COVID vaccine

Pfizer vs. Moderna Vaccines: Does One Have More Side Effects Than the Other?

Here's a breakdown of the Pfizer and Moderna vaccines, their potential side effects

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As many continue receiving their first or second doses of the Pfizer and Moderna COVID vaccines currently available, what are the potential side effects and does one cause more side effects than the other?

Chicago Department of Public Health Commissioner Dr. Allison Arwady was recently asked that very question.

There have also been reports of potential symptoms like "COVID arm" or changes in menstrual cycles.

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.

Experiencing side effects isn't necessarily a bad thing. In fact, it's a sign your body is responding.

"That's just your immune system learning the lesson of how to fight it off," Chicago's top doctor said in a Facebook Live Tuesday. "So people who have stronger side effects, it's just a sign that you have a very robust strong immune system that's learning the lesson."

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, side effects that 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."

Are certain people more likely to experience side effects?

There are also some factors that could make you more likely to experience side effects.

Chicago's top doctor said Tuesday that the biggest predictor for side effects so far has been age.

"Older people, broadly, do not have as much side effects and that's because their immune systems are not quite as robust, generally, and so they don't mount as much of a immune response," Arwady said. "It doesn't mean that they're not protected."

According to Loafman, the body's immune system is what creates the symptoms.

"That's simply a reflection of the immune response, just the way we have when we get ill," he said.

Arwady also noted that women are more likely to report side effects than men.

"Some of this is because women may just be better reporters... but there probably is something real to this too because something else interesting for those who may not know as much about immunity is that autoimmune diseases? Much, more likely in women, too," Arwady said. "And even the, like, more serious like the allergic reactions, the more serious allergic reactions? More likely in women."

Why is that?

Arwady said estrogen can elevate immune responses, while testosterone can decrease it. At the same time, she noted that "a lot of your immune modulating genes" can live on an "x" chromosome, which women have two of while men have one.

"So there's all these reasons that sort of immunity in general goes up a little bit different in women than it does in men," she said. "And so we're seeing women, a little more likely to report some of the side effects."

Data from the CDC also reported women were more likely to experience side effects than men, according monitoring from the first month of vaccinations.

From Dec. 14 through Jan. 13, more than 79 percent of side effects were reported by women, the data showed. Meanwhile, women received roughly 61.2 percent of the doses administered during that same time.

Side effects could also vary depending on whether or not you've had coronavirus.

"We have seen more likely that people will report some side effects because that is acting a little bit like a booster dose to your immune system," Arwady said. "Your immune system has already learned some of those lessons of how to protect itself, not in as long a way not as protective a way."

"That is also probably that booster effect," Arwady said.

Loafman agrees.

"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."

But not getting side effects isn't a negative, health experts say.

"If you don't get side effects it does not mean that you are not protected," Arwady said. "I want to be really clear about that."

According to Loafman, it simply means "your body didn't react with as much of an inflammatory response.

"You're still making antibodies," he said.

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 the appropriate immune response."

How effective are the Pfizer and Moderna COVID vaccines?

Questions about vaccine effectiveness have been paired with a rise in spread of multiple COVID variants.

So far, studies suggest that the vaccines currently in use can recognize the emerging variants — but they may not provide as much protection against the new strains.

Pfizer's latest study results, however, suggested that the vaccine is effective against the coronavirus variant that first emerged in South Africa.

“These data also provide the first clinical results that a vaccine can effectively protect against currently circulating variants, a critical factor to reach herd immunity and end this pandemic for the global population," Ugur Sahin, CEO and co-founder of BioNTech, said in a statement.

Moderna, citing data from its phase three clinic trial, reported its COVID-19 vaccine was more than 90% effective at protecting against COVID and more than 95% effective against severe disease up to six months after the second dose, the company said.

But boosters and new versions of vaccines that target the variants are already being explored.

Pfizer-BioNTech is testing a third booster shot of its vaccine on fully vaccinated people. Pfizer CEO Albert Bourla said people will "likely" need a third dose of a COVID-19 vaccine within 12 months of getting fully vaccinated.

"The flexibility of our proprietary mRNA vaccine platform allows us to technically develop booster vaccines within weeks, if needed," Ugur Sahin, CEO and co-founder of BioNTech, said in a release.

Late last month, the National Institutes of Health started testing a new COVID vaccine from Moderna aimed at protecting against a variant first discovered in South Africa. Moderna CEO Stephane Bancel told CNBC that the company hopes to have a booster shot for its two-dose vaccine available in the fall.

But what about without the variants?

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."

It is not known if any of the vaccines prevent the spread of the virus by people who are asymptomatic.

Monica Hendrickson, public health administrator for the Peoria County Health Department noted that the vaccines each hold a high effectiveness against death and severe illness for coronavirus.

"So, really, you're looking at a distinction that from a clinical standpoint, or from, you know, an epidemiological standpoint is very minor compared to what we really are hoping for, which is decreases in death and decreases in severe illness, where they all match up between the three vaccines," Hendrickson said. "Most important thing though is that when these vaccines come on the market, if you have an option to any of these, get one of them."

Hendrickson's message echoes one made by Dr. Marina Del Rios, emergency medicine specialist at the University of Illinois-Chicago, during NBC 5's "Vaccinated State" panel.

“Part of my messaging in the community has been that the vaccines on the market are equally efficacious and equally safe," Del Rios said. "The best vaccine you can get is the one that you can get ahold of first, and getting vaccinated earlier, sooner rather than later, protects us from getting sick ourselves and also our community, which has been so terribly devastated by this virus.”

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