Britain restricted the use of AstraZeneca's Covid-19 vaccine on Friday while Germany said it would give the shot to anyone who wants it, in a risk-management role reversal that reflects the divergent progress of their vaccination campaigns.

The shifts in guidance on the Anglo-Swedish drugmaker's vector-based shot demonstrate just how hard it is for policy makers to weigh its benefits against risks in the form of very rare - yet occasionally fatal - cases of blood clotting.

How choosy governments can afford to be depends to a large extent on how far they have progressed towards vaccinating enough people to drive down the spread of coronavirus infections and cut the number of resulting deaths...