THIS week has been described as a "turning point" in Scotland's lockdown.

On Thursday we reached the milestone when everyone over 50 - plus unpaid carers, key workers, and younger people with health conditions - should have at least been offered a first Covid vaccine dose.

Uptake has been unexpectedly high: as a whole, an astonishing 98.8 per cent of Scots aged 55 or over have had at least one inoculation, with the 50 to 54-year-olds who are still coming forward for appointments not far behind on 87.8% to date.

READ MORE: Spike in Covid hospital admissions among under-45s - 'Just because you're younger doesn't mean you're not vulnerable'

So far, so good, particularly as priority groups one to nine have accounted for 99% of Covid deaths during the pandemic.

But we know that no vaccine can offer complete protection and there are still 2.67 million people in Scotland - nearly half the total population - who have had no jag.

HeraldScotland: The majority of under 50s have yet to be vaccinated and modelling has indicated that second doses for older adults will have to be prioritised until JuneThe majority of under 50s have yet to be vaccinated and modelling has indicated that second doses for older adults will have to be prioritised until June

With growing evidence that the vaccines are curbing infections and transmission we need as many people as possible to be immunised to stop the virus spreading and reaching those individuals who either cannot be vaccinated, or whose immune system has responded weakly to it.

As we move to inoculating the under-50s there is plenty to feel positive about (most importantly the clear real-world evidence that the vaccines are preventing people from becoming sick and dying), but we should also be alert to some possible clouds on the horizon.

Firstly, there have been anecdotal reports from vaccinators of increasing 'no-shows' or people cancelling appointments.

As the Scottish Government does not publish data on attendance it is impossible to gauge how much of an issue this really is, if any.

READ MORE: Four cases of 'double mutant' Indian Covid strain found in Scotland

Younger age groups tend to be more vaccine hesitant, partly because their individual risk is low and they are more likely to engage with antivax conspiracy theories online.

Reluctance may have increased amid publicity surrounding AstraZeneca blood clots, but also -ironically - by the success of the vaccination programme itself.

Cases and deaths are a fraction of what they were back in January, which can send a misleading signal that the crisis is "over".

Complacency is a danger.

As the Herald highlighted, weekly hospital admissions for Covid have nearly doubled recently among under-45s, with more under-25s than over-75s hospitalised at the beginning of April.

HeraldScotland: Beer garden are expected to open from Monday April 26Beer garden are expected to open from Monday April 26

Over the next seven weeks, up to early June, we should see various restrictions ease: travel limits between areas have already been scrapped, and in just over a week gyms, shops, beauty salons, spas, beer gardens and restaurants should reopen - albeit without alcohol if you are dining indoors.

From mid-May, pubs can serve customers again indoors, cinemas and music venues can resume, and small groups of up to four people can meet in one another's homes - rising to groups of six by early June.

These relaxations are desperately needed - economically and mentally - but they raise unavoidable risks which may be exacerbated by a vaccine squeeze.

READ MORE: Covid vaccines are safe - but damages need to be fair for the few harmed

This is the second potential cloud: supplies.

On April 5, modelling produced for UK Sage (Scientific Advisory Committee for Emergencies) forecast that a shortfall of the AstraZeneca supplies, compared to what was expected to be delivered, was going to leave little surplus for first doses until millions of those in the priority groups were given their second booster shots.

Professor Christina Pagel, a healthcare mathematician and member of the independent Sage expert group, noted that this meant that under-50s "won't be getting their first doses in any big numbers until June and July", leaving a higher proportion of them than anticipated exposed as indoor mixing and recreation resumes.

HeraldScotland:

HeraldScotland: Although the vaccine supply modelling was specific to England, supplies are acquired by the UK Government and distributed to the devolved nations according to population - meaning that they will experience equivalent shortfallsAlthough the vaccine supply modelling was specific to England, supplies are acquired by the UK Government and distributed to the devolved nations according to population - meaning that they will experience equivalent shortfalls

It would be safer, she said, to maintain restrictions at April or May levels for longer while delaying the full reopening of the economy planned for England on June 21 (when social distancing is expected to be abandoned and nightclubs reopen) until August instead, when all adults will have had at least one dose.

Doing so would "prevent much of the summer surge" of 15-20,000 Covid deaths predicted by Sage, added Prof Pagel.

A third cloud could be variants.

While numbers remain extremely low in Scotland (25 known cases of the South African variant, and six of the Brazilian P1 strain), England is seeing an increase.

HeraldScotland: The South African variant responds less well to vaccines (Chart by Dr Duncan Robertson, Loughborough University)The South African variant responds less well to vaccines (Chart by Dr Duncan Robertson, Loughborough University)

Cases of the South African (B.1.351) variant have gone from 150 in early February to 600 now, despite efforts to contain it through sequencing and self-isolation.

Clinical trials suggest the AstraZeneca vaccine is only about 10% effective at preventing mild to moderate infections from the South African variant, which has also shown evidence of being able to escape the Pfizer vaccine.

On the upside, scientists believe it can spread less easily than the UK's own Kent variant and that vaccines should still guard against more severe disease.

Another variant to watch, however, is India's new "double mutant" strain (B.1.617) - so-called because it has two potentially worrying changes in the spike protein which have not been found together in any other type - which is now responsible for 73 cases in England and four in Scotland.

This is all the more concerning because the UK has a large South Asian population with ties to the subcontinent, and India is not on the 'red list' for travel.

India has gone from fewer than 10,000 confirmed cases per day earlier this year, when the virus was wrongly thought to have "disappeared", to over 200,000 a day now - the highest in the world.

Hospitals are so full that some patients are sharing beds.

HeraldScotland: To date, 7.4% of people in India have had at least one Covid vaccine dose compared to 47.8% of people in the UKTo date, 7.4% of people in India have had at least one Covid vaccine dose compared to 47.8% of people in the UK

Nearly two thirds of cases analysed in India between January and March were caused by this new variant, which is feared to be both more infectious and able to evade antibodies.

How well vaccines can protect against it is unknown (less than 8% of the Indian population has had one dose of any Covid vaccine) but the fact that there are nearly 100 cases here already should be a red flag that international travel and unsupervised home-quarantine will be our weakest link as the economy reopens.