Coronavirus: Delta patients are infectious without symptoms for longer - study

People who contract the Delta variant of COVID-19 spend much longer infectious before they start to show symptoms than people who contract earlier versions of the virus, new research has found. 

Asymptomatic spread of the disease has made stamping it out difficult, particularly the highly contagious Delta variant. 

Previous research has found people infected with Delta - first detected in India late last year - typically carry much larger viral loads than those who contract other variants, and now scientists in China have found another trick up its sleeve. The window in which cases are infectious without realising it more than twice as long.

"It is just tougher to stop," University of Hong Kong epidemiologist Benjamin Cowling told Nature

His team found while it took 6.3 days on average for a person infected with other strains to start showing symptoms, they didn't start testing positive for viral RNA until 5.5 days after being infected - leaving a short window of just 0.8 days where they were asymptomatic but infectious.

While Delta symptoms typically start showing up sooner - after 5.8 days - viral DNA can be detected after just four days, leaving a gap of 1.8 days.

The research, published online ahead of peer review, found 74 percent of all Delta infections happened during this window, since people typically isolate once they know they've got the virus, which has killed millions worldwide.

The research also backed up earlier studies that found Delta causes higher viral loads in patients, but vaccinations helped limit it, and made them 65 percent less likely to pass it on, if infected. 

The reproduction number for Delta in this study - the number of people the average person will infect if measures aren't taken to prevent it - was 6.4, compared to the original virus' estimated score of between two and four. 

"The higher viral load and higher risk of pre-symptomatic transmission indicated the challenges in control of infections with the Delta variant," the scientists concluded.