In the study, mathematical modelling indicated that the variant reinfected many who had been infected in the first wave in India.
Given Delta’s immune system evasion ability, the study cautioned that strong ‘infection control measures’ and boosting vaccine response would be necessary for health-care workers even in the post-vaccination era, since hospitals receive numerous vulnerable patients with poor immunity or with co-morbidities.
It looked at data across three Delhi hospitals to assess the level of ‘breakthrough infections’ – infection even after vaccination – and found Delta to be the key factor in transmitting chains, even though severe disease in fully vaccinated workers was rare.
The study also reaffirms that the Delta variant – B.1.617.2 – is faster, fitter and comes with a strong immune evasion response, allowing it to drive the second wave in India.
INSACOG – the Indian SARS-CoV-2 Genomic Consortia – is a government body set up to study and monitor genome sequencing and virus variation of Covid-19 strains.
From the Mumbai data, the study found that “the Delta variant is 1.1- to 1.4-fold (50% bCI) more transmissible than previously circulating lineages in Mumbai, and that B.1.617.2 is able to evade 20-55% of the immune protection provided by prior infection with non-B.1.617.2 lineages.”
The study – ‘SARS-CoV-2 B.1.617.2 Delta variant emergence and vaccine breakthrough – is in preprint and under submission to the reputed scientific journal, Nature. Its co-authors include Sujeet Singh, Director, National Centre for Disease Control; Anurag Agarwal of the Institute of Genomic Integration Behaviour, experts from three hospitals in Delhi, besides researchers from the University of Cambridge, and the Imperial College of London.
“Delta variant has the maximum transmissibility yet seen and reduces neutralization protection from previous infections and vaccines,” Agarwal tweeted, describing the study.
The study analyses vaccine breakthroughs in over 100 healthcare workers across three centres in India and surmises that the Delta variant not only dominates vaccine-breakthrough infections with higher respiratory viral loads compared to non-Delta infections, but also generates greater transmission between health-care workers compared to the Alpha or Kappa variants.
“There were no clusters of non-Delta infections comprising more than two individuals, whereas there were 10 such clusters for Delta variant. Importantly, the median Ct value (indicating viral load) of B.1.617.2 Delta variant infections was 16.5 versus 19 for non-Delta,” the study pointed out.
The study has also looked at the Delta variant’s ability to evade both antibodies after an infection and after vaccination.
It points to the ‘8-fold reduced sensitivity to vaccine-elicited antibodies’ in Delta versus the original variant – wild type Wuhan-1. It also notes that serum neutralising titres against the SARS-CoV-2 Delta variant were significantly lower in participants vaccinated with the AstraZeneca vaccine, compared to the Pfizer vaccine.