Delta variant a challenge for herd immunity

The latest study used genomic and epidemiological data with mathematical modelling

The severe outbreak of Covid-19 in Delhi this year showed that the Delta variant of SARS-COV-2 can infect individuals previously infected by a different variant of the coronavirus, highlighting the challenges of reaching herd immunity against the variant, according to an international team of scientists. The study, published in Science on Thursday, also found that the Delta variant was 30-70 per cent more transmissible than previous variants.

Since the first case was detected in Delhi last year, the city experienced multiple outbreaks in June, September and November 2020. The situation became worse this year, when daily cases increased to 20,000 between

March 31 and April 16.

The authors noted that

Delhi’s overall seropositivity was reported to be 56.1 per cent, which was expected to confer some protection from future outbreaks through herd immunity.

The latest study used genomic and epidemiological data with mathematical modelling. “The concept of herd immunity is critical in ending outbreaks, but the situation in Delhi shows that infection with previous coronavirus variants will be insufficient for reaching herd immunity against Delta,” said study co-author Professor Ravi Gupta from the University of Cambridge.

“The only way of ending or preventing outbreaks of Delta is either by infection with this variant or by using vaccine boosters that raise antibody levels high enough to overcome Delta’s ability to evade neutralisation,” Gupta added.

The work was led by the National Centre of Disease Control and the Council of Scientific and Industrial Research-institute of Genomics and Integrative Biology in New Delhi with collaborators from London and Denmark.