‘Silent transmission’: COVID-19 largely spread by asymptomatic, presymptomatic carriers, study says

Asymptomatic individuals are believed by many scientists to be contributing to the rapid spread of the novel coronavirus, and a new study indicates that people with COVID-19 but who do not show symptoms may be behind most infections.

Published in the Proceedings of the National Academy of Sciences, the July 6 study refers to the phenomena of COVID-19 infections by asymptomatic and presymptomatic individuals as “silent transmission.”

The study says that even if symptomatic cases are isolated — such as a person being quarantined until they have recovered or tested negative for the virus — “a vast outbreak may nonetheless unfold.”

”We further quantified the effect of isolating silent infections in addition to symptomatic cases, finding that over one-third of silent infections must be isolated to suppress a future outbreak below 1% of the population,“ according to the article abstract.

RELATED: Renters face financial cliff ahead; limited help available amid COVID-19 pandemic

The researchers used 10,000 hypothetical people representing the demography of residents for New York to populate their model.

”Transmission was implemented probabilistically for contacts between susceptible and infectious individuals in the presymptomatic, asymptomatic, or symptomatic stages,“ according to the study. ”A proportion of infected individuals remained asymptomatic through recovery,“ with those subjects having an ”average infectious period“ of five days.

”Combined with case isolation, our results indicate that 33% and 42% detection and isolation of silent infections would be needed to suppress the attack rate below 1%“ based on asymptomatic proportions of 17.9% and 30.8%, the study states. 

While health experts have frequently stated that asymptomatic individuals are contributing to the spread of the novel coronavirus, it’s been unclear just how much they may fueling new infections, as well as if asymptomatic spread may be impacting spikes in cases as states and cities in the U.S. reopen parts of their economies.

RELATED: 'A hot mess': Nationwide trend shows testing delays as coronavirus surges

Earlier in June, the World Health Organization walked back comments that one of its officials had made on how asymptomatic transmission was ”rare.“ Maria Van Kerkhove, the WHO official who made the comments, later clarified, calling the comments a ”misunderstanding.“

“The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets, but there are a subset of people who don’t develop symptoms and to truly understand how many people don’t have symptoms, we don’t actually have that answer yet,” said Van Kerkhove at a June press briefing.

It is now believed by hundreds of scientists in dozens of countries around the world that the coronavirus is airborne, meaning that individuals can contract COVID-19 simply by breathing in air in a space that was previously occupied by an infected person. The new insights prompted 239 scientists to urge the WHO this week to acknowledge that the virus can spread through the air.

On July 7, WHO acknowledged the “emerging evidence” regarding the possibility of airborne transmission, stating that the organization understands the “implications regarding the modes of transmission, and also regarding the precautions that need to be taken.”

RELATED: WHO acknowledges 'emerging evidence' that coronavirus can spread through the air

Over 3 million people in the United States were confirmed to have the novel coronavirus as of July 8, according to data from the Johns Hopkins Coronavirus Resource Center.