Some experts are concerned that a COVID-19 vaccine will be less effective in people with obesity than for those with a lower body mass.

“We know from other vaccines that have been tested in obese populations that these individuals don’t respond as well,” vaccine researcher Matthew B. Laurens, M.D., of the University of Maryland School of Medicine, and others told ABC News this week.

Obesity, which affects nearly 40% of Americans aged 60 and older, tops the list of chronic conditions that put people at risk for severe COVID-19, according to the Centers for Disease Control and Prevention.

There are theories about why these patients aren’t well protected by vaccines. These range from the idea that standard vaccine needles may be too short to reach shoulder muscle properly; that a bigger dose or a booster shot is needed to match body weight; and that chronic inflammation caused by obesity interferes with effectiveness, reported Nate Wood, M.D., an internal medicine/primary care resident who writes for ABC.

No matter the reason, one problem is that people with obesity often are excluded from vaccine trials due to chronic illness such as diabetes and high blood pressure, Wood wrote.

Laurens, who is working on a phase 3 vaccine trial funded by drugmaker Moderna and the National Institutes of Health, said this is the reason his team is including a diverse range of study participants in the trial, to “learn more about how to protect particularly the vulnerable populations, including the obese.”

In other coronavirus news

COVID-19 vaccine will not arrive in early fall, NIH director says: The head of the National Institutes of Health has made it clear that a coronavirus vaccine is unlikely to be ready for distribution by October. “Maybe November [or] December would be my best bet,” said Director Francis Collins, M.D., Ph.D., according to Inside Health Policy. The statement was made in response to an earlier announcement by the Department of Health and Human Services that a partnership with AstraZeneca could result in an October vaccine delivery.

Meanwhile, Paul Mango, HHS chief of staff for policy, said that the federal government’s Operation Warp Speed vaccine development program is seeking to distribute a vaccine in January, the news outlet reported.