Over half a century ago, a pub landlord could explicitly discriminate against anyone wishing to enter his premises – for whatever reason.  Those who oppose the Equality laws as a restraint on freedom are likely to take the same view of vaccine passports.

So it is an irony that the workings of the second will be made problematic by the provisions of the first – at least, without a state-run scheme in place (and perhaps even then).

No service provider who discriminates against those who have been advised not to vaccinate, such as pregnant women, and perhaps too against those who refuse the vaccine for reasons of conscience, would last long in court.  The same goes for employers – with knobs on.  Which can lead to only one of two outcomes.

First, our imaginary landlord doesn’t attempt his own vaccine passports at all.  Or else provides for exceptions and exemptions which make them difficult to police effectively.  While the landlord next door does something else entirely.

Boris Johnson clearly believes that a state-run scheme is preferable to this looming freelance chaos (as many voters would surely see it).  So, first things first: there is talk of governments banding together to require a uniform state-licensed document for travel abroad.  We’ll believe that when we see it.

And look forward to the domestic reaction if or when the EU, for example, insists on British citizens producing such a document as a condition of entry into member states.

But until or unless that happens, governments abroad will continue to demand, as part of their border control measures, a range of documents to cover a range of conditions – as now.  Indonesia will continue to require proof of vaccination against polio; the Phillipines, for meningitis; Brazil, for yellow fever.

And so on: so there’s no need for a new state-licensed system to enable Brits to go abroad.  So let’s turn our gaze back to home, mull such a system here, and ask: what is the question to which it would be the answer?

Ministers will not want to argue that the purpose of a scheme will be to raise vaccination rates.  This is because 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.

The Prime Minister set out a different case yesterday: that there is a trade-off between vaccine passports and full opening, at least where big events are concerned.  No passports, and the rigours of social distancing will apply: meaning fewer bottoms on seats, lower profits, less pleasure – and less “old normal”.

We understand his and Ministers’ dilemna – faced as they are with the turbulent prospect of a Covid passports free-for-all, and its consequences.  And we appreciate why that trade-off could bite hard during the next few months.

But not every adult will have been offered a first dose until the end of July.  So how could it be fair to exclude someone from a concert hall, say, between May 17 and early August if they haven’t had the chance to have even a single vaccine shot?  What about under-18s after July 31, who mostly won’t have had that opportunity either?

It is considerations like these which seem to be pushing Ministers away from earlier dates and vaccine passports towards later ones and mass testing.  This is just as well.

For any state-run scheme would be plagued by privacy, security and political problems – not to mention legal ones, with ECHR Article Eight privacy rights, GDPR and the the Data Protection Act all in play.  The long-running problems with the NHS Covid App don’t inspire confidence in the state’s capacity to rise to the challenge.

Perhaps claims of a “racist system”, since more black people than white would excluded from events, wouldn’t resonate widely.

And of “an ageist system”, too – since those minorities who refuse the vaccine are likely to be older.  Or else of an “anti-youth system”, since young people are last in the line for the jab.  Maybe, as we say, the public as a whole would take such unfairnesses on the chin.  And maybe not.

It’s true that youngsters will have the chance to receieve the vaccine as the months go on.  And the more time passes, the more likely it is that any teething problems with the App can be ironed out.

But hang on a moment.  By the autumn, the whole adult population will indeed have been offered at least one dose of a vaccine.  If these have been produced within a year, the most astounding triumph of science in modern times, they ought too to be capable of adaptation to new variants.

And if some deadly new vaccine-resistant variant comes, it will threaten to overwhelm the NHS – and we’ll be back to lockdown anyway.

Above all, how useful are vaccine passports if a vaccinated person can still carry the disease?  By contrast, the alternative means of policing entry – lateral flow tests – would swerve the forced medication problem.

They might be workable as a condition of entry to a cinema or football match or theatre.  But if these tests are to be carried out at venues, how will the process work?  (Each test currently takes about half an hour.)  And if they are not, how can it be certain that a person claiming that a test has shown him negative is telling the truth?

What about false positives – a particular problem with lateral flow tests?  And even if all these hurdles are vaulted, the matter of the passing of time remains.  Chris Whitty has suggested that in future Covid will be treated like flu.  But we don’t mass test for flu at public venues.  When do Ministers believe a point be reached?

Finally, we turn from principles and practicalities to politics, at least as far as a state-run vaccine passport scheme is concerned.

Public opinion has backed lockdowns.  The bottom line is that voters believed that lives would be lost were these not imposed.  Doubtless they will also support passports.  But as time continues to pass and deaths continue to fall (hopefully), fear of Covid will carry on fading.

At which point, public support for passports may be a mile wide, but it is also likely to be only an inch think.  The Covid Recovery Group are against them.  So are the Liberal Democrats, as they strive to make opposition their USP.

Which bring us to M’Learned Starmer.  The Labour leader is too smart to have risked crossing public opinion over lockdown.  But, as he seems to appreciate, voting against vaccine passports would offer him the chance to defeat the Government at far less electoral risk (for there will surely bound be a vote on the issue even if a Bill isn’t required).

We can hear him now: “Ladies and gentlemen of the jury, Labour has nothing against the principle of vaccine passports, but this incompetent Tory Government has messed them up because [fill in reason here].”

And if Starmer is too smart to have sailed against the wind over lockdown, Johnson is too smart to risk Commons defeat over passports.  The more you ponder the Parliamentary arithmetic, the more you see why more mass testing is likely to happen and why a state-run vaccine passport scheme is not.