Overweight people may not be receiving immunizations properly because the needles used are not long enough.
Injections against diseases such as flu and hepatitis B may not have the full effect if standard size needles are used in patients with more fat layers, researchers say.
Travel medicine expert Jane Zuckerman, from London's Royal Free and University College Medical School, said: "A standard size of needle will not guarantee successful intramuscular injection in all people".
Writing in the British Medical Journal, Dr Zuckerman said that, to ensure a vaccine is effective, a decision on the size of the needle and the site of the injection needs to be made on an individual basis for each patient.
She points out that, for example, women have significantly more fat layers under the skin than men.
A recent study of 220 adults found that the standard needle measuring 16mm (5/8 inch) would not go deep enough to deliver an injection effectively in nearly half the women and 17% of the men.
The study recommended using needles of 25mm (1 inch) and 38mm (1.5 inch) depending on the patient's weight.
These longer needles should not make the injection much more painful but would ensure that the vaccination is administered properly.
Vaccines are most effective, and cause less tissue damage when given into muscle, particularly in the upper arm.
Although injections into the buttocks have been common in the past, Dr Zuckerman says, the fat layers there "do not contain the appropriate cells that are necessary to initiate the immune response".
She concludes that for effective vaccines "a selection of non-fixed needles (pre-filled syringes that may be provided with a needle fixed on the barrel) should be available".