In an online report Thursday, Pfizer researchers compared data from early-stage testing of two vaccine candidates. Both revved immune systems similarly, and neither caused severe side effects.

But one candidate caused considerably fewer injection reactions, particularly in older adults -- symptoms such as fever, headache, chills or muscle pain that are temporary but unpleasant, Pfizer reported.

Final testing of Pfizer's lead candidate has begun as researchers recruit about 30,000 people in the US and other countries. It's one of a handful of experimental vaccines to reach end-stage tests around the world.

It generally takes years to develop a safe and effective vaccine for widespread use and distribution. US health officials hope to start offering vaccinations sometime next year.