Delta variant behind April wave in Delhi, shows NCDC study

Scientists have found this variant to be much more transmissible than other variants in the national capital, the study shows. This is the first time that a surge in any region has been directly linked to the circulation of a particular variant of the virus.

Written by Anuradha Mascarenhas

Pune | Updated: June 5, 2021 08:40 AM IST

4 min read

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The variant was isolated from nasal/throat swabs from two travellers who returned to India from the UK and Brazil. (Express file photo by Gajendra Yadav)

THE MASSIVE surge in coronavirus infections in Delhi in April was caused by the rapid spread of B.1.617.2, a sub-lineage of the variant first found in Maharashtra last year and recently named Delta, according to a new study.

In the study, scientists from the National Centre for Disease Control (NCDC) and Delhi-based Institute of Genomics and Integrative Biology (IGIB) assessed the Delta variant to be at least 50 per cent more transmissible than the Alpha variant, which was first seen in the UK and was predominant in the region in February and March.

By April, the study shows, the Delta variant had replaced Alpha as the most common variant in Delhi’s population, and accounted for nearly half of all the new infections. “Detection of B1.1.7 (Alpha variant) was minimal (in Delhi population) till January 2021, increasing to about 20 per cent in February and 40 per cent in March,” the study said.

“(The presence of) B.1.617 lineages (Delta is a sub-lineage) increased from below five per cent in February 2021 to about 10 per cent in March before overtaking B.1.17 in April and rising to about 60 per cent of all samples. The sub-lineage B.1.617.2 (Delta variant)…showed the maximum rise going from less than 10 per cent of B.1.617 (parent lineage) to about 80 per cent,” it said.

The study also said that while the Delta variant had higher transmissibility and carried greater viral load, it seemed to cause a lesser number of deaths.

“Our data indicates B.1.617.2 (Delta) shows high transmissibility and surges without any increase in the CFR (Case fatality Ratio, or the death rate). We estimate the transmissibility to be as much as 50 per cent greater than B.1.1.7. Viral load of B.1.617.2 appears to be higher than B.1.1.7, and based on the data from India and UK, so does vaccination breakthrough rate,” it said.

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The study said that the Delta variant’s impact on mortality needed to be re-examined, and that the CFR in Delhi was “significantly reduced” during the surge period.

“This may be due to the sudden decrease of B.1.1.7 (Alpha) in the Delhi region which reportedly causes high CFR. The CFR was rising again towards the end of the period (May). Since CFR may be due to an amalgamation of multiple factors, including short-term collapse of the healthcare system, there is currently no evidence linking B.1.617.2 to change in CFR,” it said.

Sujit Singh, head of NCDC and one of the authors of the study, told The Indian Express that several characteristics of the Delta variant were still under investigation. “We are studying the role of the Delta variant in breakthrough infections, or infections post vaccination, and will submit a report soon,” he said.

The B.1.617 lineage, first discovered in October last year in Maharashtra, is believed to be the main trigger for the second wave of infections in the country. Subsequently, three sub-lineages of this variant were detected, of which B.1.617.2 — the others are B.1.617.1 and B.1.617.3 — has been found to be the most dominant. A few days ago, this variant was named Delta by the World Health Organisation (WHO).

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The Indian Express had earlier reported that several scientists have acknowledged that a greater surveillance on the circulation of B.1.617 lineage during December and January could have warned the country of the impending second wave.

Anuradha Mascarenhas

Anuradha Mascarenhas is a Senior Editor at The Indian Express, based in Pune. With a career spanning three decades, she is one of the most respected voices in Indian journalism regarding healthcare, science and environment and research developments. She also takes a keen interest in covering women's issues . Anuradha has won several awards including the Press Council of India's national award for excellence in journalism under the gender based reporting category in 2019 and the Laadli Media award (gender sensitivity -2024). A recipient of the Lokmat journalism award (gender category-2022), she was also shortlisted for the RedInk awards for excellence in journalism-2021. Her debut book At The Wheel Of Research, an exclusive biography of Dr Soumya Swaminathan the inaugural chief scientist of World Health Organisation was also nominated in the Popular Choice Category of JK Paper AUTHER awards. She has also secured competitive fellowships including the Laadli Media Fellowship (2022), the Survivors Against TB – New Research in TB Media Fellowship (2023) and is part of the prestigious 2025 India Cohort of the WomenLift Health Leadership Journey.” Professional Background Education: A gold medalist in Communication and Journalism from Savitribai Phule Pune University and a Master’s degree in Literature. Author: She authored the biography At The Wheel Of Research, which chronicles the life and work of Dr. Soumya Swaminathan, the former Chief Scientist at the WHO. Key Focus: She combines scientific accuracy with storytelling, translating complex medical research into compelling public and human-interest narratives. Recent Notable Articles (Late 2025) 1. Cancer & Specialized Medical Care "Tata Memorial finds way to kill drug-resistant cancer cells" (Nov 26, 2025): Reporting on a breakthrough for triple-negative breast cancer, one of the most aggressive forms of the disease. ``Discipline, diet and purpose; How a 97-year-old professor defies ageing'' (Nov 15, 2025) Report about Prof Gururaj Mutalik, the first Head of Department at Pune's B J Government Medical College who at 97 credits his longevity to healthy habits and a strong sense of purpose. 2. Environmental Health (The "Breathless Pune" Series) "Long-term exposure even to 'moderate' air leads to chronic heart, lung, kidney issues" (Nov 26, 2025): Part of an investigative series highlighting that even "safe" pollution levels are damaging to vital organs. "For every 10 µg/m3 increase in PM2.5 level, there was 6-8% jump in medicine sales" (Nov 23, 2025): Using commercial data to prove the direct link between air quality and respiratory illnesses in Pune. 3. Lifestyle & Wellness News "They didn't let cancer, diabetes and heart disease stop them from travelling" (Dec 22, 2025): A collaborative piece featuring survivors who share practical tips for traveling with chronic conditions. ``At 17, his BP shot up to 200/120 mmHG; Lancet study flags why child and teen hypertension doubled between 2000 and 2020'' (Nov 12,2025)--A report that focusses on 17-year-old-boy's hypertensive crisis and reflects the rising global trend of high blood pressure among children and adolescents. 4. Scientific Recognition & Infrastructure "For promoting sci-comm, gender diversity: IUCAA woman prof highlighted in Nature" (Nov 25, 2025): Covering the global recognition of Indian women scientists in gender studies and physics. ``Pune researchers find a spiral galaxy like the Milky Way from early universe'' (December 3, 2025)- A report on how Indian researchers discovered a massive galaxy that existed when the universe was just 1.5 billion years old , one of the earliest to have been observed so far. Signature Beat: Health, Science & Women in Leadership Anuradha is known for her COVID-19 reportage, where she was one of the first journalists to provide detailed insights into the Covishield and Covaxin trials. She has a dedicated interest in gender diversity in health and science, often profiling women researchers who are breaking the "leaky pipeline" in STEM fields. Her writing style is scrupulous, often featuring interviews with top-tier scientists and health experts from various institutions X (Twitter): @runaanu ... Read More

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