2021 Covid Outbreak In Delhi Shows Herd Immunity Against Delta Variant Difficult: Study

New Delhi: 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 the journal Science on Thursday (October 14), also found that the Delta variant was between 30-70 per cent more transmissible than previous SARS-CoV-2 lineages in Delhi.

Also Read: Mix-and-match Vaccines Highly Effective Against COVID-19: Lancet Study

Since the first case of COVID-19 was detected in Delhi in March last year, the city experienced multiple outbreaks, in June, September and November 2020. The situation became worse in April this year, when daily cases increased from approximately 2,000 to 20,000 between March 31 and April 16. This was accompanied by a rapid rise in hospitalisations and ICU admissions, severely stressing the healthcare system, with daily deaths spiking to levels three-fold higher than previous waves.

The study 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. Herd immunity is a form of indirect protection from a disease that can occur when a sufficient percentage of a population has become immune to an infection.

Also Read: Delhi Government Launches WhatsApp Number To Provide COVID Info, Locate Vaccination Centres, Book Slots

The latest study used genomic and epidemiological data, together with mathematical modelling, to study the outbreak. 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 (CSIR-IGIB) in New Delhi with collaborators from the University of Cambridge and Imperial College London, UK, and the University of Copenhagen, Denmark.

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,” Professor Gupta added.

To determine whether SARS-CoV-2 variants were responsible for the April 2021 outbreak in Delhi, the team sequenced and analysed viral samples from Delhi from the previous outbreak in November 2020 until June 2021. They found that the 2020 outbreaks in Delhi were unrelated to any variant of concern. The Alpha variant, first identified in the UK, was identified only occasionally, primarily in foreign travellers, until January 2021, according to the researchers. Alpha 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 in April, they said.

Also Read: Decision On COVID Vaccination Of Children To Be Based On Scientific Rationale: Dr V K Paul

Applying mathematical modelling to the epidemiological and genomic data, the researchers found that the Delta variant was able to infect people who had previously been infected by SARS-CoV-2. The researchers noted that prior infection provided only 50-90 per cent of the protection against infection with Delta variant that it provides against previous lineages.

This work helps understand the global outbreaks of Delta, including in highly vaccinated populations, because the Delta variant can transmit through vaccinated or previously infected people to find those who are susceptible, said Anurag Agrawal from CSIR-IGIB, senior author and co-lead investigator of the study.

To look for actual evidence of reinfection to support their modelling work, the researchers examined a group of people 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 of the outbreak in the country. 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 breakthrough infections than non-Delta lineages. Previous study by Gupta showed that the Delta variant has most likely spread through its ability to evade neutralising antibodies and its increased infectivity.

Also Read: Meet The Man Who Cremated Over 4,000 Bodies During The COVID-19 Pandemic

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

NDTV – Dettol have been working towards a clean and healthy India since 2014 via Banega Swachh India initiative, which is helmed by Campaign Ambassador Amitabh Bachchan. The campaign aims to highlight the inter-dependency of humans and the environment, and of humans on one another with the focus on One Health, One Planet, One Future – Leaving No One Behind. It stresses on the need to take care of, and consider, everyone’s health in India – especially vulnerable communities – the LGBTQ population, indigenous people, India’s different tribes, ethnic and linguistic minorities, people with disabilities, migrants, geographically remote populations, gender and sexual minorities. In wake of the current COVID-19 pandemic, the need for WASH (Water, Sanitation and Hygiene) is reaffirmed as handwashing is one of the ways to prevent Coronavirus infection and other diseases. The campaign will continue to raise awareness on the same along with focussing on the importance of nutrition and healthcare for women and children, fight malnutrition, mental wellbeing, self care, science and health, adolescent health & gender awareness. Along with the health of people, the campaign has realised the need to also take care of the health of the eco-system. Our environment is fragile due to human activity,  that is not only over-exploiting available resources, but also generating immense pollution as a result of using and extracting those resources. The imbalance has also led to immense biodiversity loss that has caused one of the biggest threats to human survival – climate change. It has now been described as a “code red for humanity.” The campaign will continue to cover issues like air pollution, waste management, plastic ban, manual scavenging and sanitation workers and menstrual hygiene. Banega Swasth India will also be taking forward the dream of Swasth Bharat, the campaign feels that only a Swachh or clean India where toilets are used and open defecation free (ODF) status achieved as part of the Swachh Bharat Abhiyan launched by Prime Minister Narendra Modi in 2014, can eradicate diseases like diahorrea and the country can become a Swasth or healthy India.