US obesity epidemic could undermine effectiveness of a Covid-19 vaccine - CNN

But in the United States, where at least 4.6 million people have been infected and nearly 155,000 have died, the promise of that vaccine is hampered by a vexing epidemic that long preceded Covid-19: obesity.

Scientists know that vaccines engineered to protect the public from influenza, hepatitis B, tetanus and rabies can be less effective in obese adults than in the general population, leaving them more vulnerable to infection and illness.

While the exact biological mechanisms are still being investigated, chronic inflammation seems to interfere with the immune response to vaccines, possibly subjecting obese people to preventable illnesses even after vaccination.

An effective vaccine fuels a controlled burn inside the body, searing into cellular memory a mock invasion that never truly happened.

Evidence that obese people have a blunted response to common vaccines was first observed in 1985 when obese hospital employees who received the hepatitis B vaccine showed a significant decline in protection 11 months later that was not observed in non-obese employees.

The finding was replicated in a follow-up study that used longer needles to ensure the vaccine was injected into muscle and not fat.

Researchers found similar problems with the hepatitis A vaccine, and other studies have found significant declines in the antibody protection induced by tetanus and rabies vaccines in obese people.

"Obesity is a serious global problem, and the suboptimal vaccine-induced immune responses observed in the obese population cannot be ignored," pleaded researchers from the Mayo Clinic's Vaccine Research Group in a 2015 study published in the journal Vaccine.

Vaccines also are known to be less effective in older adults, which is why those 65 and older receive a supercharged annual influenza vaccine that contains far more flu virus antigens to help juice up their immune response.

By contrast, the diminished protection of the obese population — both adults and children — has been largely ignored.

"I'm not entirely sure why vaccine efficacy in this population hasn't been more well reported," said Catherine Andersen, an assistant professor of biology at Fairfield University who studies obesity and metabolic diseases.

Timothy Garvey, an endocrinologist and director of diabetes research at the University of Alabama, was among those who stressed that, despite the lingering questions, it's still safer for obese people to get vaccinated than not.

"The influenza vaccine still works in patients with obesity, but just not as well," Garvey said.