The amount of coronavirus growing inside your nose can make you more likely to transmit the virus to others, according to a recent study that looked at how the disease spread among hundreds of people in Catalonia, Spain.
The scientists analyzed 282 COVID-19 cases and hundreds of their close contacts from last spring. Around 90 of those cases resulted in an infection in at least a second person. They found that those patients with lower viral load—the quantity of virus measured in a person’s nasal swab during routine testing—had a one in eight chance of transmitting to a contact. When the viral load was high, the transmission rate doubled to one in four contacts getting infected.
If this finding sounds obvious, that’s because, in part, it is.
“Intuitively, even if you don't know anything about virology or epidemiology, it would make sense,” Michael Marks, the study’s lead author and an associate professor at the London School of Hygiene & Tropical Medicine, told Gothamist. “There's a lot of virus in my nose and throat, therefore, there's a lot of virus in the air that I cough out or breathe out.”
Yet it can take time to find the right group of patients and medical specimens to prove the connection, even though Marks said similar patterns are observed with other respiratory viruses. But he and his co-authors say theirs is the largest study yet confirming the relationship between viral load and risk of coronavirus transmission.
The findings, which were published in The Lancet Infectious Diseases on February 2nd, could also improve contact tracing and give people a sense of how their case of the disease might progress. A second part of the study found that people with more virus in their nose were also more likely to develop COVID-19 symptoms—with the risk rising from 38% to 66% between those with low to high viral loads.
Marks says such information could play into how “aggressively to pursue contact tracing” for people with higher viral loads, given they’re more likely to spread the disease. Public health authorities could direct resources to cases with a high viral load for contact tracing. A technique known as a quantitative PCR test has the ability to determine viral load and is commonly found in public health labs and at universities.
“We would expect more of that case's contacts to have become infected,” Marks said. “It's not really so much about what you or I as individuals who might get COVID-19 should do.”
Virologist Nischay Mishra, of Columbia University, agreed. The study found that viral load during an initial PCR test was a better predictor for symptoms than other variables like sex, age, diabetes, and heart or respiratory disease.
“The people who have high viral load, their contact tracing should be more strict,” Mishra. “We need to contact everyone. But scientifically, the people who are carrying more virus, they spread [it] more.”
But immunologist Gigi Kwik Gronvall wondered if the bigger priority should be public health efforts, including contact tracing, that first focus on controlling the new coronavirus variants, some of which are better transmitters or can escape our immunity. Masking, social distancing, and ventilation may be more impactful toward curbing case surges.
“The reality is that we have too many cases right now to do contact tracing effectively,” Gronvall, a senior scholar at John Hopkins Center for Health Security. “It’s really a matter of being overwhelmed because there’s just so many cases.”
The study relied on contact tracing and case data from Catalonia health officials, and a close contact was defined as being about 6.5 feet from a confirmed case for more than 15 minutes. The analysis was also biased towards women, people without many underlying conditions, and mild to moderate infection.
But Gronvall added the study results are not trivial.
“Even if you still end up being exposed, perhaps wearing a mask may have helped you not be exposed to as much virus, and so that might be an important thing for your course of illness,” said Gronvall.