For the first time since the pandemic vaccines were developed, the original Wuhan strain will not be included, underscoring how much of a viral changeling SARS-CoV-2 has become. This original safety net, which is credited with inducing some of the original deep-set immunity in T and B cells that curbed severe infections and deaths, has long been overtaken by the rapidly evolving variants that followed it.
Including the original strain that arose out of China in 2019 would only dilute the potency of the new monovalent XBB.1.5. Furthermore, immunizing again with the Wuhan strain possibly could undermine vaccine efficacy by the phenomenon of imprinting, wherein the immune system gets distracted by recognizing and responding to an old viral enemy.
According to an FDA background document for the June 15, 2023, meeting, other subvariants of concern include XBB.1.9 and XBB.1.16, which have co-circulated with XBB.1.5. However, the FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) decided to go all in on XBB.1.5, in part because it has demonstrated cross-reactivity against the other XBB subvariants of omicron.
“I certainly think that we should update this vaccine,” said VRBPAC member Paul Offit, MD, a vaccine expert at Children’s Hospital in Philadelphia. “This virus drifts, no doubt about it. It drifts away from protection against mild disease, but it does not drift away from protection against severe disease in people who are otherwise healthy and, say, [younger] than 75 [years of age]. I think we need to define — and the CDC [Centers for Disease Control and Prevention] can help us with this — who really benefits from booster dosing. Because it’s not everybody.”
Offit voted against the bivalent vaccine booster last year, saying it only should be indicated for the elderly and those with underlying conditions.
It was a high-profile defection, and with most people showing signs of vaccine-induced or hybrid immunity through natural disease, the bivalent vaccine largely was ignored by the millions of people the CDC said were indicated for use.
Still, the FDA background document for the meeting notes the continued evolution of SARS-CoV-2 creates the possibility for “increased transmissibility and adaptation to the host.”1
As has been suspected in the emergence of omicron, immune-escape variants may be selected out in prolonged infections in immunocompromised hosts.
“Thus far, the impressive plasticity, especially in spike, suggests that the virus can continue evolving by both incremental (drift-like) and saltatory (shift-like) modes, underscoring the importance of ongoing global surveillance,” the FDA stated.
In that regard, the World Health Organization seems to be lining up with the XBB subvariants, creating the possibility of a true global vaccine. Although it needs to be cleared by the CDC’s Advisory Committee on Immunization Practices (ACIP), the new COVID-19 vaccine is expected to be available sometime in September 2023.
ACIP will discuss whether this vaccine should be given to everybody or a subset of people, the committee reasoned.
Seasonal or Not?
Although the virus has been subdued, some committee members questioned whether SARS-CoV-2 will circulate as a seasonal virus like influenza, although that is the pattern other human coronaviruses generally have fallen into.
In any case, “what we end up with after today will not be final — it’s not like MMR (measles, mumps, and rubella) vaccine,” said Peter Marks, MD, PhD, director of the FDA Center for Biologics Evaluation and Research. “This is not going to be the final formulation for this vaccine forevermore. It will probably require another update at some point.”
VRBPAC chair Arnold Monto, MD, of the University of Michigan, concurred, saying, “As somebody who has been looking at the common cold coronaviruses, they are sharply seasonal. I think it’s premature to say that this virus will not become seasonal once there is antibody in the population. I agree we’re not there yet, but we may be. And to say that it’s not going to follow a familiar pattern may be premature as well.”
The committee vote was a unanimous (21-0) “yes” to the following question: “For the 2023-2024 formula of COVID-19 vaccines in the United States, does the committee recommend an update of the current vaccine composition to a monovalent XBB lineage?”
In subsequent discussion the committee recommended focusing on XBB.1.5, rather than XBB.1.16, or XBB.2. As of June 10, 2023, XBB.1.5 comprised 40% of the circulating SARS CoV-2 in the U.S. Vaccines developed against it also protect against the other XBB subvariants, and the real-world concession is that manufacturers are further along with efficacy and safety testing with XBB.1.5 than other subvariants.
“We may well, in the next month, find XBB.1.16 or XBB.2 become more dominant than XBB.1.5,” said committee member Stanley Perlman, MD, of the University of Iowa. “But that doesn’t matter because we have good data that there’s good cross-reactivity within the S protein and other parts of the virus. So, this should work just fine.”
An important logistical issue is that at some point the FDA and the CDC will have to publicly announce cessation of the current bivalent vaccine — possibly a month or two before the new vaccine is ready — to make sure the new XBB.1.5 booster can achieve full immune effect.
Research is ongoing to create a next-generation vaccine that would be effective against all strains of SARS-CoV-2, but Marx said that probably is at least two years away.
“We need a [new] generation of vaccines that would offer a better breadth, duration, depth of protection,” he said. “But I think we’re here right now with what we have.”
REFERENCE
- Food and Drug Administration. FDA Briefing Document: Vaccines and Related Biological Products Advisory Committee Meeting: Selection of strain(s) to be included in the periodic updated COVID-19 vaccines for the 2023-2024 vaccination campaign. Published June 15, 2023. https://www.fda.gov/media/169378/download
COVID-19 advisors to the FDA voted unanimously to approve a new monovalent vaccine for the coming fall containing the currently predominant omicron subvariant XBB.1.5.
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