COVID-19 Vaccine Booster Doses May Soon Be a Reality. Here’s What Experts Know So Far.

Pfizer, Moderna, and Johnson & Johnson are trying to stay ahead of highly infectious coronavirus variants.

multicolor covid 19 vaccine
Andriy OnufriyenkoGetty Images

In an attempt to ensure the effectiveness of their COVID-19 vaccines against new variants of the novel coronavirus, both Pfizer and Moderna are testing a third booster shot of their respective two-dose vaccines.

On February 25, Pfizer announced that it is studying a third booster dose in some people who received their first dose of the vaccine more than six months ago. The company specifically stated that emerging and future variants of SARS-CoV-2, the coronavirus that causes COVID-19, were the reason for the study. Pfizer also said that it is exploring the possibility of a new, “variant-specific vaccine” that would target B.1.351 (the highly infectious South African variant).

Moderna also announced that it has finished making a variant-specific vaccine to target B.1.351, and the company has begun a Phase 1 clinical trial of the vaccine. Moderna says that it will explore the use of the new vaccine as a “booster dose” for people who are already fully vaccinated, to see if it can “boost immunity against the variants of concern.”

Pfizer’s CEO Albert Bourla, Ph.D., has said that people can anticipate the need for a booster. He made the comments in an interview with CNBC, explaining that it’s “likely” people who have received the full two-dose regimen of the Pfizer-BioNTech vaccine will need to have another dose “somewhere between six and 12 months and then from there, there will be an annual revaccination, but all of that needs to be confirmed.”

Moderna’s CEO Stephane Bancel confirmed to CNBC that the company will likely have a booster dose for its vaccine ready by the fall, so that additional protection is ready ahead of peak flu season.

Both companies implied that it will be months before concrete data is available, however. In the meantime, you probably have questions about these COVID-19 booster shots—so we asked experts what they know so far.

Remind me: How do the mRNA COVID-19 vaccines work?

Both the Pfizer and Moderna vaccines use messenger RNA (mRNA) technology. The vaccines, which do not contain live virus, encode a part of the spike protein—the piece of the virus that latches onto human cells—found on the surface of SARS-CoV-2, according to the Centers for Disease Control and Prevention (CDC).

This gives your cells instructions to develop a piece of that protein that is unique to SARS-CoV-2. Your immune system recognizes these new pieces of protein as foreign invaders and mounts an immune response to fight off what it interprets as an infection (even though there is no threat). This causes you to develop antibodies specific to SARs-CoV-2, which will help you fight off future infections.

Your body eventually eliminates both the mRNA and the proteins, but the antibodies stick around. Just how long they last, though, is still being studied—the CDC specifically says that more data from both vaccines is needed, but current studies suggest at least six months for the mRNA vaccines.

How effective are the Pfizer and Moderna vaccines?

Both vaccines were found to be highly effective during Phase 3 clinical trials. Research from Pfizer’s Phase 3 clinical trial showed that its vaccine is 52% effective after the first dose, and about 95% effective after the second dose in adults ages 16 and up. Results of Moderna’s Phase 3 clinical trial, which were published in The New England Journal of Medicine, found that the company’s vaccine is about 94.1% effective against COVID-19 in people ages 18 and older

But—and this is a big but—the trials were conducted before variants like B.1.1.7, which was first detected in the U.K., and B.1.351 began spreading rapidly, says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. In other words, there is a possibility that the effectiveness of the vaccines today may be lower than what the data from months ago indicates.

With the new research being conducted, both Pfizer and Moderna are “trying to preemptively address whether the variants could impact the immunity generated by their vaccines,” says Reynold Panettieri, M.D., director of the Institute for Translational Medicine and Science at Rutgers University.

What about the Johnson & Johnson vaccine?

The Johnson & Johnson vaccine works differently than its mRNA competitors. It modifies an existing cold virus with the spike protein. The resulting adenovirus doesn’t have the ability to reproduce in your body, so it can’t cause COVID-19 or other illnesses. The modified virus is pulled inside your cells after the single dose, where it travels to the 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 copied into mRNA. This prompts an immune response, causing your body to produce antibodies to the perceived threat.

The vaccine is 85% effective at preventing a severe or critical form of COVID-19 that can lead to hospitalization or death at least 28 days after vaccination, according to data released by the company, which has been confirmed in a Food and Drug Administration (FDA) analysis.

In a February interview with CNBC, Johnson & Johnson’s CEO Alex Gorsky said that people may need to get the single-dose vaccine each year for several years, similar to the annual flu shot. “Unfortunately, as [the virus] spreads, it can also mutate,” he said. “Every time it mutates, it’s almost like another click of the dial, so to speak, where we can see another variant, another mutation that can have an impact on its ability to fend off antibodies.”

That said, the future of the Johnson & Johnson vaccine is currently unclear after the FDA and CDC temporarily paused its use in the U.S. while investigating reports of extremely rare blood clots believed to be linked to the vaccine. All six reported cases out of 6.8 million doses administered, as of April 12, occurred in women between the ages of 18 and 48, with symptoms emerging six to 13 days after vaccination. The agencies stress that the clots “appear to be extremely rare” and the step is out of an “abundance of caution” until a thorough investigation is completed.

So, will a vaccine booster dose be needed for full COVID-19 protection?

It’s hard to say at this point, given that not enough time has passed to collect data on this, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. (Remember: The first COVID-19 vaccines were distributed in the U.S. in December 2020.)

Pfizer, Moderna, and Johnson & Johnson have all suggested that they are concerned about the South African variant’s potential impact on the effectiveness of their vaccines, as well as the possibility that future variants may make their vaccines less effective.

“It’s still early days in the science,” Dr. Schaffner says. “It may well be that we could use the standard vaccine as a booster to protect against variants—if we need it. We still don’t know how long the standard two-dose vaccine will protect us.”

There are different strategies to deal with variants, adds Dr. Adalja. “One is to reformulate the vaccine, and the other is to add another booster with the same formulation,” he explains. Creating a booster could increase antibodies and T cells (a type of white blood cell that’s an essential part of your immune system) enough to help tackle variants of the original, dominant SARS-CoV-2 strain.

It’s also possible that a booster shot may make an already effective vaccine even more effective. “They may be trying to see if they can get the efficacy up closer to 100%,” Dr. Panettieri says.

Until medical experts know more, Dr. Adalja emphasizes that receiving two doses of the Pfizer or Moderna vaccine will still offer worthy protection. “The priority still should be getting people vaccinated with the original vaccine, which does have an impact on all of the variants when it comes to what matters—serious illness, hospitalization, and death,” he says.

Plus, people across the country are still waiting to get their first dose of the vaccine. “[A third dose] would be logistically difficult in the short term,” Dr. Adalja says. But, he adds, distribution should get easier with time as more vaccination systems and supplies become available.

This article is accurate as of press time. However, as the COVID-19 pandemic rapidly evolves and the scientific community’s understanding of the novel coronavirus develops, some of the information may have changed since it was last updated. While we aim to keep all of our stories up to date, please visit online resources provided by the CDC, WHO, and your local public health department to stay informed on the latest news. Always talk to your doctor for professional medical advice.

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