The most common side effects to expect after your Moderna booster: headache, fatigue, and pain at the injection site

• An expert FDA panel recommended to OK Moderna's booster shot for certain high-risk groups.
• The vaccine-maker is applying to use a half-strength dose for the booster.

A booster shot of Moderna's coronavirus vaccine has similar side effects to what people experienced after the second dose.

The Food and Drug Administration is in the final stages of reviewing Moderna's booster-shot application. The agency's expert panel unanimously recommended Thursday OK'ing a third jab for certain high-risk groups, including people 65 years and older as well as younger people at higher risk of severe disease. The FDA typically follows the advisory group's recommendations.

Moderna plans to use a half-strength dose for its third shot. The initial two injections were 100 micrograms; the third shot would be 50 micrograms. The company said this smaller dose still produced good immune responses and would increase how many doses Moderna can produce.

Even with a smaller dose, people who received the booster shot still experienced common side effects. These were predominantly mild or moderate in intensity and resolved on their own within a few days, according to clinical trial results presented by Moderna.

People under 65 years old recorded more side effects, although the frequency was typically slightly lower than rates after the second dose. For younger people, here are the most common side effects.

These are from 129 study volunteers who received a half-strength booster shot after being fully immunized:

• Injection site pain: 86%
• Fatigue: 62%
• Headache: 59%
• Muscle pain: 50%
• Joint pain: 42%
• Chills: 40%
• Injection arm swelling or tenderness: 25%

Moderna also included safety data in its application for 38 study volunteers who were 65 years and older. Their most common side effects were:

• Injection site pain: 76%
• Fatigue: 47%
• Muscle pain: 47%
• Headache: 42%
• Joint pain: 40%
• Chills: 18%

The FDA still needs to officially authorize Moderna's booster shot. The Centers for Disease Control and Prevention would then need to also sign off on its use before it can be rolled out.