Data substantiating wide asymptomatic spread of COVID-19 remains elusive

Fear of symptom-free transmission was a major driver of the highly restrictive mitigation measures imposed over the past year.

Published: April 9, 2021 11:25am

Updated: April 9, 2021 9:38pm

Hard data on the true prevalence of asymptomatic COVID-19 spread remains elusive, with scientists struggling to quantify just how often carriers of the SARS-Cov-2 virus transmit it without outward signs of illness. 

Fears that COVID-19 can spread very easily through asymptomatic transmission were a major factor behind the lockdowns and mitigation measures seen across the world over the past year. Health leaders claimed that the threat of symptom-free spread necessitated broad shutdowns of major portions of the economy, indefinite school closures, mandatory public mask-wearing, capacity limits and other social distancing measures, and various other tactics for containing the spread of COVID-19. 

Lost in the chaos of the past year is any definitive indication that asymptomatic spread of the virus is widespread or common. Scientists and public health officials have struggled to come up with meaningful epidemiological evidence to that effect, though most have continued insisting that it's a major contributor to the overall pandemic. 

In June of last year, World Health Organization official Maria Van Kerkhove encountered significant backlash when she said that, based on current evidence, asymptomatic spread appeared to be "very rare." Van Kerkhove subsequently qualified her comment, a qualification hyped as a "walkback" by some in the media. Health officials "don't actually have that answer yet" as to the prevalence of asymptomatic spread, she said in narrowing her previous claim. "I was referring to a small subset of studies." 

Van Kerkhove has not responded to queries since then regarding the evolving study of asymptomatic COVID transmission. Several other scientists, meanwhile, have attempted to answer that question with mixed results. 

One study from December found that, within households, asymptomatic carriers of the virus passed it on to housemates at the rate of about 0.7%. That investigation made headlines for its apparent demonstration that asymptomatic spread is, in effect, vanishingly rare. 

Yet at least one coauthor of the study, University of Florida biostatistics Professor Natalie Dean, has been warning since the study's publication that its findings on that front should be taken with a grain of salt. 

After its publication, "I worked with some fact-checking organizations to explain the context," Dean told Just the News. A Politifact check from December, for example, noted that the paper itself warned that the sample size of asymptomatic case studies was very small. And "the paper looks only at household transmission, not community spread," the news service also noted. 

Dean holds that the totality of the evidence of asymptomatic spread paints a complex and as--yet uncertain picture. "Individuals who never develop symptoms appear to be quite a bit less infectious," she said. Yet individuals who are "presymptomatic" instead of truly asymptomatic, she said, "can be highly infectious."

"This period right around the onset of symptoms is when viral load and infectivity maximizes," Dean explained. "So there are interesting scientific implications of the difference between these two. For example, if vaccines reduce the emergence of symptoms, do they also mean that the few people who still get infected but don’t develop symptoms are less infectious? But for other policy purposes, people without symptoms or with only mild symptoms can be very contagious." 

Dean pointed to a study published this month in the journal Clinical Microbiology and Infection that sought to better assess asymptomatic spread rates. It found "a prediction interval of 0%-10% for asymptomatic [attack rates]," potentially much higher than those found by Dean's team last year. 

Even such numbers would still come in quite a bit lower than some of the more dire estimates, such as a paper in Januaury that claimed that "persons with infection who never develop symptoms may account for approximately 24% of all transmission." 

That study, however, was less a review of hard epidemiological data and more an "analytical model" that "assessed the relative amount of transmission from presymptomatic, never symptomatic, and symptomatic individuals across a range of scenarios." The authors of that study did not respond to queries seeking information on the study's potential limitations. 

Others have come up with even higher numbers: Last summer, a review of early pandemic data from scholars at the Scripps Research Translational Institute suggested that "asymptomatic infection rate may be as high as 40% to 45%" with a "conservative estimate" of "30% or higher." Those numbers stand in sharp variance to the newer, lower estimates, though the data itself still remain incomplete. 

Asymptomatic transmission of viruses is not new; seasonal influenza can transmit asymptomatically, though reportedly not as easily as can SARS-Cov-2.

Ultimately, only more data collection — which can be a painstaking, lengthy process — can settle the question. Many are holding out hope, meanwhile, that the ongoing vaccine campaign will significantly reduce what appears to be the already low rate of asymptomatic spread of COVID. 

"[I]f the vaccine also halts asymptomatic spread, then you could potentially vaccinate your way out of the epidemic," Baylor College School of Tropical Medicine Dean Peter Hotez told NBC last month. 

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