Booster shot side effects generally similar to initial shots


Hello again, dear readers! It’s a busy and, let’s face it, sometimes stressful time of year. We urge you to take a bit of time for yourself to regroup, even if it’s just a bit of deep breathing or a quick walk outdoors. If any of you have coping mechanisms for stress that might be helpful to others, we’d love to hear them. And now, onward to your letters.

-- As booster shots roll out in greater numbers, readers are asking what to expect. “Our son had significant side effects lasting 24 hours after his second Pfizer vaccination,” a reader said. “Is it known what will happen with a third shot of Pfizer?” So far, people who have received booster shots say their reactions are similar to those of the initial two-shot or single-dose series. Fever, headache, fatigue and pain at the injection site are the most commonly reported side effects. Most were mild to moderate and rarely lasted longer than a day or so. At this time, anyone 16 and older is eligible for a booster. For Pfizer and Johnson & Johnson, the booster is the same formulation and dose as the original vaccine; the Moderna booster is the same formulation, but half the original dose.

-- In a recent letters column devoted to COVID-19 questions, we referred to the vaccines as being authorized for emergency use by the Food and Drug Administration. A reader from Spokane, Washington, wondered if we had made an error. “Is the vaccine not, in fact, FDA approved?” she wrote. “That is important, as many are using the emergency-use designation as a reason to not get vaccinated.” The status of the three coronavirus vaccines has been of great interest, and in a bit of flux. All three -- the Pfizer, Moderna and Johnson & Johnson -- started with an emergency-use authorization (EUA) by the FDA. That’s a mechanism that makes rigorously tested medications available before the formal approval process is complete. Our reader is correct that the Pfizer vaccine has received full FDA approval for use in people 16 and over. However, use of the Pfizer vaccine in those younger than 16, and as a booster shot, remains under an EUA. The Moderna and Johnson & Johnson vaccines are both still undergoing the FDA approval process.

Elizabeth Ko, MD and Eve Glazier, MD

-- In response to a column about slow-to-heal bones, a reader suggested that an important part of how bone tissue is formed had been left out of the discussion. Specifically, the calcium-binding actions of magnesium and vitamin K2. While it’s too complex to cover in a letters column, we agree that she makes an excellent point. We will revisit the question in greater depth in an upcoming column soon.

We’ll close by thanking you for taking the time to write. You’ve kept our mailboxes full. We want to keep up with as many of your questions as we can, so we’ll be writing a letters column next week, as well. Meanwhile, please remember that you can find all of our columns online in a searchable database at

To learn more about the vaccines and for the latest information visit UCLA Health's COVID-19 Vaccine Info Hub.

(Send your questions to [email protected], or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)