The Blairs' secret is out - their youngest son has had the controversial vaccine. But who told the press?

And finally... a baby boy we cannot name has had the jab that his dad says every child in Britain should have.

In a way, the story of Leo Blair's trip to the doctor's surgery to be given the controversial MMR vaccine should never have been news. But yesterday The Independent put an end to speculation surrounding the Blairs and the vaccine.

In a front-page exclusive, the paper said Leo had now had the injection, which protects against measles, mumps and rubella, in London on 22 January – not shortly before Christmas, as other reports had maintained.

But the Prime Minister's decision to guard the privacy of his children (notwithstanding the odd photo-opportunity) at a time when other parents were expressing serious concerns about the safety of the MMR vaccine, triggered the now familiar will-he-won't-he saga.

As Leo, now 20 months old, approached the age at which most British babies are given the MMR jab, the rumour mill at Westminster began to grind into action.

Against a background of an outbreak of measles which claimed the lives of two children in Dublin, and the counter-propaganda from the anti-MMR lobby concerned about the suspected links between the triple vaccine and autism, the focus turned to Mr Blair's own baby.

It was in early December that the Daily Mail was the first to ask: is baby Leo having the MMR jab? The natural answer should have been yes, but Cherie Blair's half-sister, Lauren Booth, had written just days earlier in the New Statesman that her own one-year-old daughter, Alexandra, would not be having the vaccine.

Her reasons gave rise to a belief at Westminster that Mrs Blair, like her half-sister, harboured deep concerns about the triple vaccine and that she might choose to prevent little Leo from having it.

The Tories pounced. Julie Kirkbride, a Conservative MP who had decided against giving her own baby son the MMR jab, challenged the Prime Minister outright in the Commons on 19 December. How could he ask other parents to give their children the controversial jab if it wasn't good enough for Leo? Mr Blair refused to answer.

Days later two Sunday newspapers ran stories disclosing that a member of Mrs Blair's family suffers from autism, and that another half-sister, Lyndsay Booth, was an active lobbyist about the side-effects of MMR and possible links to autism.

The case was made again. Mrs Blair must be reluctant to have Leo immunised because that shared family experience meant that she shared her sister's fears. Mrs Blair's decision to attend events held in aid of the National Autistic Society were thrown in as supporting evidence.

The Prime Minister was furious. He issued a stern statement, faxed to newspapers from his weekend retreat at Chequers. It was "horrible", he said, for newspapers to try to find out private medical details about his son, and "offensive beyond belief" that people would suggest that he would advise parents to opt for the MMR jab while deliberately refusing the same for his own child.

As far as the Blairs were concerned, they could not and would not get into the business of making public the details that could have stopped the row there and then. But it turns out that despite the Blair family's decision to have Leo immunised, illness prevented him from having the jab. Again, the Blairs did not think this was anyone's business other than their own.

But did Downing Street change its mind and leak the story to The Independent last week? As with everything at Westminster that doesn't get a straight answer straight away, foul play is now suspected.

On Friday the Public Health Laboratory Service disclosed that the take-up of the MMR jab is 82.4 per cent – the lowest figure since monitoring began in 1995 and well short of the 95 per cent World Health Organisation target. In London the figures are worse, at 73.4 per cent.

The same day the Department of Health and Downing Street were informed of a measles outbreak at a private nursery school in Clapham, south London. Three cases had been confirmed and a further 22 suspected cases were being tested. None of the children confirmed as having measles had been given the MMR vaccine. Rates of vaccination in the London borough are the lowest in the country. Experts began to talk in hushed tones about the possibility of "another Dublin" or even an epidemic.

The news was certain to provoke further discussion of Leo Blair and the MMR vaccine, particularly as the Conservatives – who accused ministers of having "failed to grasp the importance of establishing public confidence in the MMR vaccine" – were regularly being fed information from health professionals in the field claiming that the Blairs had not yet had Leo immunised. There would have been nothing simpler for Downing Street than to get Alastair Campbell to pick up the phone to The Independent and tip them the wink.

It appears that this is not what happened at all. The story came through "political sources" last Wednesday. It was put to Downing Street, which declined either to confirm or deny it. No 10 thought the story had just "gone away", little knowing that by Friday MMR and measles would again be headline news.

Asked again on Thursday, No 10 did not take an opportunity to knock down the story or to stand it up. And so by Friday, confident of its veracity and having frankly given the Prime Minister long enough to deny it, The Independent took the decision to publish.

Mr Blair has still declined to comment. He is in no position to do so now, unless he wants to see his privacy arguments crumble. But in a clear indication that the guessing game is over, the story has not been denied and Mr Blair left it to his Health Secretary to respond.

"I think the Prime Minister has clearly said throughout that MMR is the right thing to do," said Alan Milburn. "Everything we are told is that MMR is perfectly safe, indeed it is a very effective immunisation against what can be dangerous and even life-threatening diseases. But in the end the Prime Minister's children have their right to privacy."

So that's that then.