In the first place, we would like to stress that it is a duty upon Muslims to exert their utmost efforts to ward off harm of all its kinds. Surely, part of removing harm is to use vaccines to boost immunity against polio as well as other fatal diseases.
The European Council for Fatwa and Research, states the following: The Council investigated the topic mentioned above and after probing into the objectives of the Shari`ah and its outcomes, the juristic rules and the jurists’ sayings regarding what may be excused, decided the following:

First, this oral vaccine has been proved medically useful as it leads to immunizing children and protecting them against polio, Allah Almighty willing.
Moreover, there is no alternative to it yet. Therefore, its use therapeutically and preventively is permissible, for the prevention of its use would lead to major evil and destruction. The fields of fiqh are widely tolerant regarding these impurities— assuming that this liquid [Trypsin] is impure—particularly when we know that this impurity is removed through cell proliferation and washing.
Besides, this case belongs to the area of necessities or needs that are treated as necessities, and it is well known that one of the most important objectives of the Shari`ah is to bring about interests and benefits and to ward off evil and harm.
Second, the Council exhorts the imams of Muslims and those in charge of their centers not to be too strict and literal in these matters of ijtihad that would realize important benefits to Muslim children, since these matters do not conflict with the definitive texts.