UPDATE: The U.S. is recommending a “pause” in administration of the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of potentially dangerous blood clots.

In a joint statement Tuesday, the Centers for Disease Control and Prevention and the Food and Drug Administration said they were investigating unusual clots in six women that occurred 6 to 13 days after vaccination. All six cases were in women between the ages of 18 and 48. More than 6.8 million doses of the J&J vaccine have been administered in the U.S., the vast majority with no or mild side effects. Read more here. 
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Side effects are possible after receiving either one or two doses of any of the three coronavirus vaccines currently being administered in the U.S., but not everyone experiences them.

So what causes the side effects and what does it mean if you get them when others don't?

Here's what you should know:

Are side effects more common after the first or second dose?

According to the CDC, side effects after your second shot "may be more intense than the ones you experienced after your first shot." 

"These side effects are normal signs that your body is building protection and should go away within a few days," the CDC states.

In trials of both the Moderna and Pfizer vaccines, more people experienced side effects after the second dose.

What are the possible side effects?

According to Pfizer, about 3.8% of their clinical trial participants experienced fatigue as a side effect and 2% got a headache. 

Moderna says 9.7% of their participants felt fatigued and 4.5% got a headache.

Like Pfizer and Moderna's COVID-19 vaccines, the main side effects of the J&J shot are pain at the injection site and flu-like fever, fatigue and headache.

The CDC reports the most common side effects for all three authorized vaccines is at the injection site. They include:

• Pain
• Redness
• Swelling

Common side effects in the body include:

• Tiredness
• Headache
• Muscle pain
• Chills
• Fever
• Nausea

"The Pfizer-BioNTech COVID-19 Vaccine and Moderna COVID-19 Vaccine both need 2 shots in order to get the most protection," the CDC states. "You should get the second shot even if you have side effects after the first shot, unless a vaccination provider or your doctor tells you not to get it."

“When people receive that second dose, they are receiving the second booster to try and reach the maximum efficacy," said Dr. Edward Cachay, infectious disease specialist at UCSD. 

The Centers for Disease Control and Prevention advises people to stick around for 15 minutes after vaccination, and those with a history of other allergies for 30 minutes, so they can be monitored and treated immediately if they have a reaction.

Why do you experience side effects? Is it bad if you don't?

According to Dr. Mark Loafman, chair of family and community medicine for Cook County Health in Illinois, the body's immune response creates the symptoms, but there's no real way to tell if you will experience side effects.

"That's simply a reflection of the immune response, just the way we have when we get ill," he said. "From our immune response to a cold virus or a flu virus or the coronavirus, you know, it's our body's immune response that creates the illness. The virus itself is not the direct cause, it's directly causing the illness, and it's our inflammatory response making the immune system respond. And that's our body's way of destroying the virus is having inflammation and immunity to fight back. So that's the variation that's there and it's hard to know how you're gonna respond until you get it."

The good news is, if you do experience side effects, it's a sign your body is responding.

"The good news on our part is that a brisk response equals an effective response," Loafman said. "It tells us that the vaccine is working. Our body's forming a robust immune response and we feel that that's a positive thing. So we tend to see the vaccines that have a higher efficacy rate also have more of the so-called side effects or the symptoms because they work so well."

But if you don't, that's OK too.

"It's just that your body didn't react with as much of an inflammatory response," Loafman said. "You're still making antibodies. So it's not so much that sicker is better, but it just means that it's a vaccine that causes a reaction. Everybody - 95% of people who get it, almost 100% of people will get some reaction. Many of them that's an asymptomatic reaction, but others is more symptomatic."

According to Loafman, every person's response is unique.

"It's really just kind of a reflection of how unique each of our systems are, what other immunities we have," he said. "You know, a lot of the antibodies cross react and we have cross reactivity so it's really a mosaic. Each of our immune systems is a mosaic composite of all that we've been through and all that we have and all we've recently been dealing with. Our individual response varies. Everybody gets gets the appropriate immune response. The symptoms vary just like they do with with pretty much every infection you could think, from all the way from asymptomatic to potentially severe. That's all a sign of our individual immune systems responding to that to that exposure."

Are side effects an indication of whether or not you already had COVID?

There's some truth to the idea, but again, it depends, Loafman says.

"If you had COVID a while ago or you've already got some immunity, it's more like a booster," he said. "And boosters for some people are completely asymptomatic, boosters for other people trigger their immune response against it so they have some inflammation with it."

What can you do if you experience side effects?

The CDC recommends people talk to their doctors about taking over-the-counter medicines like ibuprofen, acetaminophen, aspirin, or antihistamines, for any pain and discomfort after getting vaccinated. 

"You can take these medications to relieve post-vaccination side effects if you have no other medical reasons that prevent you from taking these medications normally," the CDC states. "It is not recommended you take these medicines before vaccination for the purpose of trying to prevent side effects."

The CDC recommends you seek medical attention if you experience any of the following:

• If the redness or tenderness where you got the shot gets worse after 24 hours
• If your side effects are worrying you or do not seem to be going away after a few days
• If you get a COVID-19 vaccine and you think you might be having a severe allergic reaction after leaving the vaccination site, seek immediate medical care by calling 911.