The Paris terrorist attacks hit high yield passengers travelling on International Airlines group carriers last month.
The parent company of British Airways, Aer Lingus, Iberia and Vueling disclosed a 0.3% drop in group premium traffic in December.
Overall traffic last month was up by 10.5% year-on-year as capacity rose by more than 12%.
IAG said: “The terrorist incidents in Paris in November and the US government travel advisory had a temporary negative impact on load factors and premium traffic in December.
“However, our guidance for full-year 2015 operating profit, issued with our third quarter results, remains unchanged.”
IAG airlines, including Aer Lingus, carried almost 95 million passengers in 2015.