Covid-19 vaccines significantly reduced the number of cases of severe disease for everyone, regardless of their BMI, according to researchers at the University of Oxford.
A new study of over nine million people in England found two doses of the Covid-19 vaccine were effective against severe infection for underweight, overweight, or obese people.
In further analysis of the vaccinated participants, the results showed that people with a healthy BMI had less risk of hospitalisation than those who were underweight or overweight.
The findings provide some of the first evidence of Covid-19 vaccine effectiveness across all BMI categories and are published in The Lancet Diabetes and Endocrinology.
The researchers examined anonymised health records from more than 12 million people from 1 738 GP practices in England. Of these, 9,171,524 patients over 18 years old, had BMI data, and had not previously been infected with Covid-19 were included in the study. During the study period, which ran from December 2020 until November 2021, 566 461 participants tested positive for Covid-19.
The World Health Organisation (WHO) classification of BMI was used to determine if participants were healthy weight (18.5 and 24.9), underweight (below 18.5), overweight (25-29.9) or obese (30 and over) when they caught Covid-19. Levels were adjusted for Asian people to reflect the higher health risks at lower BMI levels in this group, and characteristics such as age, sex, smoking status, and social deprivation were accounted for in the analysis.
The researchers found that being vaccinated offered high protection across all BMI groups. Vaccination meant people were around 70% less likely to be hospitalised and approximately two-thirds less likely to die than people who had not been vaccinated.
Dr Carmen Piernas from the University of Oxford said: ‘Our findings provide further evidence that Covid-19 vaccines save lives for people of all sizes.’
However, the results showed that among vaccinated people, the vaccine was most effective in people of a healthy weight. Even with vaccination, participants who were underweight, overweight or obese were at a significantly higher risk of severe infection from Covid-19 when compared with participants with a healthy BMI.
The researchers found that people with a BMI of 17 had a 50% increased risk of hospitalisation and people with a BMI of 44 had three times the risk of hospitalisation compared to people with a healthy BMI of 23.
Dr Piernas said: ‘The cause of the increased risk among people with obesity is unknown. It is, however, consistent with the higher rate of seasonal flu infections in people with a higher BMI. We suspect that these findings may be explained, in part, by an altered immune response in heavier weight individuals, but that is just speculation at this point.
‘The reduced effectiveness of Covid-19 vaccines among people with a low BMI may also reflect a reduced immune response due to frailty or other conditions associated with low body weight. Further research is needed to explore the relationship between BMI and immune responses.’
This comes after a large observational US study published in March found that most reported reactions to mRNA Covid-19 vaccines Pfizer-BioNTech and Moderna were mild and transient.