The medical journal at the centre of a row over controversial MMR research says it should not have published the paper.
The jab protects against mumps, measles and rubella
BBC News Online examines the long-running debate over the safety of the three-in-one jab and this latest twist.
What is MMR?
MMR is a combined vaccine against measles, mumps and rubella, three common infectious diseases of childhood.
It was introduced in the UK in 1988 to replace single vaccines for each disease.
It is used in countries throughout the world, with millions of doses delivered each year.
Why are people worried about it?
A research paper published in 1998 in the Lancet was the trigger for the present MMR crisis.
This suggested a link between autism - a developmental disorder which often arises in the first few years of life - and inflammatory bowel problems.
Importantly, what it did NOT do was suggest that there was any evidence MMR was connected to either of these problems.
However, it said that studies were "underway" to pursue this theory.
In press statements following the publication of the paper, Dr Andrew Wakefield, the lead author, said he felt there might be a link between MMR and autism and said that single vaccines rather than the combined jab should be offered to parents.
What do other researchers say about MMR?
There have been a large number of research projects to examine the safety of MMR - and particularly the relationship between MMR and autism - in the wake of the Wakefield paper in 1998.
None so far has found evidence which supports the assertion that MMR might cause autism or bowel disease.
No direct link between MMR and autism yet found in the body
Studies of vaccinated and unvaccinated children show no difference in autism rates between the two
The overwhelming majority of scientists believe that MMR is safe.
The scare over MMR has been partly driven by the fact that some parents of autistic children recall that the symptoms of their child's disorder emerged shortly after being given the first MMR jab at approximately 14 months.
Autism experts say that many cases of the disorder do emerge at this age - so it is unsurprising that in some cases, the timing of the two events appears connected.
Other teams around the world are trying to replicate Wakefield's original research.
Surely these diseases are not too dangerous? I had them when I was a child and they did me no lasting harm.
It's fair to say that the chances of any individual child suffering long-term damage from measles, mumps or rubella are relatively small, although all three are unpleasant even in their mildest form.
However, measles is a potentially disabling or even lethal disease, mumps can cause miscarriage, and rubella devastating defects in unborn children.
Dangerous? Complications of measles
1 in 2,500-5,000: Death
1 in 100: Hospital admission
1 in 1,000: Meningitis
People forget just how nasty these illnesses are - in the year before MMR was introduced, even though single vaccines were available, there were 90,000 cases of measles, and 16 deaths.
Prior to rubella immunisation, hundreds of babies each year were aborted because their mothers were infected.
Dangerous? Complications of mumps
1 in 25: Deafness - usually with partial or complete recovery
1 in 30: Pancreatitis Hospital admission
Mumps during pregnancy can cause miscarriage
The boxes on the right gives the odds of serious complications of these illnesses.
While one death in 2,500 cases of measles does not sound very many, experts are concerned that if coverage of vaccines falls away, then outbreaks will again be measured in the thousands rather than dozens of cases. Then there will definitely be deaths, they say.
Experts say that it is the absence of these illnesses - due to vaccination - that makes us forget exactly how unpleasant and dangerous they are.
Dangerous? Complications of rubella
1 in 3,000: Bleeding disorders
Babies infected with rubella in the womb can suffer deafness, blindness, heart problems or brain damage
A return of measles, mumps and rubella in high numbers would bring serious consequences, they say.
A majority of parents still choose the jab. Doesn't this mean that it's OK not to get MMR as outbreaks are unlikely?
Not at all. It only requires a relatively small number of families to withdraw from the vaccination programme for highly-infectious diseases such as measles to reestablish themselves.
Statistical analysis published recently found that a drop of only a few percent in the proportion of children immunised in an area could mean the difference from small pockets of infection to endemic disease affecting many thousands.
Will measles come back?
Experts say that vaccination rates are too low
If they do not improve - bigger measles outbreaks are likely, they say
A massive outbreak in Dublin in the 1990s killed three children
While nationally, four out of five children get MMR, in some areas, such as parts of London, approximately 60% or less receive the jab.
Even the vaccination rate staying the same is no good - every year this persists, it means that thousands more unvaccinated children are being added to the nursery and school population, worsening the situation and making outbreaks more likely.
At the end of the day I've got to make a decision on this. How do I do it?
For many parents this has become one of the most gut-wrenching decisions they have to make.
But many doctors say that the risks of MMR must be compared with other risks we readily expose our children to as part of normal life.
Experts say that parents regularly make decisions which place their children at small risk of injury or death - but do it because the benefits outweigh the risks greatly.
An example: Every time you strap a child into the back of a car and drive somewhere, there's a risk involved, albeit a small one.
If you rejected all these tiny risks, you would never leave the house or cross the road, say doctors.
They insist that, while there is no such thing as a medical procedure with zero risk, the evidence shows categorically that MMR vaccination presents, at most, a miniscule danger to a child, just as any other vaccination does - and that the risks of not vaccinating are much larger.
At the end of the day, it all comes down to how you perceive the risk of MMR - do you believe the body of current medical opinion, which declares it safe, or a small number of scientists who harbour doubts about it, and parents who blame it - rightly or wrongly - for their child's autism?