• Headaches, fatigue and muscle pain were some of the most common side effects from J&J's Covid-19 vaccine, according to a new FDA report.
• The agency said the trial results were "consistent with the recommendations set forth in FDA's guidance Emergency Use Authorization for Vaccines to Prevent COVID-19."
• Roughly 40% of people reported headaches and just more than 38% reported feeling fatigued, the report said.

Headaches, fatigue and muscle pain were some of the most common side effects among people who received Johnson & Johnson's one-shot coronavirus vaccine in clinical trials, according to a Food and Drug Administration report published Wednesday.

In the report, the FDA staff endorsed J&J's vaccine for emergency use authorization, and, if approved, it would become the third vaccine given authorization in the U.S. behind Pfizer-BioNTech and Moderna.

Side effects following vaccination are common, doctors say, and they typically are indications that it's helping build protection against the disease. The FDA said it determined that the clinical trial results and safety data were "consistent with the recommendations set forth in FDA's guidance Emergency Use Authorization for Vaccines to Prevent COVID-19."

The most frequent, least severe "local" adverse reaction among those who were vaccinated was injection site pain, which was reported by nearly half of vaccine recipients, compared with roughly 17% in the placebo group, according to the report. Skin redness, or erythema, and swelling were reported less frequently, the FDA said.

The most frequent "systemic" adverse reactions, which typically occur less frequently than local reactions but are slightly more severe, included headache and fatigue, according to the report. Nearly 40% of people who received the vaccine reported experiencing a headache and just more than 38% reported feeling fatigued.

More than 33% of vaccinated people reported feeling muscle pain, roughly 14% reported nausea and under 10% had a fever, according to the report. Most of the adverse reactions were reported in younger people age 18 to 59, the FDA said, though most age groups reported similar rates of nausea.

The FDA staff found that most people who received the vaccine were able to shake off the side effects within a couple of days after their shot.

While the rate of unexpected side effects was similar between the vaccine and placebo groups, there was a slight imbalance of some reactions in the vaccine group to those who received a placebo. The vaccine group reported 15 cases of "embolic and thrombotic events," or blood clots, among 14 recipients, compared with 10 in the placebo group.

There were also six cases of tinnitus, a ringing or buzzing noise in at least one ear, among vaccine recipients compared with none in the placebo group, the FDA found.

However, the agency said current data is "insufficient to determine a causal relationship between these events and the vaccine."

The report found a "balanced" number of Bell's palsy cases, a condition that causes half of your face to droop. The FDA had previously said the condition would be monitored among recipients of other vaccines, noting that it's not necessarily a side effect but is worth watching out for.

The FDA said there were no reports of anaphylaxis — a severe and life-threatening allergic reaction that occurs rarely following a vaccination — immediately after receiving the J&J shot.

Some cases of anaphylaxis were reported in people who received either Pfizer's or Moderna's shot, though researchers say it occurred within the range of those reported for the influenza vaccine, according to a Centers for Disease Control and Prevention report published Friday.

J&J first submitted its Covid vaccine data to the FDA on Feb. 4. While the inoculation's level of protection varied by region, the company said it prevented 100% of hospitalizations and deaths.

— CNBC's Berkeley Lovelace Jr. contributed to this report.