Second Covid wave in Delhi shows reaching herd immunity difficult with Delta: Study

The severe outbreak of COVID-19 in Delhi this year showed not only that the Delta variant of SARS-CoV2 is extremely transmissible but that it can infect individuals previously infected by a different strains of the virus, according to a team of international scientists.

The team led by National Centre of Disease Control and the Council of Scientific and Industrial Research (CSIR), Institute of Genomics and Integrative Biology, India, along with collaborators from the University of Cambridge and Imperial College London, UK, and the University of Copenhagen, Denmark, used genomic and epidemiological data, together with mathematical modelling to study the outbreak.

To determine whether SARS-CoV-2 variants were responsible for the April 2021 outbreak in Delhi, the team sequenced and analysed viral samples from the previous outbreak in November 2020 until June 2021.

Their findings, published in the journal Science, showed that the 2020 outbreaks in Delhi were unrelated to any variant of concern. The Alpha variant (B.1.1.7) was identified only occasionally, primarily in foreign travellers, until January 2021.

The Alpha variant increased in Delhi to about 40 per cent of cases in March 2021, before it was displaced by a rapid increase in the Delta variant (B.1.617.2) in April.

The researchers examined a cohort of individuals recruited by CSIR. In February, 42.1 per cent of unvaccinated subjects participating in the study had tested positive for antibodies against SARS-CoV-2.

In June, the corresponding number was 88.5 per cent, suggesting very high infection rates during the second wave. Among 91 subjects with prior infection before Delta, about one-quarter (27.5 per cent) showed increased levels of antibodies, providing evidence of reinfection.

When the team sequenced all the samples of vaccination-breakthrough cases at a single centre over the period of the study, they found that among 24 reported cases, Delta was seven-fold more likely to lead to vaccination breakthroughs than non-Delta lineages.

Since the first case of Covid-19 was detected in Delhi in March 2020, the city had experienced multiple outbreaks, in June, September and November 2020.

In November 2020, the capital city had almost 9,000 cases daily, but it declined steadily between December 2020 and March 2021. However, the situation reversed dramatically in April 2021, going from approximately 2,000 daily cases to 20,000 between March 31 and April 1.

"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 Professor Ravi Gupta from the Cambridge Institute of Therapeutic Immunology and Infectious Disease.

"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," he added.

The research was also supported by the Ministry of Health and Family Welfare, Council of Scientific and Industrial Research, and Department of Biotechnology.