COVID vaccines work well for people of all body weights, but underweight and obesity remain risk factors for serious disease

More than 6 million people worldwide have died from COVID since the start of the pandemic. The good news is that we now have a wide range of treatments as well as highly effective vaccines that have helped reduce the number of severe COVID cases.

Still, some people are more likely to get very sick or die from COVID than others. Our research, conducted before vaccines became available, showed that obesity is one of the factors that increases a person’s risk of being hospitalized or dying from COVID. We’ve now followed this up with a new study looking at how body weight affects the uptake and effectiveness of COVID vaccines, and found that being underweight can also be a risk factor for serious disease.

Obesity is measured using the body mass index (BMI). This is calculated by dividing a person’s weight in kilograms by their height in meters, squared. A person with a BMI of less than 18.5 is considered underweight, in the range of 18.5-25 is considered a healthy weight, above 25 is considered overweight, and above 30 is considered obese.

These early findings about the people most at risk were used to support policies that prioritized certain groups of people for vaccination. This included people with a BMI of 40 or higher.

Other research from before the COVID pandemic has shown that obese people are less likely to take seasonal flu vaccines. There is also some evidence that some vaccines work less well in obese people, although we don’t fully understand the reasons for this.

what we did

In our new study, published in The Lancet Diabetes & Endocrinology, we used anonymous health records from more than 9 million people aged 18 and over in England. Our data covered the period from December 8, 2020 (the date the UK’s first COVID vaccine was administered) to November 17, 2021. During this time, 566,461 people out of 9 million had tested positive for COVID, of which 32,808 were admitted hospitalized and 14,389 died.

To investigate the effectiveness of vaccines at different body weights, we looked at the risk of serious COVID outcomes (such as hospitalization and death) across the entire BMI range. Within BMI groups, we compared people who had been vaccinated with people of the same age, sex and other characteristics who had not been vaccinated.

We found that COVID vaccines were highly effective against severe outcomes across all BMI categories, especially after the second and third doses. After the second dose, vaccinated people in healthy weight, overweight or obese groups were nearly 70% less likely to be hospitalized due to COVID than their unvaccinated counterparts. Similarly, vaccinated people in the healthy weight, overweight and obese groups were about 60-74% less likely to die from COVID than unvaccinated people of the same BMI.

Read more: COVID vaccines for children under five: what parents need to know

Our results indicate that COVID vaccines may be slightly less effective in underweight people. Vaccinated underweight people were about 50% less likely to be hospitalized and about 40% less likely to die than their unvaccinated counterparts. Of course, this still represents a good level of protection.

People who were underweight were also the least likely to be vaccinated initially, and this was true of all age groups. The proportion of fully vaccinated people was highest among overweight and obese people. Fully vaccinated meant getting two or three doses in our study, as not everyone was eligible for a booster at the end of our study period.

Uptake of vaccination by age and BMI

A chart showing the uptake of COVID vaccination by age group and BMI.

Adapted from The Lancet Diabetes & Endocrinology, Author provided

We also only looked at the risk of serious COVID outcomes in the people who were vaccinated with at least one dose. Although vaccination greatly reduced the risk of severe COVID-19, underweight and obese people were at greater risk of hospitalization or death from COVID than people of a healthy weight. This was also the case before vaccines were rolled out.

Risk of COVID hospitalization and death after two vaccine doses according to BMI

Two figures with curves showing increasing risk of hospitalization (left) and death (right) with increasing BMI, after two doses of COVID vaccine.

Adapted from The Lancet Diabetes & Endocrinology, Author provided

The associations became less significant in those who received a third dose. But since only a limited number of people were eligible for the third dose at the end of the study period, we need further research to better understand the effect of boosters.

The data also did not allow us to examine any differences between the different COVID vaccines, or to assess the influence of different COVID variants.

Read more: Inflammation: key factor explaining vulnerability to severe COVID

People who have received two doses of a COVID vaccine receive a high level of protection against serious consequences, regardless of their body weight. Given the slight reduction in vaccine efficacy in underweight people, targeted efforts may be needed in this group to increase vaccination coverage.

While serious outcomes were much rarer after vaccination, people in the underweight and obese groups had a significantly higher risk compared to those of a healthy weight.

Public health strategies to help people achieve and maintain a healthy weight have many benefits. An important one could be to help reduce the burden of severe COVID.

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