COVID-19 vaccines work well with all body weights BUT underweight and obesity remain risk factors
A group of British researchers compares how vaccinated people in different categories fared against the new coronavirus
Since the start of the pandemic, more than 6 million people have died from COVID-19 around the world. The good news is that we now have a wide range of treatments as well as very efficient vaccines which have helped reduce the number of severe cases of COVID-19.
Still, some people are more likely to get very sick or die from COVID-19 than others. Our research, conducted before vaccines were available, showed that obesity is one of the factors that increases the risk of being hospitalized or dying from COVID-19. We have now followed this with a new study examining how body weight affects the uptake and effectiveness of COVID-19 vaccines, and found that being underweight can also be a risk factor for severe disease.
Obesity is measured using 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, between 18.5 and 25 is considered healthy weight, greater than 25 is considered overweight, and greater than 30 is considered obese .
These early findings on those 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 more.
Other research prior to the COVID-19 pandemic has shown that obese people are less likely take seasonal flu shots. There is also evidence suggesting that some vaccines work less well in obese people, although we do not fully understand the reasons for this.
What we have done
In our new study published in The Lancet Diabetes and Endocrinology, we used the anonymous health records of over 9 million people aged 18 and over in England. Our data covered the period from December 8, 2020 (when the first COVID-19 vaccine was administered in the UK) until November 17, 2021. During this period, 566,461 of the 9 million people tested positive for COVID-19, of which 32,808 were hospitalized and 14,389 died.
To study the vaccine’s effectiveness on different body weights, we looked at the risk of serious COVID-19-related outcomes (such as hospitalization and death) across the entire BMI range. Within the BMI groups, we compared vaccinated people with people of the same age, sex and other characteristics who had not been vaccinated.
We found that COVID-19 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 the healthy weight, overweight or obese groups were almost 70% less likely to be hospitalized with COVID-19 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-19 than unvaccinated people with the same BMI.
Our results indicate that COVID-19 vaccines may be slightly less effective in underweight people. Vaccinated people who were underweight 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 get vaccinated in the first place, and this was true across all age groups. The proportion of fully vaccinated people was highest among overweight and obese people. In our study, being fully vaccinated meant having received two or three doses, as not everyone was eligible for a booster at the end of our study period.
Vaccination rate by age and BMI
We also looked at the risk of serious consequences from COVID-19 only in people who had been vaccinated with at least one dose. Although vaccination significantly reduced the likelihood of severe COVID-19, people who were underweight and those who were obese remained at greater risk of hospitalization or death from COVID-19 than people with a healthy weight. It was also the case before the deployment of vaccines.
Risk of COVID-19 Hospitalization and Death After Two Vaccine Doses by BMI
The associations became less significant in those who had 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 will need further research to better understand the effect of boosters.
The data also did not allow us to explore differences between different COVID-19 vaccines, or to assess the influence of different COVID-19 variants.
People who receive two doses of a COVID-19 vaccine have a high level of protection against serious consequences, regardless of their body weight. Given the slight reduction in vaccine effectiveness in underweight people, targeted efforts may be needed in this group to increase vaccine coverage.
Although serious outcomes were much rarer after vaccination, people in the underweight and obese groups had a significantly higher risk than people with a healthy weight.
Public health strategies aimed at helping people achieve and maintain a healthy weight have many benefits. One of the most important could be to help reduce the burden of severe COVID-19.
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