Researchers at the University of Cambridge report that obesity can hasten the decline of immune protection after a COVID-19 vaccination. The study appears in Nature Medicine. This finding adds to a growing body of work on how body weight influences vaccine effectiveness across different diseases.
It is already known that obesity can blunt the immune response to several vaccines, including those for influenza, rabies, and hepatitis. The Cambridge team notes that the impact on COVID-19 vaccines has not been thoroughly explored until now.
Vaccines work by prompting the body to produce antibodies that recognize the spike protein on the surface of SARS-CoV-2, the virus behind COVID-19. They also activate T cells that help prevent severe illness. In several studies, antibody levels after vaccination have appeared lower among people with obesity compared with individuals of normal weight.
To examine this in a large population, researchers used the EAVE II health data platform, which contains information for about 5.4 million people across Scotland. The analysis focused on hospitalizations and deaths due to COVID-19 among 3.5 million adults who had received two vaccine doses, either Pfizer or AstraZeneca.
The results show that people with severe obesity, defined as a body mass index above 40, faced a 76 percent higher risk of hospitalization or death from COVID-19 after vaccination compared with those in the normal BMI range. Those with BMI 30 to 40 also showed an elevated risk, and surprisingly, individuals who were underweight (BMI below 18.5) also exhibited higher risk relative to normal-weight peers.
According to the researchers, an additional set of experiments looked at the immune response after a booster dose in a subgroup of participants with extreme obesity. The team studied 28 extremely obese individuals and compared their antibody levels, antibody function, and immune cell counts after vaccination with those of 41 individuals of normal weight. The findings were shared by the study authors.
Although antibody concentrations were similar across groups, the functional ability of the antibodies to neutralize the virus was reduced in the severely obese group. This observation suggests that even when customarily measured antibody levels appear comparable, the quality of the immune response may be diminished in people with severe obesity, potentially influencing protection against COVID-19. The broader implication is a reminder that body weight can shape how well vaccines perform, particularly in preventing hospitalizations and severe outcomes. Ongoing follow-up studies are expected to refine our understanding of booster strategies for individuals with high BMI and to guide public health messaging in North America and beyond.
In summary, obesity appears to correlate with a weaker functional antibody response after vaccination and a higher risk of serious outcomes from COVID-19, despite full vaccination. Health professionals are encouraged to consider BMI as one of several factors when evaluating vaccine effectiveness and the potential need for tailored booster recommendations. The Cambridge findings align with broader research showing that weight-related immune changes can affect responses to vaccines and infectious diseases in diverse populations across Canada, the United States, and other regions.
These insights come at a time when vaccination remains the best defense against COVID-19 and its evolving variants. They reinforce the importance of preventive measures for individuals with obesity and underscore the value of booster campaigns to sustain protective immunity in high-risk groups. Public health programs may benefit from integrating weight management guidance as part of comprehensive vaccination strategies, aiming to maximize the protective benefits of vaccination for all communities.