The distribution of vaccine doses across countries is described on the basis of data on the actual doses administered. The reasons why the numbers may be low can vary widely and include international vs domestic factors as well as supply versus demand factors. At this moment in time, however, the most likely reason for low numbers is related to international supply bottlenecks.
An additional complication pertains to the differences in treatment protocols across vaccines. The vaccine landscape is diverse with different treatment protocols in terms of the number doses that need to be administered, the dosage of each dose and the length of time between them. Vaccines differ in terms of their efficacy and deviations from the prescribed protocol dilute efficacy in uncertain ways. This matters as countries consider to speed up vaccination by e.g. changing, reducing or delaying doses.
The approach taken here in the face of this diversity of possibilities is to express vaccines in double-dose equivalents. We convert all vaccines to a common denominator of the double-dose protocol. This means that for the purpose of considering whether someone has achieved full vaccination we require that two doses have been administered.
Double-dose equivalence is imposed on single-dose vaccines by simply multiplying the number of single-doses administered by two. In this way, when adding up single doses of single-dose protocols and single doses of double-dose protocols, we can keep the threshold of 200% vaccine coverage as indicating full coverage.