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

• Pfizer's COVID-19 booster shot yields mild to moderate side effects, much like its second dose.
• Injection-site pain is the most common, as well as headaches and fatigue.

A third dose of Pfizer's coronavirus vaccine yields mild to moderate side effects, much like the second shot, according to data released this month by the Food and Drug Administration.

The FDA has authorized a third dose, or "booster," for people 65 years and older and others at high risk of severe COVID-19. That includes people who are more likely to get sick because of their health status, as well those who are at high risk of exposure due to where they live and work - such as healthcare workers, teachers and daycare staff, grocery store workers, and residents of homeless shelters or prisons.

On Thursday, the Centers for Disease Control and Prevention recommended a booster at least 6 months after the second dose for: People 65 years and older, nursing home residents, and people ages 50 to 64 with underlying medical conditions. The agency said others who might consider a booster include younger adults with underlying medical conditions or those at increased risk of COVID-19 exposure because of their job or living arrangement.

Data from Pfizer shows that protection from its vaccine declines over time, and that a third dose is safe. In its booster trial, participants saw 3.3 times higher antibody levels one month after the booster than one month after their second dose.

The most frequent side effects were injection-site pain, fatigue, headache, muscle and joint pain, and chills. Here's how common each of these side effects was among the participants:

• Injection site pain: 83%
• Fatigue: 63.7%
• Headache: 48.4%
• Muscle and joint pain: 39.1%
• Chills: 29.1%

Only one trial participant had an adverse event after the booster that was considered to be severe: They experienced lymphadenopathy, or swelling of the lymph nodes, two days after the shot. That resolved after the fifth day. (Overall, 16 participants reported some level of swelling in their lymph nodes.)

Data from Israel suggests side effects resolve after 1-3 days

Other reports have also indicated that booster side effects are mild and fleeting. A report from Maccabi Health Services, one of Israel's four major health providers, found that booster side effects usually go away within one to three days.

Maccabi surveyed more than 9,200 adults in Israel who'd recently received a third dose of Pfizer's vaccine. In general, the following portions of adults reported these side effects:

• Injection site pain: 87%
• Weakness and fatigue: 57%
• Headache: 36%
• Muscle pain: 26%
• Swollen lymph nodes: 19%
• Joint pain: 14%
• Fever over 38 degrees Celsius (100 degrees Fahrenheit): 9%
• Fever up to 38 degrees Celsius: 8%

Half of the surveyed adults said the booster shot yielded worse side effects than the second dose. But the other half said the booster side effects were the same or milder.

In another survey from Clalit Health Services, Israel's largest healthcare provider, 88% of participants said they felt "similar or better" after their Pfizer booster than after dose two.

How the first 2 doses compare to a booster shot

The mRNA vaccines from Pfizer and Moderna instruct the body to produce a harmless viral protein, then develop antibodies against it. The first dose of Pfizer's vaccine generally produces the mildest side effects, since our bodies are being introduced to the instructions and the resulting protein for the first time.

By the time we receive a second dose, our bodies have learned to recognize that protein quickly, so are ready to attack it. That's why side effects are generally more pronounced. (If you've had COVID-19 before, though, your first dose may yield more intense side effects than your second.)

After their first Pfizer shot, 47% of adults ages 18 to 55 reported fatigue. That number rose to 59% after the second dose. Reports of a headache also rose from 42% to 52% from the first to second dose, while reports of muscle pain rose from 21% to 37%.

Scientists don't expect boosters to yield worse side effects than the second shot, nor any higher risk of severe allergic reactions.

"If you've tolerated two doses of a vaccine, you're far less likely to have an anaphylactic reaction to a third dose," Kawsar Talaat, an infectious-disease physician at Johns Hopkins Medicine, recently told STAT.

Boosters haven't been authorized for all US adults yet

US regulators recommend boosters for elderly people because their immunity from vaccines tends to wane more quickly than average. Immunocompromised people, including cancer and HIV patients, as well as those who've had organ transplants, may also mount a weaker immune response to vaccines.

But many questions remain about boosters. In addition to the CDC and FDA's slightly different recommendations, an independent group of advisors to the CDC put forth its own guidelines on Thursday. The group recommended boosters for nursing home residents, people 65 and older, and all adults with underlying medical conditions - but not for healthcare workers, teachers, or prisoners.

Boosting younger adults wouldn't do much to prevent COVID-19 hospitalizations, the group said. Other experts have similarly argued that there's no need for the widespread use of boosters yet.

In a review published in The Lancet, 18 scientists warned of potential risks of introducing boosters too frequently or too soon. In particular, the scientists worried that boosters might unnecessarily lead to higher instances of rare side effects like myocarditis, a type of heart inflammation. But Pfizer didn't record any cases of myocarditis in its booster trial.

"We continue to believe in the benefits of a booster dose for a broader population," Kathrin Jansen, Pfizer's head of vaccine research and development, said in a statement.

Hilary Brueck contributed reporting.