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

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November 25, 2024

5 min read

We need targeted public health interventions to reduce transmission from asymptomatically infected individuals. Like COVID, silently spreading pathogens can lead to far more infections and fatalities

By Joshua S. Weitz

Bymuratdeniz/Getty Images

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 … Read More