Exposure to the common cold offers protection against coronavirus, a new study published on Tuesday has found.
Building on previous studies testing this hypothesis, researchers from Yale University infected lab-grown human airway tissue with coronavirus and for the first three days viral load in the tissue doubled about every six hours.
However, in the tissue exposed to rhinovirus - the common cold virus - coronavirus was stopped dead in its tracks.
The researchers didn't just rely on lab-grown tissue, they also studied nasal swab samples from patients diagnosed close to the start of infection. In these cases, the virus also grew rapidly in the first few days of infection, before the body's defences were activated.
"There appears to be a viral sweet spot at the beginning of Covid-19, during which the virus replicates exponentially before it triggers a strong defence response," said Ellen Foxman, assistant professor of laboratory medicine and immunobiology at the Yale School of Medicine and senior author of the study.
Prof Foxman believes triggering these defences early in the course of infection could be replicated in future treatments for Covid-19.
One way to do this is by treating patients with interferons, an immune system protein which is also available as a drug.
Interferon Treatment a Matter of Timing
This treatment "all depends on the timing", Prof Foxman said. The drug is only effective in the first days of infection, when most people are asymptomatic.
Given this limitation, Prof Foxman suggested interferon could be used preventatively in high-risk people who have been in close contact with others diagnosed with coronavirus.
With social distancing and other Covid restrictions easing, there are concerns almost dormant common cold and flu viruses will come back in greater force.
Despite the work of Prof Foxman and her team, how these upper respiratory viruses will co-circulate with coronavirus remains unknown.
"There are hidden interactions between viruses that we don't quite understand, and these findings are a piece of the puzzle we are just now looking at," she said.