Why Delta Covid variant is spreading rapidly

Researchers at the Guangdong Provincial Center for Disease Control and Prevention in China found that viral load  a measure of the density of viral particles in the body - is roughly 1,000 times higher in people infected with the Delta variant than those infected with the original coronavirus strain, Nature reported.

According to current estimates, the Delta variant could be more than twice as transmissible as the original strain of SARS-CoV-2, the virus causing Covid-19. The variant, which was first identified in India in late 2020, has now become the predominant strain and has spread to at least 111 countries.

The team tracked 62 people who were quarantined after exposure to Covid-19 and tested their viral load every day throughout the course of infection to see how it changed over time. Researchers then compared participants' infection patterns with those of 63 people who contracted the original SARS-CoV-2 strain in 2020.

The findings, posted preprint, showed that the virus was first detectable in people with the Delta variant four days after exposure. On the other hand, the original strain took an average six days to be present in people. This suggests that Delta replicates much faster, said epidemiologist Jing Lu at China's CDC.

Individuals infected with Delta also had viral loads up to 1,260 times higher than those in people infected with the original strain.

The combination of a high number of viruses and a short incubation period makes sense as an explanation for Delta's heightened transmissibility, Benjamin Cowling, epidemiologist at the University of Hong Kong was quoted as saying to Nature.

Those infected with Delta carry a high number of viruses in the respiratory tract, which means that they can become superspreaders and infect more people. Moreover, a short incubation makes contact tracing more difficult, the researchers noted.

"Putting it all together, Delta's really difficult to stop," Cowling says.