“What is the question to which vaccine passports are the answer?” we asked in April.  After all, one can be double vaccinated and still carry the virus.  So providing evidence of the first as one enters a nightclub is no guarantee of not spreading the second.

The only convincing answer is: to raise vaccination rates.  But as ConHome put it then, “such a system would arguably be forced medication – which remains illegal for physical conditions, and might therefore run up against our international obligations, not least under the European Convention of Human Rights”.

Which is why we thought Ministers would be more likely to plump for requiring clubs to demand evidence that entrants are Covid-free – perhaps including lateral flow tests at the door.  Though such a scheme would have brought with it a mass of logistical and organisational problems.

So the Government has gone instead for a policy that won’t stop the spread of the virus altogether, is ethically dicey, and which will leaves it open to potential legal challenge.  Their motive is shown by the timing: Ministers aren’t requiring these passports be used immediately.

The reason?  Partly because the scheme isn’t oven-ready, as Boris Johnson would put it, but partly because although 87 per cent of the population has had a single dose of the vaccine, only 67 per cent has had two doses.

The nightmare for the Government features young people (who make up a big slice of the unvaccinated) being turned away from nightclubs not because they’ve refused to have even a single dose…but because they’ve indeed had one, but haven’t had the chance to have two.

It would be grotesquely unjust for Ministers, on the one hand, to demand two doses as a condition of entry but, on the other, not ensure that both doses have actually been offered.  Hence the delay until the end of September until passports are demanded – by which time more vaccinations will have been rolled out and more young people persuaded to take them.  Or so the Government hopes.

Perhaps the special NHS app will be cheat-proof and work flawlessly (though confidence in the system as a whole won’t have been boosted by reports of people who use the present one being “pinged through walls”).

Maybe clubbers will simply sign up to be vaccinated and go with the flow.  Perhaps claims of a “racist system”, with more black people than white excluded from events, will fall on deaf ears – and those of an “anti-youth system” will do so too.

It could be that ECHR Article Eight privacy rights, GDPR and the Data Protection Act don’t come into play.  And after all, the public as a whole supports restrictions.  Ministers would then be able to roll out the passports for other venues without mass opposition: in theatres, sporting venues, cinemas, pubs – any venue covered by the three Cs: “closed spaces, crowded places and close-contact settings”.

But even if voters go one way, MPs may go the other.  There will be a Commons vote on the plan in one form or another, and Conservative MPs are very restive about restrictions already.

On the same recent day that 24 Tory backbenchers voted against the 0.7 per cent aid reduction, amidst a mass of publicity, 31 opposed a set of Coronavirus rules.  Forty-nine voted against regulations more broadly last month.

That’s enough to defeat the Government if the opposition piles in too.  And Johnson is not in a great place with his backbenchers: add to those Conservative MPs who tend to rebel over Covid those who believe that Downing Street has no grip, and you soon reach a number larger than 49, especially since the two groups overlap.

The weekend’s chaos over whether or not the Prime Minister and Chancellor would self-isolate has been well and truly clocked by backbenchers.  Vaccine passports will add fuel to the fire, since they first seemed to be on, then were off (“we are not looking at a vaccine passport for our domestic economy, Nadhim Zahawi said in February)…and are now on again.

With the Covid Recovery Group arguing that requiring vaccine passports means creeping ID cards, Keir Starmer will be able to weigh the risk of getting on the wrong side of public opinion against the opportunity to defeat the Government.

Perhaps Johnson’s real plan is first to up vaccination rates among young people and then withdrawn the passport scheme. If not, the Labour leader will come under pressure from the party’s MPs to abandon his lawyerly caution and go in for the kill.