The coronavirus Delta variant, which has become dominant in a number of countries including the UK, most likely spreads through its ability to evade antibodies and its increased infectivity, researchers have said.
As the virus replicates, errors in its genetic make-up cause it to mutate.
Some mutations make it more transmissible or more infectious, some help it evade the immune response, potentially making vaccines less effective, while others have little effect.
One such variant is the Delta variant, was first observed in India in late 2020, and it has since spread around the world.
In the UK, it is responsible for nearly all new cases of coronavirus.
Professor Ravi Gupta, from the Cambridge Institute of Therapeutic Immunology and Infectious Disease at the University of Cambridge – one of the study’s senior authors, said: “By combining lab-based experiments and epidemiology of vaccine breakthrough infections, we’ve shown that the Delta variant is better at replicating and spreading than other commonly-observed variants.
“There’s also evidence that neutralising antibodies produced as a result of previous infection or vaccination are less effective at stopping this variant.”
Prof Gupta is also a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), which advises the Government.
He added: “These factors are likely to have contributed to the devastating epidemic wave in India during the first quarter of 2021, where as many as half of the cases were individuals who had previously been infected with an earlier variant.”
To examine how well the Delta variant was able to evade the immune response, researchers extracted serum from blood samples collected as part of the Covid-19 cohort of the NIHR BioResource.
The samples came from people who had previously been infected with the coronavirus or who had been vaccinated with either the Oxford/AstraZeneca or Pfizer/BioNTech vaccines.
Serum contains antibodies raised in response to infection or vaccination.
The team found the Delta variant virus was 5.7-fold less sensitive to the sera from previously-infected individuals, and as much as eight times less sensitive to vaccine sera, compared with the Alpha variant.
In other words, it takes eight times as many antibodies from a vaccinated individual to block the virus.
While an analysis of more than 100 infected healthcare workers at three Delhi hospitals, nearly all of whom had been vaccinated, found the Delta variant to be transmitted between vaccinated staff to a greater extent than the Alpha variant.
In laboratory experiments, the researchers found the Delta variant was more efficient at breaking into the cells compared with other variants as it carried a larger number of cleaved spikes on its surface.
They also found that once inside the cells, the variant was also better able to replicate.
Both of these factors give the virus a selection advantage compared to other variants, helping explain why it has become so dominant, experts suggest.
Dr Partha Rakshit, from the National Centre for Disease Control, Delhi, India – joint senior author, said: “The Delta variant has spread widely to become the dominant variants worldwide because it is faster to spread and better at infecting individuals than most other variants we’ve seen.
“It is also better at getting around existing immunity – either through previous exposure to the virus or to vaccination – though the risk of moderate to severe disease is reduced in such cases.”
The research, published in Nature, was supported in India by the Ministry of Health and Family Welfare, the Council of Scientific and Industrial Research, and the Department of Biotechnology, and in the UK by Wellcome, the Medical Research Council and the National Institute of Health Research.