It is very difficult to calculate the effectiveness or masks as other factors like school holidays seem to have a much bigger effect. There are also things like vaccination rate and prior infection rate to take into consideration.
Currently the Orkneys are 656 cases/100,000 and the Shetlands 262, yet they are similar areas living under the same rules.
Both are higher than Glasgow (229). You need to be able explain this level of variation before you can start deciding how effective masks etc. are.
Here's the three island local authorities over the past 90 days (plus Highland as the adjacent mainland authority) from Travelling Tabby.
Na h-Eileanan Siar which has the highest proportion of Gaelic speakers of all the Scottish local authorities went off on one during the second half of October with covid prevalence peaking at 630 cases per 100k early November. An Comunn Gàidhealach's Royal National Mod took place in Inverness from the 8th to the 16th October and it's not known as the whisky olympics for nothing... So there is a clear correlation with the spike in the Western Isles and arguably a clear indication that late night alcohol fueled indoor socialising is a particularly productive environment for coronaviruses!
There's not a such a clear link to anything for Orkney, but remaining restrictions on ferry capacity were lifted before the October school holidays, so more outward and inward travel will have occurred and Orkney is simply much more accessible than Shetland with a 90min ferry crossing with Northlink (60 minutes on the Pentland Ferries route) vs a 13 hour crossing with Northlink between Aberdeen and Lerwick for Shetland.
There is also some overlap with the discussion on the Scottish Vaccine passport thread - the argument that you can't prevent cases, only delay them. Indeed, low previous covid prevalence is likely a factor in the current high island rates vs lower Glasgow rates, but by delaying those infections the peak is coming in a highly vaccinated population with better knowledge of how to treat those who do end up in hospital despite being vaccinated.
It's also worth pointing out that the total population of these 3 island authorities is low, so a relatively small number of cases can cause big swings in the prevalence per 100k, this has been particularly noticeable throughout the panedemic with the tiny mainland council of Clackmannanshire .
