Severely overweight healthcare workers generated only half the antibodies to fend off Covid after receiving two doses of the jab, compared to healthy people.
Italian researchers said obese people may need another top-up vaccine, or bigger doses, to protect them against the disease.
Severely overweight people are already considered to be more at risk of becoming critically ill or dying if they get infected.
Scientists say this may be because they are more likely to have other conditions —such as diabetes and high blood pressure — or suffer from breathing difficulties.
Experts today said it was also well-known obesity — defined as having a BMI above 30 — hampers the effectiveness of jabs.
Previous research has suggested the flu vaccine, which is dished out every winter, could be half as effective in severely overweight people.
But the Italian study, yet to be scrutinized by fellow scientists, is believed to be the first time the same link has been found for Covid.
Carrying added weight can lead to the immune system being constantly mildly inflamed impairing its function, according to experts.
For comparison, in healthy people, it only triggers inflammation when it is fighting an infection.
Almost a third of adults in England and nearly 40 percent in the US are classed as obese, according to estimates.
In the study, 248 participants received two doses of the Pfizer vaccine at the Rome-based Istituti Fisioterapici Ospitalieri.
Scientists analyzed their blood for levels of antibodies seven days after the second dose was administered.
Antibodies are virus-fighting proteins that can stop a Covid infection. They are a major part of the immune system – but not the only part.
Results showed that while those who were a normal weight had a high concentration (325.8), those who were obese had half this level (167.1), on average.
But the study only included 26 obese people, which is considered to be too small to draw concrete conclusions.
And there was no significant difference in results between those who were obese and normal weight, meaning more tests are needed.The study was only published as a pre-print on medRxiv, and its methodology and findings have yet to be peer-reviewed.
It also considered vaccine responses by age — but found no significant difference between the different groups.
Professor Ian Jones, a virologist at the University of Reading, told MailOnline he thought the figures looked ‘suspiciously high’.
‘It’s probably the small numbers (of participants),’ he said. ‘But that an effect is seen is not surprising.
‘It would be nothing to do with the vaccine, all would be similar. It is the individual’s response that is compromised.’
He added it was a ‘general finding’ that jabs are less effective in obese people.
‘It’s not entirely clear why but it may be related to metabolic syndrome which in turn has a knock-on effect to many cell functions, including immunity,’ he said.
Metabolic syndrome is the medical term for a cluster of conditions, including high blood pressure, heart disease and type 2 diabetes.