Many parents who refuse to give their children the MMR vaccine, opting instead for separate injections, fail to ensure their children receive all three jabs, research shows.
The study by Great Ormond Street Hospital and University College London Institute of Child Health found 48% of children given single measles, mumps and rubella vaccines, which are unlicensed in the UK, did not receive all of them.
Researchers looked at 14,578 children born in the UK from September 2000 to January 2002 and found 88.6% had been immunised with MMR by age three, 5.2% had received at least one of the single jabs, and 6.1% were unimmunised.
But of the 634 children who received at least one jab, only 52% had received all three.
The study, published online in the British Medical Journal (BMJ), found that, although MMR coverage is relatively high, "it remains lower than the estimated level required to ensure herd immunity (over 95%), leaving a substantial proportion of children susceptible to avoidable infection".
Uptake of the MMR vaccine fell in the 1990s following concerns fuelled by research by Dr Andrew Wakefield suggesting a link between the jab, autism and bowel disease.