymptomatic and asymptomatic spread could be a major driver of the pandemic, perhaps responsible for 44% of all transmission. The CDC’s “best estimate,” last updated in September, puts transmission prior to symptom onset at 50%. But instead of relying on the leap of logic that viral loads alone are completely deterministic of transmission, we should consider relevant data from population-based studies, as the CDC did in their report on school transmission (albeit buried deep down where lazy reporters won’t find it). Contact tracing studies have considerable limitations, but they get us much closer to examining “real transmission,” as it happens in real life, as possible. If computer models contradict observable transmission rates, it’s the models that must be disregarded, not the the tracing studies.
Results from studies of asymptomatic and presymptomatic transmission rates aren’t entirely one-sided, but many with good sample sizes and methodology point to much lower asymptomatic/presymptomatic transmission than modeling studies propose. As Mark K. Slifka and Lina Gao note in an article on the subject, “asymptomatic/presymptomatic transmission measured by direct contact tracing studies is lower than that predicted by COVID-19 transmission models.” Results for asymptomatic/presymptomatic spread range from 31% of contacts in one small study to just 4.2% in another, and even 0% in a study of household contacts. In a study of 157 locally acquired cases in Singapore, presymptomatic spread accounted for just 6.4% of the cases.
As I noted, contact tracing studies have their limits, with isolation and quarantine potentially disrupting chains of transmission that may have occurred otherwise and asymptomatic cases potentially being underrepresented. But one particular study conducted between January and March in Guangzhou, China, though small, is worth examining closely, as the results from its careful tracking of presymptomatic/asymptomatic spread cut directly against the conventional wisdom that this group “easily” spreads the virus.
The authors examined the secondary attack rate of 38 presymptomatic individuals who were not exhibiting symptoms at the time they passed the virus to others. Each had a median number of 4 close contacts, with 369 close contacts studied in total. Close contacts were defined as those who were in contact with the index case within the two days prior to their positive COVID test, and who weren't consistently wearing a mask while in contact. These weren’t people who just happened to come within six feet of the index cases at the grocery store; but rather belonged to categories such as family members, caretakers, people who shared a vehicle or a meal, or caterers and entertainers in a “closed environment” (see the appendix).
The 38 presymptomatic individuals infected only 12 people, or about 3.3% of their close contacts. The rate of secondary infections was less than a third of the total number of index cases, meaning if the study were representative of presymptomatic behavior generally, then presymptomatic spread has not been driving the growth in cases - at least not at that time and that region. But here’s the kicker: according to the study’s appendix, almost all of the secondary cases lived with the index case. The contact of those two cases who weren’t were classified as “social interactions.”
One could argue that almost all the secondary cases happened to be cohabitators with the index cases because people in Guangzhou were wearing masks and distancing everywhere else, so they couldn't infect anyone but close contacts. But this doesn't explain why presymptomatic people only spread it to 12 close contacts out of 369, or why data from across the U.S. and the world shows no consistent reduction in transmission from distancing and masks.
If presymptomatic people were “super spreaders,” it’s arguable they should have infected their close contacts at an R0 well above 1, or more than 38 individuals, and that more of those cases would have been casual interactions with people who didn’t live with them--though it should be noted not every single close contact of the index cases was reached, and the presymptomatic period wasn’t well-defined (e.g., how many days before symptom onset was the contact made?).
But the same limitations on tracking close contacts applied to index cases with symptoms, and that data lines up well with results from other studies. One study of 468 COVID cases from China pegged presymptomatic transmission at 12.6%. The aforementioned meta-analysis of secondary attack rates in households found asymptomatic and presymptomatic cases transmitted to far fewer contacts than symptomatic cases (0.7% compared to 19.9%)--not anywhere close to the CDC’s “best estimate” that they are 75% as infectious as symptomatic cases and responsible for up to half of COVID transmission. The argument that if you feel fine and go shopping without a mask, you might kill someone’s grandma, is therefore cut at the knees —especially given that the vast majority of COVID-19 cases are not severe.
Academic M.D. Andrew Bostom, whom I asked to review this article for medical accuracy and coherence and who has been following the studies coming out about COVID-19 transmission, pointed me to another study conducted in Geneva. It modeled transmission based on the results of antibody tests of over 4,000 participants and found “asymptomatic infections are far less likely to transmit than symptomatic ones,” and risk of transmission from outside the household was four times lower than risk of transmission from a household member. Assuming residents of Geneva were staying home while sick, it buttresses the findings of contact tracing studies finding similarly low rates of asymptomatic and presymptomatic transmission.
In the interest of intellectual honesty, it bears repeating that not all studies show drastically lower transmission for presymptomatic people. A large study conducted in Taiwan, for instance, found “299 contacts with exclusive presymptomatic exposures” might have potentially equal or greater infection risk than symptomatic contacts. The presymptomatic SAR (secondary attack rate) was just 0.7% with a 95% confidence interval of 0.2%-2.4%, compared to 0.7% with a 0.4-1.1% confidence interval for those exposed after symptom onset. Only two people from the group exposed before symptom onset were actually infected, however.
Other studies have indicated a correlation between severity of symptoms and transmission rates, which also favors supports the idea that transmission from people without symptoms is far lower than what the CDC and many media outlets claim. A study of 3410 close contacts of 391 index cases in Guangzhou found secondary attack rates escalated with severity of symptoms: “0.3% (CI, 0.0 to 1.0%) for asymptomatic to 3.3% (CI, 1.8% to 4.8%) for mild, 5.6% (CI, 4.4% to 6.8%) for moderate, and 6.2% (CI, 3.2% to 9.1%) for severe or critical cases.” The previously discussed smaller study found a similar pattern, though the SAR for severe cases was slightly lower than moderate cases (perhaps due to isolation measures).
Notably, that study also found household transmission highest (10.3%), compared to public transportation (0.1%), healthcare settings (1.0%), and entertainment venues or workplaces (1.3%). Grocery and retail stores, places government officials and corporate leadership insist masks, distancing, and those utterly unscientific one-way aisle stickers are crucial, were not on the list.
Given the weakness of the case that COVID-positive people without symptoms are driving viral spread, we shouldn’t be surprised that lockdowns and mask mandates haven’t demonstrated consistent, statistically significant reduction in COVID transmission. Yet eleven months later, our leaders are still making the same mistake they did in the beginning: limiting essential freedoms and destroying our economy and social fabric based on suppositions and computer-modeled “doomsday” scenarios rather than solid evidence. It is past time we demand evidence-based approaches from our leaders before this precedent solidifies and our rights become permanently contingent on pseudo-science and fear.
and Fauci and Von Kerkhove said that asymps don't contribute much to the spread...but we are living in big-lie times.
Excellent and thorough. If only Americans received such research and reporting from mainstream media outlets with their big tech algorithm newsfeeds.