What to know about the new bivalent Covid-19 boosters

Jacqueline Raposo
Jacqueline Raposo
Sep 13
A white senior woman with short gray hair wearing a medical mask and red t-shirt is given an injection on her upper arm by a medical technician with curly hair wearing a dark long-sleeved shirt and scrubs.

On August 31st, the FDA approved the emergency use of updated Pfizer-BioNTech and Moderna Covid-19 boosters that target Omicron BA.4 and BA.5—the most contagious variants we’ve come up against yet.

The U.S. government has over 170 million doses of the boosters, which are guaranteed to be available for free through the COVID-19 Public Health Emergency. Some are concerned that it's currently slated to end on October 13th. But public health officials expect the Biden administration to extend by another 90 days, and an HSS spokesperson has promised 60-days notice. 

“At each stage of the pandemic, people have faced different challenges to safely and affordably managing their health,” says our founder Cyrus Massoumi. “In the earliest phases of the vaccine rollout, Dr. B worked to match vaccine providers with patients who were at highest risk. Now, at this stage—as federal funding begins to scale back—we offer Visitless Prescription services so that those with an active Covid-19 infection can get antiviral treatments as efficiently and affordably as possible.”

How excited should we be about what might be the last vaccine we’re guaranteed to get for free? It’s complicated. So let’s break it down: What exactly is in the new recipe? When should we get boosted? And what other considerations are key to know?

What is new about this booster?

The original vaccines from Pfizer-BioNTech and Moderna authorized in December 2020 are monovalent—they target the messenger RNA (mRNA) coding for the spike protein of SARS-CoV-2 that emerged from Wuhan, China. 

The updated boosters are bivalent. They contain half of the original recipe, believed to encourage a broad immune response against Covid-19. The other half contains the new formula, which targets the mRNA components of Omicron BA.4 and BA.5. These currently make up about 90% of infections in the U.S. So even a modest surge in protection could reduce transmission rates—excellent news as we head into the colder months.

Why is the first vaccine especially relevant?

The first vaccines were rigorously tested. Through the Delta and Omicron waves, they continued to reduce hospitalization and death rates by 80-90% (for those fully vaccinated). We have massive amounts of data on their safety, side effects and efficacy through the millions of people who’ve gotten up to four doses. And a few months after being approved for emergency use, both were given full FDA approval—further proof of their safety and efficacy.

The production of the new bivalent boosters is the same, and tweaks to the mRNA targeting are small. So we can assume a good deal about their safety from this prior data. (This is also how flu vaccines are updated.)

Were they really not tested on humans?

The new bivalent boosters have not been tested on humans. Clinical trials begin in October, so we’ll know much more in the coming months.

But in mice, they encouraged an increased immune response to Omicron variants BA.1, BA.2, BA.2.12.1, BA.4 and BA.5. This is significant because scientists genetically modified mice to express the human version of ACE2—the protein SARS-CoV-2 binds to. Mice also have a “well characterized'' immune system. When infected with SARS-CoV-2, they mirror symptoms experienced by humans. Their vaccine response paints a picture of how human bodies may also respond.

Further, bivalent Pfizer-BioNTech and Moderna boosters targeting Omicron BA.1 have undergone small clinical trials in humans. In those studies, BA.1 bivalent boosters encouraged a better immune response in participants than the original monovalent booster. They also proved as safe as the original vaccines.

What about side effects?

Study participants who received the BA.1 bivalent boosters reported side effects similar to the monovalent boosters—general pain, redness or swelling at the injection site, fatigue, headaches, etc. Again, they’re the same kind of vaccine, only with tweaks made to their recipe.

Concerned about myocarditis? It’s been such a rare side effect amongst millions of vaccinated people that even robust clinical trials would be too small to identify it.

Still, should we be concerned? 

Most public health officials aren’t worried about the safety of the new boosters. But they question how much they’ll change things for the better.

We don’t yet know their ability to reduce infection or transmission rates. Omicron incubates so quickly that the immune system has little time to prepare a response, which is one of the reasons those rates remain high. But even a modest improvement would make a difference for individuals and communities—high infection rates mean regular disruptions to schools, the workforce and pretty much all social spaces. We’ll have to wait and see how this pans out.

The lack of clinical trials may also contribute to vaccine skepticism. Only 52% of Americans 18-65 years old have gotten their first booster—and ten states don't have 60% of residents fully vaccinated. If not socially accepted, the new boosters may not make enough of a population dent to curb the expected winter surge.

Who is eligible for the bivalent booster?

As it’s a half-dose (booster) vaccine, only those who have received 2 full doses of the Moderna or Pfizer-BioNTech monovalent vaccine are eligible for the new booster. Moderna is approved for those over 18 and Pfizer-BioNTech for those over 12. It must be at least two months since you've completed your primary vaccination or gotten a booster.

Who should get boosted ASAP?

According to CDC director Rochelle Walenksy, the CDC’s message is clear: “You need to get your fall booster vaccine.” Health professionals agree this is especially important for those over 50 years old and those at high risk for severe illness which includes those with diabetes, obesity, asthma or heart conditions and those who are immunocompromised.

Otherwise, opinions differ. With the goal of using vaccines to prevent serious illness, some believe younger people with healthy immune systems can wait a few months longer between boosters, especially if they have hybrid immunity from vaccination and infection. (If you’ve recently had Covid-19, they suggest you wait at least three months.) 

Others argue that boosters protect you and those around you. The U.S. has had far higher Covid mortality rates during Omicron than comparable high-income countries. Preventing even mild cases can lower transmission to high-risk people, keeping them out of the hospital. We can do better by those most at risk.

Other key points to know:

Again, the Covid-19 Health Emergency will likely be ending in the coming months, with many speculating the transition sometime in January 2023. This doesn’t mean the cost of getting vaccinated or boosted will fall entirely onto individuals’ shoulders—under the Affordable Care Act, most insurance companies must cover recommended vaccinations and boosters. But how this will work for the 87,000 providers currently offering free vaccinations is unclear. And while there’s hope that we’ll soon move to a once-yearly Covid-19 vaccine, we’re not there yet.

To be safe, get fully vaccinated and boosted before this transition begins. And let our Visitless Prescription and No-Cost Care programs help you navigate a current infection.

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