Confronting the Dangers of Silent Spread Is Necessary to Prevent Future Pandemics

Confronting the Dangers of Silent Spread Is Necessary to Prevent Future Pandemics

By Joshua S. Weitz

The telltale runny nose of a common cold, or the fever and aches associated with the flu, mark the way we classify respiratory illnesses—with their symptoms. Public health messaging relies on these symptoms, urging those who are symptomatic to stay home and avoid others. That makes sense. It reduces the risk that one case becomes many.

But what if transmission is not necessarily linked to symptoms? COVID has shown that diseases can lead to catastrophic societal harm when they spread without symptoms. Hence, preventing future pandemics requires greater investment in targeted public health interventions to reduce transmission—including from infected individuals who feel fine.

Indeed, asymptomatic transmission was essential to COVID’s transition from a fast-moving outbreak in Wuhan, China, in early 2020 into a global pandemic that led to more than one million reported fatalities in the U.S. by May of 2022. People who felt fine transmitted their infection to others before developing symptoms (during a presymptomatic phase) or even if they never developed symptoms. Comparisons of early outbreak data revealed that approximately half of infected individuals were asymptomatic. That would be good news if asymptomatic infections were nontransmissible. But that wasn’t the case.

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On February 23, 2020, researchers from China, France and the U.S. released a joint analysis of more than 450 COVID transmission events in 93 cities in China. The analysis focused on the serial interval: the time between when someone exhibits symptoms and when the person they infect exhibits symptoms. Counter to expectations, the analysis showed that COVID’s serial intervals were often less than zero, meaning individuals exhibited symptoms before the person that infected them. These statistics were evidence of rampant presymptomatic transmission. Public health experts tried to raise the alarm that efforts to stop transmission via symptom screening (e.g., testing for elevated temperature or shortness of breath) were bound to fail and that “unprecedented measures” were needed to fight back.

The deadly consequences of asymptomatic transmission soon arrived in the U.S. On March 10, 2020, the Skagit Valley Chorale gathered outside Seattle for a rehearsal. Despite efforts to limit physical contact, within a few days it was evident that someone in the group had unwittingly infected others. Ultimately, 53 of the 61 attendees were infected, and two died. This superspreading event revealed that COVID could spread in the air in the absence of symptoms. Yet the relevance of asymptomatic transmission remained contested. On June 8, 2020, a top WHO official declared that asymptomatic transmission was “very rare.” The pandemic was raging, but we were losing precious time to confront silent spread. The consequences were grave. As head of the White House Coronavirus Task Force, Anthony Fauci noted in August 2020: “I’ve never seen a viral disease in which you have such a wide breadth of symptoms, ranging from no symptoms at all, in 40–45 percent of cases, to severe enough to kill you.” Asymptomatic transmission represents a double-edged sword. Individual outcomes may be better, but silent spread leads to many more infections that can lead to worse outcomes for the population.

What can be done to reduce asymptomatic transmission? Initial responses to the pandemic involved limitations on gatherings and stay-at-home orders. But COVID’s unusual mix of severe and asymptomatic outcomes catalyzed a diverse group of stakeholders to invest in unconventional approaches to reduce the risk of silent spread. These approaches include real-time risk assessment, large-scale rapid testing, context-specific masking and improved indoor air quality. Each of these has a complementary role in reducing silent spread, and if implemented at scale, they can be essential weapons in the ongoing fight against pathogens of pandemic potential.

In the absence of symptoms, real-time risk assessment powered by outbreak models and disseminated via mobile-accessible dashboards could function as a threat forecast. These dashboards could provide mapped information on a variety of infectious disease risks, including upsurges of COVID reported in wastewater. People could then decide to avoid events whose risk exceeded their tolerance. However, even if someone attends an event, the use of on-site rapid testing and masks could limit infections. This could be an enormous force for good, especially in nursing homes and long-term care facilities, which had a disproportionately large fraction of overall COVID fatalities. Irrespective of individual action-taking, infrastructure investments in indoor air quality (via improved filtering, air turnover rates and upper room UV-C sterilization) could improve health outcomes.

Finally, we must commit significant resources to the development and effective dissemination of vaccines both in the U.S. and globally—especially in developing countries. The production of billions of doses of vaccines just a year after COVID’s emergence represents an incredible validation of the power of basic research and public-private partnerships. However, producing vaccines does not always translate to getting shots into arms. Public health agencies must improve messaging to explain both why individuals can benefit from vaccines, when they should get vaccines (and boosters) and what each vaccine is meant to do. In the case of COVID, mRNA vaccines were shown to reduce rates of symptomatic illness by more than 90 percent. Yet these vaccines do not prevent all infections. This means vaccinated individuals can still get infected, test positive and infect others—but their risk of severe outcomes decreases. This is precisely the point. But the fact that vaccines did not provide perfect protection against infection (asymptomatic or otherwise) has accelerated the rampant spread of misinformation that threatens to diminish vaccine uptake—not just for influenza and COVID but also for preventable childhood diseases, including measles.

Nearly five years have elapsed since early warning signs emerged of a novel coronavirus spreading in Wuhan fueled by asymptomatic transmission that would soon lead to a global pandemic. At the time, the risk to public health and socioeconomic stability seemed far removed. Since then scientists, public health experts, government agencies and the biotech sector have developed a suite of countermeasures to confront the dangers of silent spread—yet there is more to do, including identifying the consequences of the silent spread of avian influenza in wild and domestic animals. Translating this momentum into data-driven threat assessments, high-impact interventions (spanning testing and air quality improvements), faster vaccine deployments and more effective messaging from doctors and public health agencies is essential to reduce the ongoing burden of COVID; these actions will better prepare the world to identify, prevent and respond to threats of pandemics to come—before it is too late.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.