People with a high body mass index, or obesity, have strong protection against adverse outcomes when vaccinated against COVID-19, according to a new study from the United Kingdom. But the shots provided slightly less protection for underweight individuals, potentially due to frailty, investigators report.

Obesity is a top risk factor for severe COVID-19. A high BMI also has been linked to reduced immune responses to influenza vaccinations, according to the authors of the new study. To see whether the same was true for COVID-19 vaccines, they looked at BMIs and COVID-19 outcomes in a database of 9 million patients aged 18 years or older.

Results across the range of BMIs were compared between vaccinated and unvaccinated patients. Those who were vaccinated had received the Pfizer-BioNTech, Moderna or AstraZeneca vaccines. 

Across all BMI categories, patients who had received a second vaccine dose had 40% to 74% lower odds of hospital admission or death than their peers who were not vaccinated. But vaccinated, underweight patients appeared to have lower vaccine-derived immunity than those in other BMI groups. What’s more, underweight patients also were less likely to be vaccinated than their peers, the researchers reported. 

The lower vaccine efficacy in the underweight group could be due to frailty or underlying conditions such as cancer, which could be linked to immunosuppression, they theorized.

The prevalence of frailty, and vulnerability to COVID-19, is high in U.S. nursing facilities. Although COVID-19 risks rise with age, one early-pandemic study found that frailty better predicted COVID-19 survival than age. 

The rising trend of obesity among Americans has also  posed increased care challenges for some nursing home operators, experts say.

The risk remains

Despite being better defended against hospitalization and death than their unvaccinated peers, underweight patients and those with obesity were more likely to experience these adverse events than people at a clinically “healthy” BMI of 23 kg/m, the researchers reported. This was true even after a second vaccine dose.

Another large study from the Centers for Disease Control and Prevention has linked high BMI to higher likelihood of invasive mechanical ventilation and death from COVID-19. 

Full findings for the current study were published in the Lancet Diabetes & Endocrinology.

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