Delving into Delta: Virus slowly giving up its secrets

There are still a lot of unknowns with the Delta variant, but what we do know is that it has a much higher R0 value than previous strains

First identified in India in December 2020, Delta (a lineage B virus) has quickly become the most dominant coronavirus variant worldwide.

While questions still remain about its ability to cause more severe illness than previous variants, data suggest it is more than twice as infectious as the original virus arising in Wuhan (a lineage A virus) which has had an enormous influence on the course of the pandemic.

The Delta variant is one of four current “variants of concern” identified by the WHO. Largely, it is a variant of concern because it has features of these emerging mutants – that is we know it has increased transmissibility, something that has been experienced by many by countries across the world.

Already in Australia and New Zealand we are seeing it 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).

Unlike the 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.
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.

There are still a lot of unknowns with the Delta variant, but what we do know is that it has a much higher R0 value than previous strains. The R0, as we know, 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 earlier on in the pandemic, 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 viruses, and could be due to the fact that this strain is known to produce large amounts of virus.

It appears the Delta variant has some changes in its genome which makes it more infectious than other strains. Not only does it seem able to replicate more in the body, it is replicating at a much faster rate than has previously been seen and could explain why it is able to spread more efficiently. We know that this variant carries multiple mutations, including within the spike protein which binds the virus to human cells.

Recently, the P681R mutation has gained a lot of attention as it seems to be associated with more efficient cleavage. It is thought that this helps the variant infect more cells in a given amount of time, and therefore, make more copies of itself overall.

This mutation probably does contribute to the increase in spread, but it is certainly not the only cause and it is very important to remember that single mutations are usually associated with a number of other mutations in the virus genome.

The question still remains exactly how Delta affects the human body differently than other variants. 

To start with, the symptoms appear to be different from the traditional COVID symptoms we were told to look out for at the start of the pandemic. Previously fever, persistent cough and loss of taste or smell were the most common symptoms to look out for. But now, a headache is reported to be the most common symptom, followed by a sore throat, runny nose and fever.

It is still unclear whether the Delta variant causes more serious illness than previous strains. Certainly, people with Delta are getting pneumonia and severe illness and some are dying – but the question we need to answer is, are they dying at the same rate as before? We would hope not.

What is clear, however, is that the highest spread of virus and severe outcomes is in those places with low vaccinations rates. This clearly highlights the importance and immediate benefits of achieving the highest possible COVID-19 vaccine uptake, especially in outbreak areas.

The problem we face at the moment is that it has become clear that you can be fully vaccinated yet still have infection with the Delta variant. Almost universally, the virus does not cause severe illness in those that are vaccinated, but the vaccine still allows the virus to transmit.

That is not to say that you are not protected once you are vaccinated, the vaccines are still very effective, but they are not preventing transmission as much as we had hoped.

This has led to recommendations in some countries for a third booster dose of the mRNA vaccines (Pfizer, Moderna). The reality remains that if we have sufficient people vaccinated then we will be able to limit the global spread and as long as people continue to get vaccinated, then we will avoid big waves of hospitalisations.

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

Thanks to the Royal Australasian College of Pathologists for help in production of this article