IS the objective of the Scottish Government's vaccine passport plan infection control, or a last-ditch bid to boost uptake among the young?

Whether through genuine reluctance, ambivalence, or procrastination, one in four people aged 18 to 29 in Scotland are yet to be even partially inoculated - a figure unchanged now for weeks.

During August, just 96,761 first doses were administered (compared to nearly 878,000 in March) and nearly two thirds of those were given to newly-eligible 12 to 17-year-olds.

Infections have already quadrupled, from less than 9000 a week before Beyond Zero to more than 42,000 in the past seven days and, with the return of universities looming, memories of how campus outbreaks helped to kick start 2020's second wave have spurred ministers to try to find a way of putting a lid on the epidemic without reimposing measures such as physical distancing.

The question is whether the passport policy will actually achieve its goal - or be undone by its own contradictions.

On uptake, there is a danger that it backfires among those already more suspicious of the state - particularly ethnic minorities or people living in deprived neighbourhoods - whose hesitancy will only become more entrenched as a result.

Research due to be published later this month in the Lancet journal EClinicalMedicine found exactly that. In a survey of 16,527 people, the vast majority (88%) said passports would neither encourage nor deter them from getting vaccinated.

READ MORE: How can Scotland increase vaccine uptake in young people?

But in the remaining 12% of respondents, two thirds said passports would actually put them off - with non-English speakers, younger age groups, and Black British people more likely to express this view.

As behavioural psychologist Professor Stephen Reicher noted, these initiatives fare best in societies like Denmark with high "social trust" where citizens believe authorities will do what they say.

The UK has much lower levels of social trust - though Scotland tends to rank higher than England.

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A second issue is whether limiting the passports to vaccination-only and to a very narrow range of settings undermines both the aim and messaging of the policy.

In other countries where these 'health passes' are in place, eligibility is usually based on three criteria: being fully vaccinated; proof of a negative Covid test; or "presumed immunity" where someone has recovered from the infection.

On Wednesday, Nicola Sturgeon stated the purpose of limiting entry into nightclubs, adult entertainment venues, and some large events to the vaccinated-only is "to control the spread of the virus, as we head into the autumn and winter".

But we know that people who are vaccinated can both catch and spread the virus, especially with the Delta strain.

At the same time, recent research from Public Health England found that people who had recovered from Covid actually had better immunity against Delta than those given two doses of AstraZeneca (by 77% to 70%) - though it must be said getting vaccinated after prior infection provided even greater protection.

HeraldScotland: Football stadiums will also be covered by the vaccine passportFootball stadiums will also be covered by the vaccine passport

Admittedly, if everyone in a football stadium, nightclub or concert venue is vaccinated the odds of an outbreak is much lower than if they are mixing together in a space which also includes some unvaccinated, infected individuals i.e. our current scenario.

READ MORE: Why vaccine passports are replacing lockdown from Israel to Australia

Lateral flow tests are prone to false negatives, so there is a reasonable argument against relying on them on their own - but if we really want to minimise virus spread, shouldn't fully vaccinated people also be asked to provide evidence of a recent negative test in a sort of 'belt and braces' approach?

The bottom line is that limiting the criteria to vaccination-only sends the message that the policy is more about jags than infection control, and while vaccination in itself will help to curb the spread of the virus (around half of known Covid cases in the past week were in the un- and under-vaccinated 0-25 age group) this risks undermining that all-important 'social trust'.

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The same is true of targeting passports to a few specific settings (most popular with young people who also happen to be the most unvaccinated) when most other countries have applied them across the board to cinemas, gyms, restaurants, swimming pools - even to classrooms in Italy, where teachers are banned without a Green Pass.

The virus can spread just as easily in any of these environments, so there is also a risk that the policy is seen as overly discriminatory at the same time as being less effective than it might have been.

It is also worth remembering that as restrictions have eased and cases soared, some of the most vulnerable in our society - such as immunosuppressed cancer patients - have felt their freedoms recede as environments such as cafes or gyms become potential danger zones.

Making all of these spaces as safe as possible for everyone is a question of fairness not only for businesses, but for those marginalised by health conditions.

READ MORE: Warning hospital numbers could hit 4,500 within two weeks unless transmission slows

Finally, do vaccine passports actually work? In terms of protecting the NHS, a surge in Covid hospitalisations is already 'baked in' as a result of recent record cases.

How bad it gets depends on what happens next, with modellers forecasting anything from 1000 to 4,500 hospital beds occupied by Covid patients by mid-September depending on whether transmission continues rising, plateaus, or falls back to earlier August levels.

But in any case, the passports won't take effect until the end of September.

HeraldScotland: Covid cases per million per day. Israel reintroduced its Covid pass at the end of July but cases continued rising sharply. Denmark introduced its coronapas in April and, in France, cases have fallen since its pass became mandatory in a wide range of venues in early AugustCovid cases per million per day. Israel reintroduced its Covid pass at the end of July but cases continued rising sharply. Denmark introduced its coronapas in April and, in France, cases have fallen since its pass became mandatory in a wide range of venues in early August

The picture internationally is mixed.

In France they were credited with a surge in vaccine uptake and their rollout has coincided with a steep decline in infections.

In Denmark, the policy is due to be scrapped next week, having helped to keep virus rates stable until over 70% of its total population was fully vaccinated.

Israel declared its own Green Pass redundant in June after cases fell close to zero, only to reinstate it at the end of July as the Delta variant swept the country.

Since then there has been little sign of its explosive outbreak slowing.

For the best results though, passports should be fair, trusted and cover a wide range of indoor settings.

Right now, Scotland's scheme may come up short.