"They tell you that you could feel side effects like body soreness or a problem where the injection was," said McLure. He doesn't know whether he got the vaccine or the placebo, but says he didn't experience any side effects during his participation in the Moderna two-dose vaccine trial. His experience and that of the tens of thousands of people in the Moderna and Pfizer vaccine trials are what experts are using to answer today's most frequently asked questions. The few short term side effects reported included fatigue, headache and flu-like symptoms within few hours to the first few weeks.