"As obesity rises, we need to be aware that the standard of care may have to change to protect obese youth," study co-author Dr Amy Middleman of Baylor College of Medicine in Houston is reported to have said.

The study

Over three years Middleman's team vaccinated 22 young women and two young men in the shoulder, randomly assigning them to be injected with either a 1-inch or a 1.5-inch needle.

Once injected, vaccines trigger production of small molecules called antibodies, which kick-start our immune system if we are ever attacked by the virus again.

The findings

The two groups turned out to have different antibody counts depending on the needle used. In those injected with the short one, the number was almost halved.

Although everyone in the study had enough antibodies to be considered protected against hepatitis B, a lower count generally means a less robust response.

"It gives us more evidence of the importance of choosing the right needle length," said Middleman, "because we just don't know what the impact could be in other vaccines."

Fat makes a difference

For years, doctors have known that vaccines tend not to work as well in heavy people. Whether the obese have weaker immune systems, or fat keeps shorter needles from reaching muscles, where the vaccines can affect immune cells, was unclear.

The introduction of the hepatitis B vaccine in the 1980s offered some clues. Soon after doctors began using the vaccine, they realised that it was failing to protect some female nurses.

At the time, the shot was given in the buttock and was thwarted by the padding there.

Instead of entering the muscle as it was supposed to, the vaccine apparently was broken down in the fat tissue, where it had little chance to affect immune cells. So doctors began giving the shot in the less-padded shoulder.

With the obesity epidemic now adding extra insulation to the shoulder the needles simply have to be longer.

Women are more affected by needle length than men, because their fat distribution is different. But even obese women shouldn't be overly concerned until more research has been done, Middleman cautioned.

"Should you go back and get all your immunisations repeated?" she said, "No, I don't think so."

Pic: Vangelis Thomaidis