IT has gone from being placed on the UK’s amber travel list in June to being the world’s most vaccinated population.

On October 1, Portugal finally put an end to Covid restrictions which had kept its bars and nightclubs closed since March after surpassing a target of fully inoculating at least 85% of its population.

Nearly 100% of the eligible population, aged 12 and over, have had both doses, but some mitigations remain:customers entering entertainment venues, now back at full capacity, must show proof of vaccination or a negative test, and facemasks remain compulsory on public transport and in premises such as hospitals, shops and care homes.

Even now, coverage continues to climb and currently exceeds 87% in comparison to around 74% in Scotland.

HeraldScotland: Portugal has the world's most highly vaccinated population against Covid in the world, with only the UAE close behindPortugal has the world's most highly vaccinated population against Covid in the world, with only the UAE close behind

How Portugal achieved this feat, and what its experience tells us about herd immunity, is intriguing governments and scientists worldwide.

On the former, there is a consensus that Portugal benefits from low vaccine scepticism. Only around 3% of its citizens are vaccine ‘deniers’ in comparison to hesitancy rates estimated at 10-20% in the UK.

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Portugal may also have benefitted from a comparatively older population than, for example, previous vaccination leader, Israel.

In Portugal, only 13% of the population are under 14 (and thus mainly exempt from immunisation) against 29% of the Israeli population.

Nonetheless, even with its world-beating uptake rates, the reopening of Portugal’s economy has gone hand-in-hand with a 24% uptick in infection rates during October - albeit from a low base and still remaining nine times lower than the UK’s rate.

More than anything this is testament to the Delta variant's phenomenal capacity to spread, even in a very highly vaccinated population.

HeraldScotland: Portugal has gone from reporting around 629 cases per day on October 1 to 775 per day nowPortugal has gone from reporting around 629 cases per day on October 1 to 775 per day now

As Adam Kleczkowski, professor of mathematics and statistics at Strathclyde University, recently put it in a piece for the Conversation: "The more infectious something is, the more people need immunity to it and the stronger that immunity needs to be to stop it spreading."

Since we cannot alter the transmissibility of the Delta strain (though of course ongoing mitigations and restrictions help to artificially curb its R number), that really only leaves two options: vaccinate more people, or develop even more effective vaccines.

Pfizer and Moderna have already developed customised booster vaccines designed to target the Delta and Beta (South African) variants, but these remain in clinical trials.

On the former, we must keep plugging away to encourage uptake among unvaccinated adults (there are 364,000 in Scotland) as a matter of priority - but could children under-12 be next in line?

US authorities are expected to give final approval next week on using the Pfizer-BioNTech vaccine in children aged five to 11, after FDA advisors concluded that the benefits outweigh any other health risks.

If so, millions of young American children to could begin being vaccinated against Covid for the first time as early as November 3.

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To date, very few countries - China, the UAE and Bahrain - have opted to vaccinate under 12s, but none with the Pfizer vaccine.

What happens in the US, however, is much more likely to be mirrored in the UK and Europe than what happens in the Middle East or China.

Nonetheless, UK advisors on the JCVI have proven to be extremely cautious in striking a balance between benefits to harms - so much so that it effectively passed the buck to chief medical officers to rule on a wider remit than health grounds alone. The UK was both late to vaccinate teenagers, and an outlier in offering them only a single dose.

HeraldScotland: Schoolchildren aged five to 11 could be offered Covid vaccinations for the first time next weekSchoolchildren aged five to 11 could be offered Covid vaccinations for the first time next week

A decision on five to 11-year-olds will be even more fraught.

Right now, the 0-14 age group has the highest case rate per 100,000 compared to any other in Scotland, but the lowest risk of serious illness.

Perhaps vaccines will be limited to clinically vulnerable children? Or a decision reached on the basis of education, Long Covid, or their propensity to spread the virus (unlikely, though, given this was excluded from the decision-making on adolescents).

There is another issue, however: can we justify, ethically or rationally, vaccinating low-risk children when 62% of the world's population remains unvaccinated?

HeraldScotland: Children aged 0-14 have the highest Covid rates, but the number of infections has been in steep decline since September 7Children aged 0-14 have the highest Covid rates, but the number of infections has been in steep decline since September 7

HeraldScotland:

Vice Admiral Gouveia e Melo, the military chief who spearheaded Portugal's rollout, told Australia’s ABC News: “We are over-vaccinating in richer countries and then there is zero vaccination in poorer countries. I can’t agree with that, not only due to ethics and morals, but [also] because it’s not the best strategy and rational attitude.”

The arguments are well-rehearsed: leaving the bulk of the world unvaccinated while we give boosters to double-dosed adults and inoculate low-risk children is both morally indefensible and contrary to our own self-interest, because it increases the danger of a new variant which might unravel everything we've gained.

READ MORE: What game theory tells us about Britain's reaction to coronavirus

But anyone familiar with the 'Prisoner's Dilemma', a hypothetical scenario in game theory, knows that completely rational individuals will tend to act in their own self-interest even if the consequences - collectively - backfire.

It goes: two prisoners are separated and told that they, individually, will be set free if they testify that the other committed a given crime.

If they cooperate with one another and stay silent, their sentence will be reduced to one year. If each betrays the other, both will go to prison for two years.

Acting selfishly appears like the only logical move, because it is the only one which can possibly lead to freedom.

Plus, you mistrust the other to act in the collective interest. The end result? Both end up worse off.

READ MORE: Why the Delta variant has left herd immunity through vaccination alone impossible

It explains everything from our bizarre panic buying behaviour at the outset of the pandemic, to our failure - so far - to tackle climate change.

So yes, Portugal - and the US, and the UK - arguably are "over-vaccinating" our populations - but it is difficult to envisage any other outcome. And yet, with Delta, even 100% might not be enough.