I have recently seen an interview with a Romanian State Secretary at Health Ministry about the COVID-19 vaccination. The interview touched on the sensitive point of insufficient vaccine doses and the reporters asked about how the actual doses Romania receives are actually computed.
There was no direct answer, but an educated guess that the algorithm is not clear and most certainly is heavily influenced by member state population size and infection incidence rate. This last factor is particularly sensitive for Romania, due to a rather small amount of tests being performed.
This document suggests that the population size is the major (only?) factor, but the final distribution is not clear:
Agreements contain a provision on the equal distribution of vaccine doses to the Member States, which will ensure that each country receives doses based on a pro-rata population distribution key, unless otherwise agreed between the participating Member States in the course of implementation of the Advance Purchase Agreements
Is the European Commission algorithm (used to compute vaccine doses to be received by each country) public?
A very similar question was posted here (thanks Hulk for providing this reference).