COVID vaccine

Vaccine Side Effects: CDC Looks Into Myocarditis Reports in Few Young Recipients

Recent data has shown "relatively few reports of myocarditis" following vaccination from mRNA vaccines, the CDC's Advisory Committee on Immunization Practices reported last week

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A few young COVID vaccine recipients have experienced cases of myocarditis following their vaccination, according to the U.S. Centers for Disease Control and Prevention's advisory panel.

Recent data has shown "relatively few reports of myocarditis" following vaccination from mRNA vaccines, the CDC's Advisory Committee on Immunization Practices reported last week. The group did not specify a particular vaccine, though the two mRNA vaccines currently approved for emergency use in the U.S. include both Pfizer's and Moderna's vaccines, with Pfizer being the only one authorized for use in children between the ages of 12 and 17.

According to the preliminary reports, the condition has been seen mainly in adolescents and young adults and has affected males more than females.

The majority of cases have largely been reported after the second dose, typically within four days after vaccination.

The panel notes that "most cases appear to be mild, and follow-up of cases is ongoing," but it did not offer details on how many cases have so far been reported.

"Within CDC safety monitoring systems, rates of myocarditis reports in the window following COVID-19 vaccination have not differed from expected baseline rates," the panel stated. "However, VaST members felt that information about reports of myocarditis should be communicated to providers."

Myocarditis is an "inflammation of the heart muscle that decreases the ability of the heart to pump blood normally," according to Harvard Health. It can be caused by a number of things, including viral infections, bacteria, lyme disease, chagas' disease and certain medications.

Symptoms include chest pain, fatigue, shortness of breath, and arrhythmias, the Mayo Clinic reports, adding that when children develop myocarditis, they might have other symptoms including:

  • Fever
  • Fainting
  • Breathing difficulties
  • Rapid breathing
  • Rapid or abnormal heart rhythms (arrhythmias)

It remains unclear if the vaccine is related to the myocarditis, however.

"Because people are getting vaccinated, it may be overlapping," said Dr. Richard Novak, chief of infectious diseases with University of Illinois Health. "Right now, they’re not seeing a great number of myocarditis than would be expected in a young population.”

In fact, “COVID itself could cause myocarditis in young kids," according to Dr. Wayne Franklin, a pediatric cardiologist with Loyola University Medical Center.

Novak also noted that summer months bring with them a number of viruses that can cause myocarditis.

“There’s a whole host of viruses that are more common in the summertime - we call them enteroviruses - that are associated with myocarditis," he said.

Here's what we know so far about the two mRNA vaccines and their side effects:

What are the potential side effects?

Side effects are possible after receiving any COVID vaccine currently being administered in the U.S.

The CDC reports the most common side effects for the 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 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.

What about other uncommon but potential side effects?

Recent reports have brought to light some other unexpected, but so far not serious, reports at could be related to the vaccines, experts say.

As more and more Americans receive their first or second doses of the Pfizer or Moderna COVID vaccines every single day, some people who menstruate are reporting changes to their periods after getting vaccinated.

Dr. Kate Clancy, an associate professor of anthropology at the University of Illinois, posted her experience on Twitter in February and received hundreds of reports from those experiencing what she pondered could be a little-publicized response to the two available mRNA vaccines.

Arwady was asked about the possibility of vaccinations impacting menstruation in a Facebook live broadcast last week.

"Two hundred and twenty million Americans have gotten a first dose of vaccine, right? So, among 220 million Americans, there are people who will have, you know a herpes outbreak, who will have a changed menstruation cycle, etc.," she said. "What has been interesting, I think, is that, that this has really raised some awareness for wanting to make sure that we're asking questions about things like changes in menstruation, right? Most vaccine trials, most trials in general actually, unless they're really focused on women's reproductive health, may not even ask questions like that and that perhaps points to some biases in terms of how, you know, trials in general for medications, etc. are set up."

"I have not seen anything, to be very clear, that suggests that there is any concerning side effects in the way that would last, I know and there's a local researcher who is looking at some of this related menstruation, but very clear there's not been any link to, you know, problems with fertility, you know, anything that's long-lasting but, you know, the goal of getting a vaccine is for your immune system to learn how to protect yourself against COVID and your immune system can interact, can interface with your, you know, your hormonal levels, etc. and so there is at least some biological plausibility that you could have, you know, some change in terms of a heavier period or a lighter period for example right after getting the vaccination," Arwady continued.

Health experts have noted that menstrual changes have been documented in recent months outside of vaccinations as well.

Dr. Whitney Lyn, a family medicine attendee for Cook County Health, acknowledged the possibility of changes post-vaccination, but also noted that stress can play a role in a woman's cycle.

"Women's menstrual cycle can, you know, always change month to month for various reasons," Lyn said. "And so one of the things that causes a woman's menstrual just to change is stress and so right now we're seeing a very stressful time. And so every time someone gets the vaccine, they're a little stressed out. So sometimes can that make your flow a little heavier or a little lighter? Yes. And so, I think it's a normal response, but I don't think it's a reason not to take the vaccine."

Even without contracting COVID or getting vaccinated, menstrual changes have been reported possibly stemming from the overall pandemic environment itself. A Washington Post report from August found that several gynecologists "confirmed that many of their patients are reporting skipped periods or have noticed increases or decreases in cycle length, blood volume and level of menstrual-related pain."

There have also been reports of what's known as "COVID arm," a term used to describe delayed skin reactions such as rashes, which appear days after injection.

"If it is going to arise, it usually appears about a week after your vaccine,” Dr. Brita Roy, an internal medicine physician and director of population health for Yale Medicine said. “It‘s a red, swollen area at the site of the shot."

The skin reactions gained attention when a letter was published in the New England Journal of Medicine earlier this month detailing some patients who experienced varying degrees of arm rashes following their first dose of the Moderna vaccine.

“It’s not super common, but it’s not uncommon. It’s a delayed hypersensitivity, similar to what you may see if you get poison ivy,” Roy said. “You maybe came into contact with the poison ivy in your yard, but some people won’t get a rash until a few days later.”

The CDC acknowledged reports "that some people have experienced a red, itchy, swollen, or painful rash where they got the shot," which it identified as "COVID arm."

According to the CDC, the rashes can start within a few days to more than a week after the first shot and "are sometimes quite large."

"If you experience 'COVID arm' after getting the first shot, you should still get the second shot at the recommended interval if the vaccine you got needs a second shot," the CDC noted. "Tell your vaccination provider that you experienced a rash or 'COVID arm' after the first shot. Your vaccination provider may recommend that you get the second shot in the opposite arm."

The CDC said those who experience COVID arm can take an antihistamine.

"If it is painful, you can take a pain medication like acetaminophen or a non-steroidal anti-inflammatory drug (NSAID)," the CDC recommends.

Does one vaccine cause more side effects than the other?

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.

But experts say data shows the two are similar and that side effects depend more on the person than shot itself.

"I would not try to make a decision between one, you know, between Moderna and Pfizer in particular, based on side effects," Arwady said Tuesday. "I think get the one that's available to you and do definitely get that second dose."

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

With the two-shot vaccines, people are more likely to report side effects after their second dose, experts have said.

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.

Arwady noted that a good indicator of whether you'll experience side effects after your second dose is how your body reacted to the first.

"The biggest predictor between first and second doses is what your own reaction was," she said. "So if you didn't have much of a reaction after the first dose, you're unlikely to have a big reaction after the second dose. We do see people having a little more side effects after the second dose than the first but usually not a huge, huge amount of difference."

She added that Johnson & Johnson's vaccine "does have a lower rate of the side effects in those first few days than the other two do."

But that doesn't mean that you shouldn't get your second shot if you get side effects after your first, experts say.

“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 CDC also noted that both shots are needed.

"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."

How effective are the Pfizer and Moderna COVID vaccines?

In clinical trials, Moderna's vaccine reported 94.1% effectiveness at preventing COVID-19 in people who received both doses. The Pfizer-BioNTech vaccine was said to be 95% effective.

A new CDC study reported that a single dose of Pfizer's or Moderna's COVID vaccine was 80% effective in preventing infections. That number jumped to 90% two weeks after the second dose, the study on vaccinated health care workers showed.

"These findings indicate that authorized mRNA COVID-19 vaccines are effective for preventing SARS-CoV-2 infection, regardless of symptom status, among working-age adults in real-world conditions," the U.S. agency wrote in the study. "COVID-19 vaccination is recommended for all eligible persons."

Pfizer's vaccine, the only one currently authorized for use in children as young as 12, also showed heightened effectiveness among adolescents.

Pfizer in late March released preliminary results from a vaccine study of 2,260 U.S. volunteers ages 12 to 15, showing there were no cases of COVID-19 among fully vaccinated adolescents compared with 18 among those given dummy shots.

More intriguing, researchers found the kids developed higher levels of virus-fighting antibodies than earlier studies measured in young adults.

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