Increased Infectivity And Immune Evasion Drive The Spread Of
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Findings suggest infection control measures against variants will need to continue in the post-vaccination era. The Delta variant of SARS-CoV-2, which has become the dominant variant in countries including India and the UK, has most likely spread through its ability to evade neutralizing antibodies and its increased infectivity, say an international team of researchers.

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 neutralizing antibodies produced as a result of previous infection or vaccination are less effective at stopping this variant.

To examine how well the Delta variant was able to evade the immune response, the team extracted serum from blood samples collected as part of the COVID-19 cohort of the NIHR BioResource. The samples came from individuals who had previously been infected with the coronavirus or who had been vaccinated with either the Oxford/AstraZeneca or Pfizer vaccines. Serum contains antibodies raised in response to infection or vaccination.

The team found that the Delta variant virus was 5.7-fold less sensitive to the sera from previously-infected individuals, and as much as eight-fold 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. Consistent with this, an analysis of over 100 infected healthcare workers at three Delhi hospitals, nearly all of whom had been vaccinated against SARS-CoV-2, found the Delta variant to be transmitted between vaccinated staff to a greater extent than the alpha variant.

Using 3D airway organoids - 'mini-organs' grown from cells from the airway, which mimic its behavior - the team studied what happens when the virus reaches the respiratory tract. Working under secure conditions, the team used both a live virus and a 'pseudotyped virus' - a synthetic form of the virus that mimicked key mutations on the Delta variant - and used this to infect the organoids.

They found that 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. 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.