(CNN) — States may be scrambling for vaccines now, but the United States could see additional Covid-19 vaccines by the end of March, if not sooner.

Millions of vaccine doses could be added to the pipeline in the coming months, perhaps easing the squeeze that Americans have felt as vaccines slowly start rolling out.

Janssen, the vaccine arm of Johnson & Johnson, plans to have results from its late-stage clinical trials ready for the US Food and Drug Administration to consider within weeks.

“Johnson & Johnson is right around the corner … right around the corner means that they’re probably a couple of weeks away from getting the data looked at, to have the FDA evaluate whether or not we’re in a situation where we could move ahead and start thinking about getting it out into the public,” Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN’s Chris Cuomo on Thursday.

Dozens of vaccines are being tested in labs around the world. At least 11 are in late stage clinical trials, according to Bio, a global trade association representing biotech companies.

In the US, there are three frontrunners.

Johnson & Johnson

Johnson & Johnson is the most likely to nest seek emergency use authorization from the FDA.

Many experts say J&J’s Covid-19 vaccine has several advantages. While J&J is running a separate trial to test its vaccine using two doses, as Moderna’s and Pfizer’s do, the current trial uses just one dose.

“This advantage goes up in neon,” said Dr. William Schaffner, an internist and infectious disease specialist with Vanderbilt University’s Department of Health Policy. “This would be the first one to be given in a single dose.”

A single does would mean more people could be vaccinated, as none would need to be set aside to give someone a second shot. That would be a huge advantage at a time when vaccines are hard to come by.

J&J’s other advantage is that it can be stored at regular refrigerator temperatures, unlike the Pfizer vaccine, which needs special deep freezers.

“If they’re successful, these vaccines would especially be popular in the developing world, because they would be easy to store and administer,” said Dr. Rafi Ahmed, the director of the Vaccine Center at Emory University.

Schaffner said adding a vaccine like this would “really accelerate” vaccination efforts in the US too.

“In other words, we could bring the vaccine to the people,” Schaffner said, “rather than bringing the people to the vaccine.”

Johnson & Johnson’s Covid-19 vaccine was made through a collaboration of J&J’s Belgium-based vaccine division, Janssen Pharmaceutical and Beth Israel Deaconess Medical Center. It is what is known as a non-replicating viral vector vaccine, using a common cold virus called adenovirus 26.

Scientists made this vaccine by taking a small amount of genetic material that codes for a piece of the novel coronavirus and integrated it with a weakened version of adenovirus 26. J&J scientists altered this adenovirus so it can enter cells, but it doesn’t replicate and make people sick. AstraZeneca uses a similar platform, but its adenovirus comes from a chimpanzee.

The adenovirus carries the genetic material from the coronavirus into human cells, tricking them into making pieces of the coronavirus spike protein – the part it uses to attach to cells. The immune system then reacts against these pieces of the coronavirus.

“So you’re not being infected with the virus that can give you Covid-19 when you get this vaccine. It just has some of the harmless Covid virus proteins on its surface,” explained Schaffner. “So essentially it’s a sheep in wolf’s clothing, and when your immune system sees it, it responds to it and creates protection against it and in the future, against the real virus that causes Covid-19.”

Dr. Paul Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said the Moderna, Pfizer and J&J Covid-19 vaccines all take a similar approach, but there is a small difference with the J&J approach.

“In the case of the Moderna and Pfizer vaccine you’re just giving the gene in a lipid nanoparticle or a fat droplet,” Offit said. “In the case of J&J you’re giving the gene in a virus that can’t reproduce itself.”

The technology used in the Covid-19 vaccine has worked with Janssen’s Ebola vaccine.

AstraZeneca

The AstraZeneca vaccine is already being used in a few countries. It was authorized in the UK in December, and in Brazil in January, but the FDA will likely want to use US data for any emergency use authorization in the US, Moncef Slaoui, chief adviser to the Trump administration’s Operation Warp Speed, said last Tuesday.

The AstraZeneca vaccine is also a vector vaccine. Like the Johnson & Johnson vaccine, it employs an adenovirus to carry the genetic instructions for making spike protein from the coronavirus into cells to generate antibodies. AstraZeneca’s vaccine, developed with Oxford University, uses a virus that infects chimpanzees but doesn’t make people sick.

As with the Pfizer and Moderna vaccines, this Covid-19 vaccine would require two shots.

The trials so far have had some confusing results, but show it could provide up to about 70% protection. The FDA paused US trials last year after questions about adverse reactions in volunteers, but it cleared up the questions and resumed.

The AstraZeneca vaccine could potentially go up for authorization around the end of March.

Novavax

The Novavax Phase 3 trial started in the US in December. The trial is enrolling up to 30,000 volunteers at 115 sites in the US and Mexico.

The Novavax candidate is a protein-based vaccine created out of the genetic sequence for the novel coronavirus.

The company uses virus-like nanoparticles as a base and scientists cover the base with the genetically engineered pieces of the coronavirus spike protein.

This vaccine is given in two doses, 21 days apart.

This kind of vaccine has been successfully used with other diseases like the shot for HPV and Sanofi’s flu vaccine.

Novavax said it expects to have results with its Covid-19 vaccine by early first quarter of this year. The first quarter ends March 31.

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