Some random thoughts on caused for the rise of the #SARSCoV2 'UK lineage' (B 1.1.7).

If its increase in frequency in the UK were caused by intrinsically higher contagiousness. It will largely displace all other #SARSCoV2 lineages in circulation, in no time, and globally.
1/

If its rise in frequency in the UK were primarily a demographic event, i.e. lineage being at the right place at the right time ('hyperspreader' event), it is not expected to significantly increase in frequency in the many other countries where it is already has a foothold.
2/

If its rise in frequency were caused by an ability to bypass, at least in part, immunisation provided by prior infection, its relative rise in frequency in different countries should be proportional to the fraction of the population previously infected with #SARSCoV2. 3/

Data to test those hypotheses should be available early next year. The 3 hypotheses are not mutually exclusive, and the truth may be any mix thereof. Also, higher transmission can be many things e.g. higher viral load, lesser symptoms for same viral load, longer contagion ... 4/