COVID-19 vaccines give effective protection to people who are underweight, overweight, or who have obesity, according to a new study by a team led by researchers at the University of Oxford. However, those with a very low or very high BMI are still at increased risk of severe illness from COVID-19 compared to people of a healthy weight, and this mirrors disparities see before vaccines became available.
The data also suggested that underweight people were less likely to be vaccinated, although the reasons for this are not well understood.
Obesity was flagged as a risk factor for severe COVID-19 early on in the pandemic and people with a BMI over 40 were prioritised during the vaccine rollout in the UK. Previous studies have indicated that people with obesity have a slightly reduced benefit from the flu vaccine, and are less likely to get the vaccine to start with. However, prior to this study, little was known about the effectiveness of the COVID-19 vaccines for overweight people and those with obesity.
The study, published in The Lancet - Diabetes & Endocrinology, used anonymised health records from nine million people across the UK across 1,738 GP practices in England taking part in QResearch – a secure database of healthcare information available to verified researchers. It used the World Health Organisation classifications of body mass index (BMI), with levels adjusted for Asian people to reflect the higher health risks at lower BMI levels in this group. Characteristics such as age, sex, smoking status, and social deprivation were also accounted for in the analyses.
Lead author Dr Carmen Piernas of the University of Oxford’s Nuffield Department of Primary Care Health Sciences said:
Our findings provide further evidence that COVID-19 vaccines save lives for people of all sizes. Our results provide reassurance to people living with obesity that COVID-19 vaccines are equally as effective for them as for people with a lower BMI, and that vaccination substantially lowers their risk of severe illness if they are infected with COVID-19.
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.
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.
The researchers found that being vaccinated offered high protection across all BMI groups, but that the effect was slightly lower in underweight people. For those who were underweight, being vaccinated reduced the likelihood of being hospitalised or dying by about a half, compared with people of a similar BMI who were unvaccinated.
But for all other BMIs, being vaccinated lowered the risk of being hospitalised by 70%.
Among vaccinated people, there was a significantly higher risk of severe disease for people who were underweight, overweight or had obesity, compared to people with a healthy BMI, even though vaccination reduced the overall risk of severe COVID-19.
For example, a very low 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. This is similar to the increased risk of severe COVID-19 outcomes shown prior to the vaccination programme.