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

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?

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.

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.

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

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.

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.

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 and cautioned against trying to make a decision between the two vaccines based on side effects.

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.

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

"The Pfizer-BioNTech COVID-19 Vaccine and Moderna COVID-19 Vaccine both need 2 shots in order to get the most protection," the CDC said. "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. Medical experts say the biggest predictor for side effects so far has been age.

Older people do not tend to experience as many side effects because their immune systems aren't as robust and they don't mount as much of an immune response.

Women are also more likely to experience side effects than men. Some of this is due to the fact that estrogen can elevate immune responses, while testosterone can decrease it.

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% of side effects were reported by women, the data showed. Meanwhile, women received roughly 61.2% of the doses administered during that same time.

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

But not getting side effects isn't a negative and does not mean that you are not protected, health experts say. It simply means your body didn't react with as much of an inflammatory 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," Sahin said in a release.

The National Institutes of Health has already 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.