Why We’re Returning to Monovalent COVID-19 Vaccines This Fall

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Just when you thought you nailed down the latest type of vaccine — bivalent vaccines — the script is shifting in the effort to prevent COVID-19 infections. 

A panel of advisors for the U.S. Food and Drug Administration (FDA) recently voted unanimously to update the COVID-19 vaccine to target Omicron emerging subvariants. In particular, the new vaccine will target the subvariant XBB.1.5, which currently makes up about 40 percent of new infections in the United States.  

This new vaccine would be a monovalent vaccine (targeting one strain), rather than the currently available bivalent vaccine, which targets both the original strain of the virus and the Omicron subvariants that took over last winter.  

The reasoning: There doesn’t appear to be a particular advantage to the bivalent vaccines now that the original strain is no longer circulating.  

“It makes sense to re-formulate the vaccine to no longer include the ancestral strain,” says Amesh A. Adalja, MD, a Senior Scholar at Johns Hopkins Center for Health Security. “Some of us advocated that they do this with the prior Omicron bivalent booster as well. It’s unclear that there’s any benefit at all by including a strain that is basically extinct.” 

What’s more, including that ancestral strain may negatively affect how the vaccine works in your body. “There is also some evidence that by including the ancestral strain, antibodies against the newer strains may not form as the immune system primarily reacts to the older strain that it has seen before,” says Dr. Adalja.  

FDA scientists said vaccine manufacturer data show that an updated monovalent formulation targeting XBB subvariants elicits stronger neutralizing antibody responses against those strains than current bivalent vaccines, reports NPR.  

In the alphabet soup of COVID-19 variants, XBB.1.5 is currently considered the most transmissible strain so far, per Yale Medicine. That means it’s more efficient and contagious than predecessors, which tends to be the case as variants mutate.  

XBB.1.5 was first identified in New York in October 2022 and is nicknamed “Kraken,” after a mythical, giant sea monster. There’s currently no evidence that it causes more severe disease than other Omicron strains.  

While making this shift ahead of the fall season was a unanimous decision by the committee members, there was debate over whether the COVID-19 vaccine should be treated like the flu vaccine (which is reformulated each year in anticipation of the flu season). 

Although many people already understand the flu shot’s schedule, experts like Dr. Adalja point out the need for more data before a parallel is drawn between the two.  

“I think it is premature to determine what the optimal schedule of boosting or updating may be,” says Dr. Adalja. “While people like to draw comparisons with influenza, coronaviruses are a completely distinct, viral family with different characteristics. It is also unclear if the majority of individuals will benefit from annual shots against COVID-19 as protection against severe disease is intact.” 

Currently, the U.S. Centers for Disease Control and Prevention states that everyone ages 6 and older should get one updated Pfizer or Moderna COVID-19 vaccine to be up to date (referring to the vaccines currently available). People aged 65 or older may get a second dose, while those who are moderately or severely immunocompromised may get additional doses.  

In anticipation of fall demand, Pfizer, Moderna, and Novavax all began developing vaccines targeting XBB.1.5 months before this decision, reports CNBC. Preliminary data shows that their shots produce strong immune responses against all XBB variants.  

As for next steps, the FDA will consider the committee’s decision and likely issue an official recommendation shortly, reports NPR. The new vaccines are on track to become available by late September or early October.  

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