Abbreviations or Flags only = worst comprehension
Using abbreviations only could cause uncertainties because users might not know the abbreviation for the particular country and leave them guessing. The same goes for flags only.
Abbreviations + Flags = better comprehension
You might get a bit better comprehension when you use flags + abbreviations because if the user doesn't know the abbreviation, he might know the flag, and the opposite.
Full country name = high comprehension
Displaying only the country name is sufficient for most user to recognize the country.
Flags + full country name = very high comprehension
However if you want to maximum comprehension of the country displayed I suggest using flags + full name of country. This way uncertainties about which country is this is going to be cut to minimum.
Use the pattern that is most suitable to your case: high comprehension or saving screen space.
Why do I need to add references? All of this is logical.
Every person knows limited and different amount of country abbreviations, flags and country names which do not overlap completely. Lets say, one person knows 38 country abbreviations, 50 flags, and 100 country names. According to Wikipedia there are approximately 195 countries total in the world.
Therefore, if you present only abbreviations to that particular person she will be able to recall only ~19.5% (38/195 countries) of the available countries in the world. However, if you present a combination of abbreviations and flags the chance that the user will know the country raises to a minimum of ~25.6% (50/195) because she knows at least 50 flags. But the percentage increases if the knowledge for flags and abbreviations doesn't overlap completely. Meaning user knows abbreviations for which she doesn't know their flag and the opposite.
So, there is higher chance for recall of the country if you present a combination of flags, names, or abbreviations.
I'm presuming that people know more country names than abbreviations because abbreviations made for utility. It's counter logical to say I know which country is CZ, but have never heard of Czech Republic. This is why I've put higher chance of recall for country name than its abbreviation.