Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
According to the most preferred position of scholars in Islam, receiving a flu-shot does not invalidate the fast since it does not fall under the category of consumption of food or drink, which we are ordered to refrain from while fasting.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
According to the most preferred position of scholars in Islam, receiving a flu-shot does not invalidate the fast since it does not fall under the category of consumption of food or drink, which we are ordered to refrain from while fasting.
Therefore, you need not worry about your fast, but if you broke it, assuming that it did break your fast, then you must make up the day after Ramadan.
It is, however, necessary to point out that the above ruling should not be extended to receiving injections of nutrients or glucose, which do, in fact, break the fast.
If, therefore, a person must receive such injections during the daytime during the month of Ramadan because of health reasons, he or she may skip fasting on the day of the injection and make it up after Ramadan.
Almighty Allah knows best.