The Johnson & Johnson option differs from Pfizer and Moderna in a few key ways. Watch the video above for Q&A with ABC7 News correspondent Dr. Alok Patel, or keep reading below for everything you need to know about the rollout.
How is the Johnson & Johnson vaccine different from Pfizer and Moderna's vaccines?
Perhaps the most significant difference is that the Johnson & Johnson vaccine is a single dose, while the other two approved so far need two doses to be fully effective.
Another positive is that it doesn't need ultra-cold storage; the J&J vaccine can stay good in a refrigerator for a few months. That makes it easier to distribute, especially to more remote areas.
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"This can actually stay in the refrigerator so it would actually be much like your annual flu shot which you can just get at your local pharmacy," says Dr. Philip Grant of Stanford Health Care.
It's a bit in the weeds, but the J&J vaccine also works a bit differently than Pfizer and Moderna. It's based on the technology the company developed to create its Ebola vaccine; it basically carries the spike gene into the body, prompting it to make harmless copies of the protein to prime the immune system in case the real virus comes along.
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There also appears to be a difference in terms of efficacy (more on that below).
When will the Johnson & Johnson vaccine be available?
Johnson & Johnson plans announced Thursday that it had submitted its application to the FDA for emergency use authorization. The company says it's ready to ship doses right away as soon as it's approved. The company expects to supply 100 million doses to the U.S. by June - and a billion doses globally by year's end.
Remember, because it's a one-shot vaccine, 100 million doses means vaccinating 100 million people.
How effective is the Johnson & Johnson vaccine?
The efficacy of the vaccine varies pretty substantially based on the country where the trial was conducted. Overall, it's about 66% effective at preventing moderate to severe symptoms. But doctors are highlighting that it's better -- about 85% effective -- at preventing severe COVID-19 symptoms. That means widespread use of the vaccine would still make a big impact when it comes to overloaded hospitals and death rates.
The vaccine worked better in the U.S. - 72% effective against moderate to severe COVID-19 - compared with 66% in Latin America and 57% in South Africa, where a more contagious mutant virus is spreading.
To compare, the Pfizer and Moderna shots were 95% protective in large U.S. studies. However, some experts say it's not fair to compare those efficacy rates side-by-side, because the Pfizer and Moderna trials weren't conducted during this winter surge and during a time when there are multiple variants around the world.
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Is the Johnson & Johnson vaccine effective against the new coronavirus variants?
The variation in efficacy country-to-country seems to indicate that it may not be as effective against new variants, such as the one found in South Africa.
"It's not only a concern, it's even more proof that vaccine manufacturers will have to pay attention to these variants in the future if we need boosters or tweaks on the vaccines," says Dr. Patel.
Hear more from Dr. Patel on the Johnson & Johnson vaccine in the video at the top of this story.
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The Associated Press contributed to this report.