Is it permissible to get a COVID-19 vaccination while fasting in the month of Ramadan if the duty-bound person knows or thinks there is a strong likelihood of experiencing side-effects like nausea or pain that will make fasting difficult?
It is permissible to get the vaccination [during the month of Ramadan], and it might even be obligatory under certain conditions such that delay is not warranted. Getting the vaccine (via intramuscular injection) does not invalidate the fast, even if it subsequently results in nausea or pain, because it is not considered to be one of the fast-nullifiers. If the person is forced to break their fast because they are unable to bear it then it is alright to do so and they must make it up later.