While Pfizer and Moderna both have COVID-19 vaccines authorized for emergency use in the U.S., other vaccine candidates are still in the works, including a single-dose option from Johnson & Johnson, which has about 45,000 people enrolled in ongoing phase 3 clinical trials. According to early data just released by the company, this vaccine also shows major promise.

Interim phase 1/2a data were published on Jan. 13 in the New England Journal of Medicine, and the results show the company’s vaccine candidate created an immune response in patients for at least 71 days—the full length of time measured in the study so far.

The vaccine was also “generally well-tolerated” in study participants, Johnson & Johnson said in a press release. While the Pfizer and Moderna vaccines are similar, Johnson & Johnson’s vaccine also has plenty of differences. Here’s what we know so far, plus what lies ahead.

How does the Johnson & Johnson COVID-19 vaccine work?

Johnson & Johnson has an andenovector vaccine, which uses double-stranded DNA to promote an immune response in the body. This technology works differently than the mRNA vaccines available from Pfizer-BioNTech and Moderna, which both use single-stranded RNA.

In the Johnson & Johnson vaccine, researchers added a piece of genetic material from the novel coronavirus’ spike protein (the piece that latches onto human cells) into another virus, Adenovirus 26, which was modified so it has the ability to enter cells but not reproduce inside of them. Adenoviruses are common viruses that usually cause cold-like symptoms, but because the one used in the vaccine was altered and cannot replicate, it can’t make you sick. (Other COVID-19 vaccines, including Oxford and AstraZeneca’s candidate, uses similar adenovirus technology.)

When you get the Johnson & Johnson vaccine, the modified adenovirus carrying a piece of the spike protein latches onto the surface of your cells. It’s pulled inside, where the modified virus travels to the cell nucleus, home to its DNA. The adenovirus then puts its DNA into the nucleus, the spike protein gene is read by the cell, and it’s then copied into messenger RNA (mRNA).

After that, the mRNA leaves the nucleus and serves as a set of instructions for other cells, so they begin making spike proteins. Those are then recognized by your immune system, and your body reacts by producing antibodies to the perceived threat (even though no threat exists).

Your immune system cells then remember how to fight the distinct piece of SARS-CoV-2, the novel coronavirus, so if you come into contact with it in the future, your body will have the capability to fight it more efficiently.

This technology is unique but Johnson & Johnson has a lot of experience with it, as it’s already been used for its Ebola vaccine. “They’ve given hundreds of thousands of doses of this similar vaccine,” which has had no major safety issues, says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine.

While it’s still being tested, Johnson & Johnson’s COVID-19 vaccine may only require one shot rather than two. Its trials so far have found that giving both one or two doses of the vaccine spurred an effective immune responses against SARS-CoV-2 in study participants, but nothing is set in stone until phase 3 clinical trials are complete and the company has enough data to support its single dose.

How effective is the Johnson & Johnson COVID-19 vaccine?

It’s not entirely clear at this point. Published data from the early stage trials found that more than 90% of people who were vaccinated developed neutralizing antibodies (which are expected to stop SARS-CoV-2 from infecting your cells) 29 days after they received the first dose of the vaccine. Two months after the first dose, all participants had developed neutralizing antibodies, which stayed put for at least 71 days.

What are the side effects of the Johnson & Johnson COVID-19 vaccine?

According to the data so far, it may cause “mild-to-moderate side effects typically associated with vaccinations,” similar to those expected from the Pfizer and Moderna vaccines. This includes cold-like symptoms, like a headache, body aches, pain at the injection site, and a fever—a normal sign that the body’s immune response is being primed.

How is the Johnson & Johnson COVID-19 vaccine stored?

    One of the biggest perks of the Johnson & Johnson vaccine is its durability. Because it doesn’t harbor delicate mRNA like the Pfizer and Moderna vaccines (which need to stay frozen), it’s much less fragile and can stay stable in a normal refrigerator (36–46°F) for up to three months.

    “That’s a big advantage,” says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. Safely storing the other available vaccines, particularly the Pfizer vaccine (which needs to be kept at a frigid -94°F), presents challenges for the average doctor’s office or pharmacy, as most locations don’t have specialty freezers that reach those temperatures.

    When will it be granted an emergency use authorization by the FDA?

    “It’s too soon to say because we don’t have phase 3 clinical data yet,” says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.

    However, he’s hopeful, because “the phase 2 clinical trial results look strong.” Dr. Russo agrees that “as of right now, there are no major concerns with the safety signals.”

    Johnson & Johnson’s phase 3 clinical trial is expected to wrap up by mid-February. If everything checks out, the company can apply for emergency use authorization from the Food and Drug Administration (FDA), Dr. Russo says. Once the FDA grants its approval, it’s possible that the vaccine could be authorized sometime in March.

    In August, the company signed a $1 billion contract with the federal government, pledging to produce 12 million doses of its vaccine by February and 100 million doses by the end of June. However, The New York Times reports production may be about two months behind schedule.

    Will you get to choose which COVID-19 vaccine you get?

    At this point, that doesn’t seem likely. “In this initial phase of vaccinations, there’s probably not going to be much of a choice for people,” Dr. Adalja says. Rather, the health department or agency administering the vaccine will make the decision, mostly based on which vaccine is readily available in a specific area.

    But a lot of this really comes down to what the data will say. “Exactly how effective is this vaccine?” Dr. Schaffner says. “If there’s a noteworthy difference, that might change things.”


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