Why Does a Rapid Screening Test for Covid-19 Work in One Situation But is Suspect in Another?

The answer to this 'riddle' is, that the rapid Lateral Flow Test is equally dodgy (it's a poor predictor of infection) in both situations. Political expediency, makes it, however, a 'solution' in one case and a 'cause of concern' in the other. It seems that the plan for UK University students to return to their alma maters, after Christmas, has, for now, been put on hold (dismissed by some as 'half baked'). The students were supposed to get a negative test shortly before making their staggered return to their University halls (so that testing at these institutions would not be overwhelmed). It has now been recognised that a negative test result guarantees very little and that students could pick up an infection on their actual return travels (https://www.theguardian.com/education/2020/dec/24/covid-ads-to-prepare-students-for-university-return-delayed-amid-rapid-test-concerns). This is a serious consideration, when the country is facing widespread post-Christmas lockdowns in an attempt to limit the transmission of the 1.1.7. Covid-19 variant. There are Universities in most relatively large towns and cities and their students are currently very widely dispersed. It looks like a recipe for extremely rapid and widespread viral transmission (as does the plan for re-openning schools after Christmas, using the same Lateral Flow Test). The Lateral Flow Test is, however, 'a welcome solution' for the impasse in Kent. This followed the initial French banning of truckers (possibly carrying the seemingly highly-infective variant of Covid-19), returning via Dover and the Eurotunnel to Calais. It is now proposed to move the estimated 10,000 trucks, currently marooned in that county, after rapidly screening the drivers. To help the process, 26 French firefighters (pompiers) have now arrived carrying 10,000 tests. These will operate in addition to the UK's 4 current mobile test sites, with a further 5 being set up (https://www.theguardian.com/world/2020/dec/24/french-firefighters-arrive-in-dover-with-10000-covid-tests-for-lorry-drivers ). It all sounds a bit like overkill (although the entente cordiale is welcome). So why the difference? In the case of the Universities, it will be very evident, if a botched scheme results in a serious and widespread rise in infections. In the case of the truckers, however, appearance of the 1.1.7.Covid-19 variant in France (inevitably the case) can never be tied to a particular test on a particuar driver. And, anyway, the UK Transport minister has promised to solve the Kent trucking blockade 'by Christmas'. And we know how reliable their Christmas promises are!