The latest vaccines are designed to target the currently circulating Omicron variants—and head off a winter surge.
RNA vaccines are getting an update. The new booster shots rolling out across the US and Europe this week are specifically tailored to currently circulating Omicron variants. Health officials think these will provide better protection against newer versions of SARS-CoV-2 than earlier shots, which were designed to target the virus that was first detected in late 2019.
Since its emergence, the coronavirus has been constantly changing. These mutations have allowed it to spread more easily and to better evade the immune system response provoked by the original vaccines and boosters. Although Omicron and its subvariants are the most transmissible yet,“Basically, we’re trying to catch up with a virus that keeps evolving,” says Ross McKinney, chief scientific officer at the Association of American Medical Colleges.
Both BA.4 and BA.5 are subvariants of Omicron. As of September 3, BA.5 accounted for an estimated 88.6 percent of all Covid-19 cases in the US, while BA.4 comprised 2.8 percent,. Meanwhile, a new descendant of BA.4, known as BA.4.6, now represents about 8.4 percent of cases. Made separately by Moderna and Pfizer, the new US formulations target the ancestral strain of the virus, BA.4, and BA.5. Called bivalent vaccines, they contain two pieces of messenger RNA that instruct cells in the body to make the distinctive “spike” protein of the original virus strain and of those two subvariants. The spike proteins of BA.4 and BA.
“Over time, the virus has progressively evolved, so it looks less and less like the virus that started in the human population,” says Robert Schooley, professor of infectious diseases at the University of California San Diego School of Medicine. “If our vaccines continue to look like the older variants, we’ll be stimulating the human immune system to recognize those variants, but not the new ones.
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