A coronavirus vaccine spearheaded by Pfizer and BioNTech bolstered immune response to the virus in some, but in others — especially in large doses — the experimental drug caused fevers and other side effects, a study released Wednesday by Pfizer revealed.

You Might Like

Researchers reported “moderate increases” in immune response to the COVID-19 pathogen in those that received low and medium doses of the vaccine. The serum was administered in two doses and “substantially greater” results were observed after a second dose, according to the study, which has not been peer reviewed yet.

Those in the study gained nearly triple the antibodies than those who recovered from the virus naturally, according to the study. But the experimental serum is not without flaws.

Forty-five people were selected to be part of the experiment where participants were split into groups and injected with either 10 microgram (μg), 30 μg, or 100 μg of the vaccine in two doses over a 21-day period. Nine participants in the study received a placebo. The results of the study, which ran from May 4 to June 19, concluded that side effects like fever, chills and muscle pain resulted mainly in the first injection given to the 100 μg or the largest-dose group.

However, around 8% of the 10 μg group experienced fevers after the second injection, while 75% of the 30 μg group had the same symptom after their subsequent dose. Scientists ceased a follow-up injection for the 100 μg due to high incidence of fever following the first dose.

The experiment included both male and female patients and the average age was approximately 35 years old.

A total of 14 COVID-19 vaccines are being tested on humans, according to CNBC.

The world coronavirus case number stands at over 10.5 million with over 512,000 deaths. The U.S. has over 2.6 million cases and over 127,000 deaths, according to Wednesday afternoon calculation by the John’s Hopkins sick map.