European Medicines Agency concern boosters could weaken the immune response. - January 11, 2022

Here it is in Reuters article: "The EMA official raised concerns that a strategy of giving boosters every four months hypothetically poses the risk of overloading people's immune systems and leading to fatigue in the population."
https://www.reuters.com/business/healthcare-pharmaceuticals/eu-drug-regulator-says-more-data-needed-impact-omicron-vaccines-2022-01-11/