How does a variant-specific Covid booster work?

The Covid-19 messenger RNA vaccines are getting an update. Launching this week in the US and Europe, the new booster shots are specifically tailored for the Omicron variants currently in circulation. Health officials believe these will offer better protection against newer versions of SARS-CoV-2 than previous shots that were intended to target the virus, first discovered in late 2019.

Since its emergence, the coronavirus has constantly changed. These mutations have allowed it to spread more easily and better evade the immune system response induced by the original vaccines and boosters. Although Omicron and its subvariants are the most transmissible to date, our vaccines have remained the same.

“Basically, we’re trying to catch up with a virus that’s constantly evolving,” said Ross McKinney, chief scientific officer at the Association of American Medical Colleges. “And while we cannot predict the future, there is hope that the next variant will be an offshoot of BA.4 or BA.5. So it will be useful to have antibodies to protect you from that.”

Both BA.4 and BA.5 are subvariants of Omicron. As of Sept. 3, BA.5 accounted for an estimated 88.6 percent of all Covid-19 cases in the United States, while BA.4 accounted for 2.8 percent, according to the Centers for Disease Control and Prevention. Meanwhile, a new descendant of BA.4, known as BA.4.6, now accounts for about 8.4 percent of cases.

The new US formulations, manufactured separately by Moderna and Pfizer, target the BA.4 and BA.5 virus progenitor strains. Known as bivalent vaccines, they contain two pieces of messenger RNA that instruct cells in the body to make the distinctive “spike” protein of the original strain of the virus and these two subvariants. BA.4 and BA.5’s spike proteins are identical, but dozens of mutations in this protein have made it easier for them to slip past disease-fighting antibodies generated by previous vaccines or infections, allowing them to enter human cells .

“Over time, the virus has evolved to look less and less like the virus that originated in the human population,” said 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 will prime the human immune system to recognize those variants, but not the new ones.”

On August 31, the Food and Drug Administration granted emergency use approval for Moderna’s and Pfizer-BioNTech’s variant-specific boosters. Shortly thereafter, the CDC approved the recordings for US citizens. Meanwhile, the European Medicines Agency and the UK Health Authority have approved a bivalent version targeting the original virus and BA.1, the omicron variant that became dominant last winter.

Currently, anyone 12 years and older can receive a new bivalent booster dose if they have had a previous booster dose or their primary vaccine series. (That means two doses of the Moderna, Pfizer, AstraZeneca, or Novavax vaccines, or one dose of the Johnson & Johnson vaccine.) The CDC recommends getting the new booster shot at least two months after a previous vaccine dose. People who have recently contracted Covid-19 can delay their booster dose by three months after their symptoms start, according to the CDC.

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