Explained: Two vaccine doses highly effective across people of different body weights, study suggests

A new study published in The Lancet Diabetes and Endocrinology has found that body size was not a factor in the effectiveness of vaccines — two doses are highly effective against severe disease for people who are underweight, overweight, or who have obesity.

A woman receives the Moderna vaccine during a COVID-19 vaccination at the Isabel Zendal Hospital in Madrid, Spain. (AP)

Obesity has been identified as a risk factor for severe Covid-19 since early in the pandemic, but little was known until now about the effectiveness of the vaccines for people with obesity. Now a new study published in The Lancet Diabetes and Endocrinology has found that body size was not a factor in the effectiveness of vaccines — two doses are highly effective against severe disease for people who are underweight, overweight, or who have obesity. In fact, the benefits were found to be slightly lower among underweight people compared to people with higher body mass index (BMI).

The sample

This is the largest study of BMI and Covid-19 vaccine effectiveness. Carried out in the UK, it covered 9,171,524 adult patients who had BMI data and had not previously been infected with SARS-CoV-2. They were grouped based on their BMI: healthy weight, underweight, overweight and obesity. During the study period from December 8, 2020 (date of the first vaccine given in the UK) to November 17, 2021, over 5.6 lakh of the 9 million participants tested positive. Of those, 32,808 were admitted to hospital and 14,389 died.

At the end of the study period, 23.3% of the healthy weight group, 32.6% of the underweight group, 16.8% of the overweight group and 14.2% of the group with obesity had not received any dose of any vaccine.

The researchers compared risk of severe disease in vaccinated versus unvaccinated people at least 14 days after a second dose. They found that being vaccinated offered high protection across all BMI groups, but the effect differed slightly in different BMI groups.

The findings

* In the underweight group, vaccinated people had around half the likelihood of being hospitalised or dying compared with unvaccinated people.

* In the healthy and high BMI groups, the vaccinated people were 70% less likely to be hospitalised than those unvaccinated, and two-thirds less likely to die those not vaccinated.

* Among vaccinated people only, of the fewer Covid-19 cases recorded, people of very low and very high BMI were more likely to experience severe disease. For example, following two doses, a BMI of 17 was linked to a 50% increase in risk of hospitalisation compared with a healthy BMI of 23, and a very high BMI of 44 had three times the risk of hospitalisation compared with a healthy BMI.

Takeaway for underweight

Story continues below this ad

“These data also highlight the need for targeted efforts to increase vaccine uptake in people with a low BMI, where uptake is currently lower than for people with a higher BMI,” lead author Dr Carmen Piernas of the Nuffield Department of Primary Care Health Sciences, University of Oxford, said in the report.

In a linked comment, Professor Annelies Wilder-Smith and Professor Annika Frahsa of the University of Berne (who were not involved in the study) wrote: “There was a higher vaccine uptake by persons with obesity… These findings should prompt a shift towards more targeted and differentiated public health messaging to also address underweight persons who may perceive themselves at lower risk in order to enhance vaccine uptake in this group.”

Newsletter | Click to get the day’s best explainers in your inbox

The study was found by the UK Research and Innovation and National Institute for Health Research Oxford Biomedical Research Centre.

Obesity as a risk factor

Story continues below this ad

One in five individuals worldwide are at increased risk of severe clinical outcomes after SARS-CoV-2 infection due to underlying health conditions, and there is now consistent evidence showing that obesity is a significant independent risk factor, according to the report. While the cause of the increased risk among people with obesity is unknown, there are several plausible explanations. These include fat deposited around the airways that could reduce functional lung capacity, obesity-related conditions that could exacerbate the pathology of Covid-19, higher viral load and prolonged and increased viral shedding that could affect recovery time, as well as factors that could impair T cell function. In addition, fat in the chest wall and abdomen in people with obesity could make ventilation more difficult, airways more prone to collapse, and could require higher pressures to maintain airways, which can lead to increased ventilation-induced damage.

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 . 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. Awards and Recognition 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.” 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. ... Read More