Health reporter, BBC News
The first MMR dose is given at 13 months
UK researchers have raised concerns over the monitoring of the safety of single measles and rubella vaccines.
It comes after an unexpectedly high number of cases of anaphylactic shock after single vaccines given at private clinics in south west England.
Although the figures are likely to be an anomaly, poor data on vaccines given at private clinics is preventing proper scrutiny, they warn.
Anaphylaxis is rare, the Archives of Diseases in Childhood study stressed.
Public confidence in the triple MMR vaccine dipped following research - since discredited - which raised the possibility that the jab may be linked to an increased risk of autism.
It has been estimated that between 2000 and 2002, 5.2% of UK children were vaccinated with single vaccines by private clinics.
Four cases of anaphylaxis after single vaccines were referred to the team in Southampton between 2003 and 2007.
Although seemingly small numbers, the researchers said they would be surprised to see even one case as anaphylaxis is so rare after vaccination.
It led them to try and work out the rate of anaphylactic reactions after single vaccines.
But unlike vaccines given on the NHS, there is no record of how many immunisations are done in private clinics.
Using data on how many measles and rubella vaccines were imported into the UK, they estimated the rate of anaphylaxis - which can be fatal - was 18.9 per 100,000 doses for measles and 22.4 per 100,000 doses for rubella.
This compares with 1.4 per 100,000 for MMR, they said, and likely to be an underestimate because there may be other cases they did not know about and not all the imported vaccine would have been administered.
Study leader Dr Mich Lajeunesse, a consultant in paediatric allergy in Southampton, said: "It is so unusual that if you saw one case of anaphylaxis to vaccines you would be surprised.
"We can't think of any reason why it would be higher for single vaccines and it's probably an anomaly.
"The issue is that people go for these vaccines because they are uncertain about how safe the MMR is but we know how safe the MMR is because we have lots of data."
He added that the Healthcare Commission should insist private clinics are subject to the same standards of vaccine data reporting as is expected of the NHS.
"They are more than likely safe but the issue it raises is about how that is monitored."
Dr Lajeunesse is currently doing a UK-wide study through the British Paediatric Surveillance Unit (BPSU) to gain a better understanding of how common anaphylaxis is after vaccination.
Immunisation expert Dr David Elliman, a community paediatrician at Great Ormond Street Hospital also said there was "no logical reason" why the rates of anaphylaxis should be so high with the single vaccines.
"The important message is not about the rate but the fact that it does occur and this idea that the single vaccines for some magical reason are safer is nonsense."
He added that private clinics should have to report to the PCT whenever they vaccinate a child in the same way GPs do in the NHS.
A spokeswoman for the Healthcare Commission said immunisation providers are not required to report how many vaccines are used within their service but they would be expected to have systems in place for dealing with and reporting any adverse reactions.