With the approval of Johnson & Johnson’s COVID-19 vaccine, three different coronavirus vaccinations are available in the U.S. While the Pfizer and Moderna shots have a lot in common, the newer Johnson & Johnson vaccine has some notable differences.

In a Twitter poll, abc27 digital reporter Avery Van Etten asked which of the available vaccines people would prefer to get if given the chance to choose:

Joseph Kontra, chief of infectious diseases at Penn Medicine Lancaster General Health, says, “All of the vaccines that are available are safe, and they’re very effective.”

On top of the three vaccines already deployed in the U.S., the country is testing a fourth vaccine that may enter the mix in the next few months. The AstraZeneca vaccine, already used in dozens of other countries around the world, could soon be available in the United States, as well.

The various shots are all doing the same job of mitigating the harms of COVID-19, but the way they do that job differs between vaccines.

How they work

The Pfizer and Moderna vaccines use mRNA technology to build immunity to the novel coronavirus, as described in this abc27 article. In contrast, the Johnson & Johnson shot is a type of viral vector vaccine, explains Kontra.

“It’s kind of a Trojan horse mechanism,” says Kontra. Johnson & Johnson’s vaccine involves the insertion of a piece of COVID-19 genetic material into the shell of an adenovirus, or a cold virus. Kontra notes that this adenovirus is just a shell, so it cannot replicate or cause an infection.

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The immune system takes up this deceptive adenovirus, and then the genetic material inside it starts producing the spike protein that covers the surface of the COVID-19 virus. This prepares the immune system to recognize the COVID-19 spike protein and ward off the coronavirus if an individual becomes infected with the real disease, explains Kontra.

Unlike the two-shot Pfizer and Moderna vaccines, only one dose of the Johnson & Johnson vaccine is necessary for it to effectively combat the coronavirus. Researchers are currently studying whether a second dose could make it more effective in the future, Kontra says.

The AstraZeneca vaccine also uses a viral vector mechanism, similarly to the Johnson & Johnson immunization, explains Kontra. However, it uses a different adenovirus to transport the COVID-19 genetic material. 

Like the Moderna vaccine, the AstraZeneca shot is administered in two doses that are 28 days apart.


The Pfizer and Moderna coronavirus vaccines are about 95% effective at preventing COVID-19 after both doses have been received. The vaccine made by Johnson & Johnson is 66% effective overall at preventing moderate and severe disease, but it’s 85% efficacious at preventing severe disease, says Kontra.

And, Kontra says, the initial trial of the Johnson & Johnson vaccine showed that it was 100% effective at preventing hospitalization and death from the coronavirus four weeks after the shot had been administered.

Although the Johnson & Johnson vaccine’s 66% or 85% efficacy may sound worse than that of the Pfizer and Moderna inoculations, Kontra says the comparison isn’t really apples-to-apples.

One reason for this is that the vaccines “were studied at very different times during the pandemic,” says Kontra. The COVID-19 variants circulating today weren’t around when the Moderna and Pfizer vaccines were being tested.

Another reason is that the parameters of the vaccines’ studies were not identical, notes Kontra.

The AstraZeneca vaccine has an efficacy similar to Johnson & Johnson’s, which Kontra says is to be expected as they work using similar mechanisms. U.S. trial results found that the vaccine is 76% effective against symptomatic COVID-19 and 100% effective against “severe or critical disease and hospitalization,” according to AstraZeneca’s website.

Unlike the Moderna vaccine, which was only tested in the United States, the AstraZeneca vaccine was tested internationally, including in Europe, North America and South America.

Kontra says that all of the vaccines appear to do what they’re intended to, which is prevent severe complications, hospitalization and death due to the coronavirus. “I encourage people not to quibble about the efficacy…because the bottom line is all the vaccines are effective and will protect you against COVID-19,” says Kontra.

Dealing with variants

The UK variant of COVID-19 is now present in almost every state in the U.S. and is expected to become the dominant strain of the virus in upcoming months, Kontra says. The Pfizer, Moderna, Johnson & Johnson and AstraZeneca vaccines are effective against this variant, he notes.

Although there is currently no variant that is completely resistant to the vaccines, they are less effective against the Brazilian and South African strains, which are currently less common in the U.S., Kontra adds. 

“Of the vaccines that are out there, the AstraZeneca vaccine is probably the weakest against the South African variant,” says Kontra. He explains that it is only 10% effective at preventing symptomatic infection in South Africa, but adds that the company is working on a modification of the vaccine that will have improved effectiveness against the variant.

Variants of the coronavirus are constantly adapting to their environments and evolving to become better at infecting people and spreading. When they have more opportunity to replicate, the likelihood of them developing resistance to the vaccine increases. “That’s part of the importance of getting people vaccinated as quickly as possible,” says Kontra.

Side effects

The side effects of the Johnson & Johnson shot are similar to those felt after the first dose of the Pfizer and Moderna vaccines, says Kontra. They could include soreness around the injection site, fatigue or body aches.

The more intense side effects associated with the second doses of the Pfizer and Moderna vaccines are not a concern with the single-dose Johnson & Johnson vaccine, as the most recently approved vaccine only requires one shot.

More than 6.8 million Americans have received the Johnson & Johnson shot as of mid-April, but U.S. distribution of the Johnson & Johnson vaccine was paused on April 13 after six women developed blood clots, one of which was fatal, within two weeks of receiving the vaccine. The CDC and the Food and Drug Administration are looking into a possible connection between the Johnson & Johnson vaccine and the rare blood clots.

Preliminary data shows similar side effects — muscle aches, fatigue, headaches, fever, swelling at the injection site — for the AstraZeneca vaccine. In Europe, officials have also expressed concern about a possible link between the AstraZeneca shots and blood clots.

The European Medicines Agency calls blood clots a “very rare” side effect of the vaccine. In Germany, one in 100,000 people inoculated suffered a blood clot. For comparison, experts note that this is less than the likelihood of developing blood clots from taking birth control pills. 

Fewer blood clots were reported in the U.K. than in Germany, and, Kontra says, “There was no evidence of any blood clot problems with any of the formal Phase III trials that AstraZeneca ran and has put out data about.”

The Associated Press quotes EMA executive director Emer Cooke as saying, “The reported cases of unusual blood clotting following vaccination with the AstraZeneca vaccine should be listed as possible side effects of the vaccine,” but also, “the risk of mortality from COVID is much greater than the risk of mortality from these side effects.”


The Johnson & Johnson vaccine may play an important role in vaccinating rural and underserved areas. It has already been used to quickly inoculate teachers in Pennsylvania.

One reason for this is that storing the Johnson & Johnson vaccine is easier than storing the Pfizer and Moderna shots, which helps simplify distribution, Kontra explains. While the finicky Pfizer vaccine needs to be kept at extremely cold temperatures, Johnson & Johnson’s vaccine is stable in a standard refrigerator.

“So the transport out to rural areas, for example, would be much simpler. And if you’re only giving one shot, you don’t have to worry about bringing everybody back; you can go out to underserved areas and really get the vaccine out to more people,” says Kontra.

While the AstraZeneca vaccine does require two doses to effectively combat the coronavirus, it is also much easier to store and distribute than the Pfizer and Moderna vaccines, Kontra explains. It’s stable at normal refrigeration temperature for months.

AstraZeneca’s “bumpy road”

From misreporting U.S. trial data to factory mix-ups to blood clot concerns, “AstraZeneca’s had a bumpy road,” Kontra says. Experts worry that the problems may have damaged the vaccine’s credibility and potentially undermined confidence in coronavirus vaccines in general.

However, Kontra notes, the AstraZeneca vaccine is already being used in dozens of other countries and is approved for emergency use by the World Health Organization. He adds that “the alleged problems with AstraZeneca…none of those have proven to be real in the data sets that have been presented.”

(Since abc27 spoke with Kontra, concerns about the vaccine’s link to blood clots have persisted, with European experts now saying they should be considered a “possible” but “very rare” side effect of the shot.)

Kontra says, “I have no doubt [the AstraZeneca vaccine] will be approved in the U.S. The question will be, how much do we really need it because the federal government has secured a sufficient supply of the other three vaccines to vaccinate everybody in the United States?”

At the beginning of April, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to President Biden, said that the U.S. may not need the AstraZeneca vaccine to meet the country’s needs, Kontra notes.

So if there is already enough vaccine in the county to inoculate all Americans, what would be the benefit of adding a fourth option? “It would get us to our finish line faster,” says Kontra.