A recent study conducted by the Korea Disease Control and Prevention Agency (KCDA) and reported by Reuters has found that the highly transmissible Delta variant has a viral load around 300 times higher than the original COVID strain.
Delta variant presents the highest viral load
When a person becomes infected with coronavirus, the virus begins to replicate itself inside the body. The total amount of virus particles in an infected person is then measured as viral load or viral burden.
According to the South Korean study, the Delta variant has presented the highest viral load of the COVID-19 strains at 300 times more than the original strain and is highest when an infected person first shows symptoms. The viral load then gradually decreases on par with other variants around ten days post-infection peak.
To conduct the study, researchers compared the viral load of 1,848 Delta positive patients to 22,106 people infected with other strains. Results showed that the viral load of the Delta variant was still 30 times higher than the original COVID-19 strains four days after peak infection.
A small Chinese study published last month also found that Delta contained the highest viral load. The study compared the results of 62 Delta-infected people, with 63 of those infected with the original strain and suggestedviral load of Delta was 1000 times higher than the original variant.
Results also revealed that the Delta variant - the UK’s dominant strain - was first detectable four days after infection, while the original SARS-COV-2 was detected later, at six days post-infection.
Does this mean the Delta variant is 300 times more infectious?
A high viral load does indicate that the virus is more contagious. But researchers behind the South Korean study stressed that just because the Delta variant’s viral load is 300 times that of the original coronavirus strain, it doesn’t make the variant 300 times more transmissible:
We think its transmission rate is 1.6 times the Alpha [Kent] variant, and about two times the original version of the virus.
Delta’s viral load is high even in vaccinated adults
The study comes after an analysis of UK data revealed fully vaccinated people infected with the Delta variant also present a viral load just as high as those unvaccinated. However, researchers are still unsure of the implications of the results on Delta variant transmission. Sarah Walker, a professor of medical statistics and epidemiology at the University of Oxford, told The Guardian:
We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time, but the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped.
The fact that even vaccinated people can have a high viral burden suggests that herd immunity may no longer be possible. However, it must be stressed that this doesn’t mean the coronavirus vaccines are ineffective as they still significantly reduce the chances of severe infection and hospitalisation.