Despite a U.S. factory mixup, more Johnson & Johnson doses are becoming available in the U.S., marking the only single-shot vaccine option.
So how effective is it and how does it compare to the Pfizer and Moderna vaccines?
According to medical experts, the three vaccines currently available in the U.S. each offer some level of protection.
Here's a breakdown:
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How effective is the Johnson & Johnson vaccine?
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."
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 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.
"If you kind of do a comparison for influenza vaccines, which we do annually, influenza's efficacy can range anywhere from 40% to 60% - and that's a good year," Monica Hendrickson, public health administrator for the Peoria County, Illinois, Health Department said last month. "So again, something that's high public health was, you know, we were hoping for something above 40%. To get to 95%, even 65%, that's a homerun."
Hendrickson 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.”
Dr. Arnold Monto of the University of Michigan, who chaired an FDA advisory panel that unanimously voted that the Johnson & Johnson vaccine’s benefits outweigh its risks, said the evidence shows no reason to favor one vaccine over another.
“What people I think are mostly interested in is, is it going to keep me from getting really sick?” Collins said. “Will it keep me from dying from this terrible disease? The good news is all of these say yes to that.”
There has been concerns surrounding emerging variants of the COVID-19 virus.
So far, studies suggest that the vaccines currently in use can recognize the emerging variants — but they don't provide as much protection against the new strains.
Pfizer and BioNTech said Thursday that trials suggest their vaccine is effective against a coronavirus variant that first emerged in South Africa, which some experts worry might evade existing shots.
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.
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.
"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.
Moderna is also testing a potential third dose of its current vaccine, and a possible booster shot specifically targeting the South Africa variant.
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."
How are they different?
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 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.
The Johnson & Johnson and another vaccine from AstraZeneca also rely on instructions for creating the spike protein that gives the coronavirus its distinctive shape and which it uses to enter a cell. But they make use of DNA inside an adenovirus, a common virus.
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 must also be kept frozen, while the J&J shot can last three months in the refrigerator, making it easier to handle. AstraZeneca's vaccine, which has not yet been approved in the U.S., is made similarly and also requires refrigeration but takes two doses.
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.
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.
The CDC reports the most common side effects for all three authorized vaccines is at the injection site. They include:
Common side effects in the body include:
- Muscle pain
"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 there other options?
Additional vaccines from AstraZeneca and Novavax could also be potentially approved in the U.S.
"In addition to the three that have been approved or have been authorized here in the U.S., we're keeping our eye on AstraZeneca," Chicago Department of Public Health Commissioner Dr. Allison Arwady said Thursday. "That is of active use, right? It's already in use in Europe and some other countries and it's been, you know, it's been having its studies here... we'll see. They weren't done yet, but there's been some speculation that, you know, possibly in April we could see that vaccine."
But AstraZeneca’s COVID-19 vaccine is currently at the center of some scrutiny in Europe as several countries suspended use over reports of dangerous blood clots in some recipients, though the company and European regulators have said there is no evidence the shot is to blame.
"I want to be really clear that, based on everything I've seen, there's not been a clear link at this point related to blood clots and AstraZeneca" Arwady said. "But there have been, you know, decisions made by some countries to sort of put that that particular vaccine on hold, or to not offer it at this time if they've got another vaccine available while they're looking into that some more."
Arwady noted there were also concerns it may not be as effective against the variant that emerged in South Africa.
Novavax, which is the lesser known of the remaining vaccines, Arwady said, is another one she's monitoring.
"Hundreds of companies have been sort of looking at vaccines, but I'd say Novavax is the other one here in the U.S. and I'm kind of keeping my eye on going further behind in the trials, but their initial data looked good," she said. "Could that be sort of a may for another vaccine? I think possibly."