The ‘silent spreader’—a patient without symptoms—is the big mystery about Covid. That puzzle actually starts with the symptoms because there’s such a wide range of them, and many of them so mild that you may not even notice. On the US Centers for Disease Control and Prevention (CDC) website, there’s a list of 11 symptoms—nearly half of these, like ‘new loss of taste or smell’, were added in April, weeks after the global outbreak had begun. These symptoms could appear 2-14 days after exposure to the virus.
By now, there’s fair consensus that a large portion of Covid patients could be ‘paucisymptomatic’ or experiencing mild symptoms—again, falling into different categories. But how many of them truly don’t give out any clues at all of having an infection while they have it? There’s no precise statistical answer at the moment—estimates range wildly, from 6 per cent 41 per cent, globally. But the actual riddle researchers are trying to crack is: Can they infect others? Yes, warned a group of Chinese scientists back in February. They described the results of a comparative study they had conducted to the New England Journal of Medicine thus: “The viral load detected in (an) asymptomatic patient was similar to that in symptomatic patients, which suggests transmission potential.” An NEJM editorial reinforced this view two months later, calling asymptomatic transmission “the Achilles’ heel of current strategies”. It cited a US study that also confirmed high viral loads in the upper respiratory tracts of asymptomatics: “Quantitative SARS-CoV-2 viral loads were similarly high in the four symptom groups” (those with typical symptoms, atypical ones, the presymptomatics, and asymptomatics).
Using this as a benchmark would alter testing strategies around the world. Currently, since there’s no doubt that people with symptoms are indeed infectious, testing has largely been centred on them and their contacts. But this follows a line of epidemic control that perhaps stems from the SARS virus which, while genetically highly similar to Covid-19, behaved differently in terms of onset of symptoms, a quickly peaking/subsiding transmissibility, and a lower viral load in the nose and upper respiratory tracts. That’s why, “despite the deployment of similar control interventions, the trajectories of the two epidemics have veered in dramatically different directions. Within eight months, SARS was controlled after SARS-CoV-1 had infected approximately 8,100 persons in limited geographic areas. Within five months, SARS-CoV-2 has infected more than 2.6 million people and continues to spread rapidly around the world,” said the NEJM edit in April.
Virologist Shahid Jameel elaborates on that. In the case of the SARS epidemic in 2003, he says, people were shedding the virus only when they were symptomatic. “But this virus starts shedding before symptoms appear. And that is one of the reasons why this outbreak has expanded so rapidly,” he says. Pre-symptomatic patients being infectious isn’t new: measles and chicken pox too behave that way. HIV too is a classic example of that, says Jameel. “With HIV, you don’t show symptoms of disease up to 8-10 years after infection but you are transmitting. So there are various examples.”
And yet, doubts about that are also occasionally lobbed into the air—even if not always convincingly. WHO epidemiologist Maria Van Kerkhove this week had to clarify her claim about it seeming rare for an asymptomatic person to transmit the disease on to a second individual. There’s no answer yet to the complex question of asymptomatics spreading infection, she said a day later, explaining that she had referred to a subset of studies. “Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but available evidence from contact tracing reported by member states suggests asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms,” says a WHO document. Even if this dovetails with the anxiety of governments to open up, there’s science behind it—and the science on both sides is incomplete. The question, clearly, is far from settled. Do asymptomatics then hold out possible clues—to immunity and even cure? Some think so. A silver lining, perhaps.