U.S. Drug company Pfitzer and Germany’s BioNTech which are working on an experimental coronavirus vaccine in partnership reported positive results on Wednesday from the early stages of trials.

Pfitzer conducted trials on 45 patients, with some people receiving a placebo. Twenty-four people between the ages of 18 and 55 who received two doses of the vaccine had “significantly elevated” antibodies within four weeks of their first injection. The 100-microgram dose caused fevers in half of patients. These patients were not given the second dose. Groups who were given a smaller dose had the second injection three weeks after: 8.3% of those given 10-microgram dose and 75% of those given 30-microgram dose developed fevers. The side effects, however, were not deemed serious or life-threatening because it did not require hospitalization.

The patients who received the single 100-microgram dose were reported to show fewer antibodies than patients who have been injected with two shots of the lower doses.

“We still have a ways to go and we’re testing other candidates as well,” said Philip Dormitzer, the chief scientific officer for viral vaccines at Pfizer’s research laboratories. “However, what we can say at this point is there is a viable candidate based on immunogenicity and early tolerability safety data.”

Pfizer and BioNTech are testing four different versions of the vaccine, but only one will go through a further trial. BioNTech and Pfitzer use a method of RNA (mRNA) technology to develop a vaccine.

After the first trial results, BioNTech and Pfizer are planning to move to a larger sample group, 30,000 participants within weeks.

Fourteen other companies including Inovio, CanSino, AstraZeneca and Moderna, are conducting clinical trials of the coronavirus vaccine. In total, 178 vaccines are in various stages of development.