• Experts say you’re more likely to have stronger side effects after a second dose of the COVID-19 vaccine than the first dose.
  • The side effects can include fatigue, arm soreness, and fever.
  • Experts note these side effects are usually mild and disappear 48 hours after vaccination.

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Some people who have received the COVID-19 vaccination report they feel ill after getting their first shot.

Even more say the second shot can be followed by symptoms such as fever, fatigue, headache, and soreness at the injection site on their arm.

In most cases, that’s actually a good thing.

These common symptoms are typically signs that the vaccine has triggered a response by your immune system.

And that’s what it is supposed to do.

“When you feel sick or have a fever, that’s largely your body responding,” Dr. Debra Powell, the chief of infectious diseases at Tower Health in Pennsylvania, told Healthline. “It’s usually a very short-term thing and much better than getting COVID and being sick for 2 weeks or in the hospital.”

To date, more than 55 million doses of the COVID-19 vaccine have been administered in the United States, according to the Centers for Disease Control and Prevention (CDC).

About 15 million people have already received a second dose.

Slightly more Americans have received the vaccine developed by Pfizer-BioNTech (28 million) than the Moderna-developed vaccine (26 million).

Both shots are mRNA vaccines, which activate the immune system by getting it to identify a type of protein found in the virus that causes COVID-19.

“The first vaccine teaches your body how to react to the virus,” said Powell.

Armed with antibodies and memory T cells that recognize the virus protein from the first shot, the immune system response tends to be more robust when the second dose is delivered, she noted.

In applying for emergency use authorization from the Food and Drug Administration (FDA), both Pfizer and Moderna disclosed side effects experienced by participants during clinical trials for the vaccines.

The real world experience with the vaccine seems to closely reflect what researchers observed.

For starters, side effects tend to be more common after the second dose of the vaccine.

“In general, it is more common to see reactions with dose two,” Dr. Sharon Nachman, chief of pediatric infectious diseases at Stony Brook Children’s Hospital in New York, told Healthline. “This is what we see with tetanus vaccine as well as other commonly used adult vaccines like zoster vaccines. However, we have heard from many patients who had a reaction to dose one and were prepared to get a worse reaction to dose two, and had nothing.”

In addition, significant numbers of people experience milder side effects after vaccination.

“It is more common to see fatigue and local reactions than to see fever,” said Nachman.

Mild side effects of the COVID-19 vaccination tend to clear up within 48 hours of injection.

Side effects are typically more pronounced among younger people than older people, likely because their immune systems are more robust.

“In general, the older the patient is, the less likely the reaction is to be significant or severe,”said Nachman.

Serious side effects, such as anaphylactic allergic reactions, are rare.

“We have seen local allergic reactions to Moderna more than we have seen with Pfizer,” Nachman noted. “These are local to the shot, and do not need to be treated.”

According to research data submitted to the FDA, 7 percent of people between the ages 18 and 55 receiving the first dose of the Pfizer vaccine reported fever, compared to 31 percent who reported a fever after the second dose.

With the Moderna vaccine, 1 percent of those ages 18 to 64 reported fever after the first dose, and 17 percent reported getting a fever after the second dose.

Reports of fatigue — the most common side effect — as well as headache and chills tended to be more likely after the second dose than the first for both vaccines.

Significantly fewer recipients reported diarrhea or vomiting as a result of the first or second shot.

People over age 55 receiving the Pfizer vaccine were less likely to report any of these side effects. Only 3 percent of older recipients, for example, reported fever after the first dose, with 21 percent reporting fever after the second dose.

While some people worry about side effects, Powell emphasized that you shouldn’t be concerned if you don’t feel sick after your first or second dose of vaccine.

“For most people, it’s very mild, and some don’t get anything,” she said.

She noted that side effects vary from person to person and that the lack of symptoms doesn’t mean your immune system isn’t working properly.

Powell also stressed that the fear of side effects should not discourage people from getting the full course of vaccination, pointing out that while the first dose of vaccine improves your immunity by about 50 percent, it’s only about 2 weeks after the second shot that vaccine protection against symptomatic COVID-19 rises to 95 percent.

“People really need to get that second dose,” she said.