The Delta variant is one of four current “variants of concern” identified by the World Health Organization (WHO). Largely, it is a variant of concern because it has features of these emerging mutants – that is, we know it has increased transmissibility. Professor William Rawlinson is a Senior Medical Virologist and is the Director of the Serology and Virology Division (SAViD) at NSW Health Pathology SEALS Randwick, and South Eastern Sydney and Illawarra Health Service. He has been at the forefront of Australia’s response to COVID-19 and explains what we know about the Delta variant.

“In Australia and New Zealand, we are already seeing the Delta variant spread through our communities much more rapidly than has previously been seen; around two to three times faster than the original sequenced version of the virus (the Wuhan/WH04/2020). There are still a lot of unknowns, but what we do know is that the Delta variant has a much higher R0 value than previous strains. The R0, is the number of people a SARS-infected person will infect when the entire population is vulnerable to the virus. Instead of an R0 of 2-3 that we were seeing with earlier strains of the virus, the Delta variant seems to have an R0 of about 5-6. That’s around 5.5 people infected for every infected person index case, which is twice what was seen previously with earlier virus variants.

“Unlike earlier forms of the virus, we have also found that people infected with the Delta variant may not have COVID-19 symptoms until two days after they start shedding the coronavirus. This in itself makes the virus much more difficult to control as it can be spread to others before people are even aware that they are infected,” said Prof Rawlinson.

As the Delta variant is relatively new, being detected first in India in December 2020, researchers are just beginning to understand how it spreads so efficiently, whether it causes more serious illness, and why there are more breakthrough infections as it spreads through populations even where large numbers of people are vaccinated. The question also still remains exactly how Delta affects the human body differently than other variants.

“When it comes to the biology of this SARS-CoV-2 strain, it appears it does have some changes in its genome which makes it more infectious than earlier forms. Single mutations are usually associated with other mutations on the SARS genome, and we already know that this variant carries multiple mutations, including within the spike protein which binds the virus to human cells. Not only does it seem able to replicate more in the body, it is also replicating at a much faster rate than has previously been seen, which could explain why it is able to spread more efficiently. Put simply, higher levels of virus in the body means greater transmissibility.

“Certainly, people with Delta are getting pneumonia and severe illness and some are dying – but at the moment, we do not know if this is at the same rate as before. What is clear, however, is that the highest spread of virus and severe outcomes is in those places with low vaccinations rates. said Prof Rawlinson.

Health experts are advising that if people want to get back to some sense of normality, then a significant portion of the population needs to be vaccinated. As long as large numbers of people remain unvaccinated across the world, new strains of the virus will continue to develop.