The report has been submitted for publication in a scientific journal but not yet reviewed.
With its other potential candidates still in the earliest stage of testing, Pfizer aims to open a large-scale study this summer but can't yet say which shot is best to include.
But researchers didn't administer a second shot of the highest dose initially tested, sticking with the low and medium doses. The higher-dose shot caused more injection reactions without apparent added benefit.
About 15 different COVID-19 vaccine candidates are in human testing worldwide, with several poised to begin huge, last-stage studies to prove if they really work.
Different companies are pursuing different types of vaccines, boosting the odds that at least one approach might work although there's no guarantee. The Pfizer and BioNTech candidates use a piece of the coronavirus genetic code to prime the body to recognize and attack the virus.
Earlier this week, Inovio Pharmaceuticals issued a news release saying its gene-based vaccine candidate showed encouraging results in similar early testing in 40 volunteers.