Pfizer, Moderna, J&J: Which COVID Vaccine is Best For You? Here's a Breakdown

As vaccinations continue across the U.S. and with children as young as 12 now eligible, what should you know about the COVID vaccines authorized for emergency use in the U.S.?

According to medical experts, the three vaccines currently available each offer protection.

Here's a breakdown:

How Long Does the Pfizer Vaccine Last? What About Moderna?

Pfizer and BioNTech said previously that data from their Phase 3 trial showed high levels of protection against COVID six months after second doses.

Research has also suggested the protection the Moderna vaccine gives against COVID-19 lasts for at least six months.

Both reports were based on follow-up tests in dozens of people who received the shots during studies that led to the vaccines’ use. Those studies were done before troubling new variants, or versions of the coronavirus, had emerged and started to spread.

How effective are the 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.

In mid-March, CNBC reported the J&J shot demonstrated 64% efficacy in South Africa, where the more contagious and virulent B.1.351 variant is rapidly spreading.

Still, White House chief advisor Dr. Anthony Fauci said during an interview with MSNBC's Medhi Hasan last month that people may need to get booster shots in a year. 

Pfizer CEO Albert Bourla also previously said people will "likely" need a third dose within 12 months of getting fully vaccinated.  

The need for a booster shot likely won't depend on how effective any of the three COVID vaccines are, medical experts say. Rather, another shot could aid in protecting against any possible new variants that could arise during the summer.

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.  

"The flexibility of our proprietary mRNA vaccine platform allows us to technically develop booster vaccines within weeks, if needed," BioNTech's Sahin said in a release.

Moderna is also testing a potential third dose of its current vaccine, and a possible booster shot specifically targeting the South Africa variant. Citing early data, the company recently said the booster vaccine generated a promising immune response against the B.1.351 and P.1 variants first identified in South Africa and Brazil, respectively.

Meanwhile, Johnson & Johnson CEO Alex Gorsky said during an interview with CNBC's "Squawk Box" earlier this month that the company is well-positioned to adapt its vaccine for variants, and is working on developing software that will "help address some of these new and emerging variants."

"We continue to try to project what will happen," Illinois Department of Public Health Director Dr. Ngozi Ezike said this week. "Again, the boosters, we're not even sure, you know, when that will be, if that will be, if it'll be new vaccines or the same vaccines."

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."

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.

The FDA said J&J’s vaccine offers strong protection against what matters most: serious illness, hospitalizations and death. One dose was 85% protective against the most severe COVID-19 illness, in a massive study that spanned three continents — protection that remained strong even in countries such as South Africa, where the variants of most concern are spreading.

The CDC reports J&J/Janssen vaccine was 66.3% effective in clinical trials at preventing COVID-19 illness in people who had no evidence of prior infection 2 weeks after receiving the vaccine.

"The vaccine had high efficacy at preventing hospitalization and death in people who did get sick," the CDC notes. "No one who got COVID-19 at least four weeks after receiving the J&J/Janssen vaccine had to be hospitalized."

It is not known if any of the three vaccines prevent the spread of the virus by people who are asymptomatic, though the CDC noted that "early evidence suggests that the J&J/Janssen vaccine might provide protection against asymptomatic infection."

While the overall effectiveness data may suggest the J&J candidate isn't quite as strong as the two-dose Pfizer and Moderna options, all of the world’s COVID-19 vaccines have been tested differently, making comparisons nearly impossible, CNBC reported.

How are they different?

Arguably one of the biggest difference between the three vaccines is the number of doses needed. The Johnson & Johnson vaccine requires just one dose, while both Pfizer's and Moderna's vaccines are two shots.

The Pfizer-BioNTech and Moderna vaccines also differ from traditional vaccines in their use of mRNA. Instead of introducing a weakened or an inactivated germ into your body, this vaccine injects mRNA, the genetic material that our cells read to make proteins, into your upper arm muscle. It teaches your body how to make the protein that triggers antibody production so if the real virus later enters your body, your immune system will recognize it, according to the federal Centers for Disease Control and Prevention. 

Unlike the Pfizer and Moderna shots, the J&J vaccine uses a cold virus, called an adenovirus, to carry the spike gene into the body. J&J uses a human adenovirus to create its vaccine while a similar vaccine not yet approved in the U.S., AstraZeneca, uses a chimpanzee version.

J&J’s shot uses a cold virus like a Trojan horse to carry the spike gene into the body, where cells make harmless copies of the protein to prime the immune system in case the real virus comes along. It’s the same technology the company used in making an Ebola vaccine.

The Pfizer and Moderna vaccines were also required to be kept frozen, while the J&J shot can last three months in the refrigerator, making it easier to handle.

Moderna said last month, however, that new data showed its vaccine could remain stable at temperatures found in refrigerators for up to three months. A change in the storage guidelines would require authorization from the FDA.

Pfizer's vaccine is also currently the only vaccine authorized for emergency use in children as young as 12. Both Moderna and J&J's vaccines can only be used on people 18 and older.

Pfizer isn’t the only company seeking to lower the age limit for its vaccine. Results also are expected by the middle of this year from a U.S. study of Moderna’s vaccine in 12-to -17-year-olds. Pfizer is currently authorized for use on people ages 16 and up, while Moderna is authorized for people 18 and older.

What about younger kids?

According to experts, those under 12 likely won't be able to receive the COVID vaccine until later this year or early next year.

Both Pfizer and Moderna have started studies in the U.S. surrounding the vaccine and children ages 6 months to 11 years. Results on those studies could come sometime in the fall.

What are the side effects for each vaccine?

According to Pfizer, about 3.8% of their clinical trial participants experienced fatigue as a side effect and 2% got a headache. The side effects remain similar among young adults and adolescents.

Younger teens receive the same vaccine dosage as adults and have reported the same side effects, mostly sore arms and flu-like fever, chills or aches that signal a revved-up immune system, particularly after the second dose.

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. These side effects usually start within a day or two of getting the vaccine, according to the CDC.

But the Johnson & Johnson vaccine was recently paused by U.S. regulators as they investigated reports of potentially dangerous blood clots known as cerebral venous sinus thrombosis, or CVST, in a small number of recipients.

The CDC and FDA said in April 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. At the time, more than 6.8 million doses of the J&J vaccine had been administered in the U.S., the vast majority with no or mild side effects.

Ten days later, the agencies recommended resuming use of the vaccine, stating the benefits of the inoculations outweighed their risks.

The CDC's advisory panel didn't recommend restricting use of the vaccine to a particular age or gender, but did propose the possibility of adding a warning label for women under the age of 50.

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.

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 CDC says 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.

Medical experts say younger people are more likely to experience side effects because they have more robust immune systems, which is what creates the symptoms.

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.

Why is that?

Estrogen can elevate immune responses, while testosterone can decrease it. At the same time, many immune modulating genes can live on an "x" chromosome, which women have two of, while men have one.

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

But not getting side effects isn't a negative, health experts say. It simply means your body didn't react with as much of an inflammatory response, medical experts say.