The first of four experimental COVID-19 vaccines being tested by Pfizer and its German partner BioNTech showed encouraging results in very early testing of 45 people, the companies said Wednesday.

Study volunteers given either a low or medium dose, in two shots about a month apart, had immune responses in the range expected to be protective, when compared to some COVID-19 survivors, according to the preliminary results.

Side effects were typical for vaccines, mostly pain at the injection site and fever.

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.