Obesity can complicate the course of COVID-19. Now, a new study says it can also reduce the effectiveness of COVID vaccines.

The researchers also found that the two vaccines used in their study triggered different levels of immune responses in severely obese people. They found, too, that prior infection had an impact.

"These results provide new information on the antibody response to SARS-CoV-2 vaccines in people with severe obesity and reinforce the importance of prioritizing and increasing vaccine uptake in this vulnerable group," study co-author Volkan Demirhan Yumuk said in a news release from the European Congress on Obesity. Yumuk is a professor at Istanbul University in Turkey.

The study included 124 severely obese adults (average age 42 to 63) and 166 normal weight adults (average age 39 to 47) in Turkey. The obese group had a body mass index (BMI) — a measure based on height and weight — of more than 40.

The participants received two doses of either the Pfizer/BioNTech vaccine or the CoronaVac vaccine made by the Chinese company Sinovac.

Four weeks after their second dose, blood samples were collected from the participants to measure their levels of antibodies against the coronavirus. In addition, their infection history was checked, and 70 were found to have previously been infected with COVID-19.

Among participants who had no previous infection and received the Pfizer/BioNTech vaccine, antibody levels among those who were severely obese were more than three times lower than those with normal weight, the study revealed.

Among those who had no previous infection and received the CoronaVac vaccine, antibody levels among the severely obese were 27 times lower than those with normal weight.

Among those who'd previously been infected and received either Pfizer/BioNTech or CoronaVac, antibody levels were similar in both severely obese and normal weight people, according to the study. The findings were to be presented at this week's European Congress on Obesity in Maastricht, the Netherlands.

The findings have implications for the United States, where more than four in 10 people are obese, according to the U.S. Centers for Disease Control and Prevention. More than 9% are severely obese.

"Our study confirms that immune memory induced by prior infection alters the way in which people respond to vaccination and indicates that two doses of Pfizer/BioNTech vaccine may generate significantly more antibodies than CoronaVac in people with severe obesity, regardless of infection history," Yumuk said.

However, Yumuk added, "further research is needed to determine whether these higher antibody levels provide greater protection against COVID-19."

Data and conclusions presented at meetings should be considered preliminary until published in a peer-reviewed medical journal.