Since the beginning of the SARS-CoV-2 pandemic, the popular assumption was that people displaying severe symptons were also the most contagious. But new data from a controversial Imperial College London study published on Tuesday found the two participants that were the biggest “superspreaders” of the virus both displayed only minor symptoms. 

“How symptomatic a person is has often been assumed to indicate their contagiousness,” said the authors of the ‘challenge’ study, published in The Lancet.  The study, involving healthy volunteers who were deliberately infected with COVID at a time when no vaccine or treatments existed, found that the severity of symptoms experienced by the volunteers “did not influence the extent of viral emissions.” 

At the time of its launch in 2021, one leading expert called it “dumb and dangerous“, in an interview with Health Policy Watch.

But the data collected from the 34-person study paid off.  There was no direct relationship between symptom severity and viral load among the 18 volunteers who went on to develop COVID, and were monitored for two weeks from a hospital bed. The variability is something the researchers suggested may be attributable to the diversity of physiological factors such as breathing mechanics or mucous acidity. 

“It’s that variability among humans that has made this virus so difficult to control,” Monica Gandhi, an infectious-diseases expert at the University of California told Nature, in a review of the findings.    

Ethics and risk

When the Imperial College study was announced in 2021, Dr Ken Kengatharan, co-founder and chairman of the California-based biotech firm Renexxion, told Health Policy Watch that a COVID-19 challenge study was “as dumb and dangerous an idea as it gets”.

“SARS-CoV-2 is an atypical coronavirus (without any comparable out there or historically, and we are just learning about its mode of action,” Kengatharan said at the time.   

Many of Kengatharan’s apprehensions have since been proven right. Around 36 million people in Europe – one in 30  – may have developed long-COVID over the first three years of the pandemic, the World Health Organization (WHO) announced Tuesday.

“Clearly much more needs to be done to understand it,” WHO European Director Dr Hans Kluge said. “Ultimately, the best way to avoid long-covid is to avoid COVID-19 in the first place.”

Understanding the role of “superspreaders” in the COVID pandemic could be and important component of effective policy making frameworks for future outbreaks.  Superspreader patterns were also identified during earlier coronavirus outbreaks, such as the SARS outbreak that began in 2002, and the outbreak of Middle East respiratory syndrome coronavirus a decade later.  

WHO’s symptomatic criteria for testing were ill-founded

The authors of the study reckon the unique perspective their data provides on several key public health questions about the COVID-19 virus justifies the risks taken by the volunteers.

The unpredictable nature of COVID-19 has led scientists and average people alike to speculate that pre-symptomatic infections were a big reason the virus proved so hard to contain. Scientific modelling has estimated that at least 30-50% of community transmission occurred before people became visibly sick, but models are only accurate to the extent of the assumption that underpin them.

The data released by the Imperial study is also the first to quantify pre-symptomatic viral emissions in a real-world setting, and tells a different story: just 10% of virus emissions recorded occurred before the onset of symptoms. 

In that respect the study also challenged the usefulness of the WHO’s suspected case criteria, observing that over one-third of virus particles emitted by participants were shed before symptoms met the WHO guidelines.

The UN health agency’s criteria, used by governments around the world to determine eligibility for COVID-19 testing at the height of the pandemic, are “relatively poor definers of the onset of contagiousness,” the study said.

In contrast to the inefficacy of the WHO criteria, lateral flow tests were able to identify infections in most participants before symptoms and viral shed began – demonstrating their potential power for containing future outbreaks.  

“A heightened awareness of early symptoms prompting self-testing could identify a large proportion of infectiousness,” the study said.

Researchers said further challenge studies on newer variants of COVID-19 such as Omicron are planned for the near future.

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