As recently as last week, many public health experts were fiercely opposed to the Biden administration’s campaign to roll out booster shots of the coronavirus vaccines to all American adults. There was little scientific evidence to support extra doses for most people, the researchers said.
The Omicron variant has changed all that.
Scientists do not yet know with any certainty whether the virus is easier to spread or less vulnerable to the body’s immune response. But with dozens of new mutations, the variant seems likely to evade the protection from vaccines to some significant degree.
Booster shots clearly raise antibody levels, strengthening the body’s defenses against infection, and may help offset whatever advantages Omicron has gained through evolution.
Many of the experts who were opposed to boosters now believe that the shots may offer the best defense against the new variant. The extra doses may slow the spread, at least, buying time for vaccine makers to develop an Omicron-specific formulation, if needed.
“Based on what we know about the potential for immune evasion, I would err on the side of giving the booster,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center who had opposed the Biden administration’s boosters-for-all push.
The administration isn’t waiting for scientific consensus. Alarmed by the preliminary reports about Omicron, officials at the Centers for Disease Control and Prevention said on Monday that all American adults should receive booster shots.
The first confirmed Omicron infection in the United States was reported on Wednesday in San Francisco, in a traveler who returned to California from South Africa on Nov. 22. The individual had been fully vaccinated — but had not received a booster — and showed mild symptoms that were said to be improving.
The Omicron variant, first identified in southern Africa, has been discovered in at least 20 countries, and the World Health Organization has warned that the risk posed by the virus is “very high.” After news of the variant’s spread in South Africa, countries around the world have curtailed air travel to and from southern Africa.
Omicron carries more than 50 genetic mutations, more than 30 of them on the virus’s spike, a protein on its surface. Vaccines train the body’s immune defenses to target and attack these spikes.
Until now, experts like Dr. Gounder had argued that while the vaccines’ potency against infection with the Delta variant seemed to be waning, they still shielded most people from severe illness, hospitalization and death. Booster doses should be recommended only for adults older than 65 and those who are in long-term care facilities or have weak immune systems, they said.
If Delta were the only threat, boosters would still not be warranted, Dr. Gounder and other researchers said. But Omicron may be a more formidable foe.
“If it is highly resistant to antibodies, which seems likely but unproven, then additional doses are appropriate,” said John Moore, a virologist at Weill Cornell Medicine in New York.
“I’d like to see more data, but it won’t do anybody any harm to have additional protection.”
Even before Omicron’s arrival, some experts were coming around to boosters for all adults, as cases in the United States inched upward again in recent weeks.
“It’s impacting things like elective procedures at multiple hospitals in Massachusetts and elsewhere,” said Dr. Camille Kotton, an infectious disease physician at Massachusetts General Hospital and an adviser to the C.D.C. “We really need to put an end to this.”
“Now, more than ever, is a great time to get vaccinated for people who are not yet vaccinated, or to go get boosters,” she said.
Dr. Kotton’s initial hesitation was partly rooted in a dearth of research regarding the safety of booster shots in young adults. Given certain rare heart problems in young men after receiving the second dose of an mRNA vaccine, it was not clear that the benefits outweighed the risks.
But the data now available has eased her concerns, she said — so much so that she has urged her college-age sons to receive booster doses.
“Oh yeah, I changed,” she said. “Thinking about risks and benefits, it is a really good idea to get booster doses for people who qualify.”
Greater support for boosters among scientists may eventually complicate efforts to deliver limited supplies of the coronavirus vaccines to poor countries. The World Health Organization has said for months, long before Omicron’s appearance, that the clamor for extra doses in rich countries was robbing poorer nations of the first doses they desperately need.
Despite the W.H.O.’s designation of Omicron as a high risk, the organization has not changed its position on boosters.
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“Right now, there is no evidence that I’m aware of that would suggest that boosting the entire population is going to necessarily provide any greater protection to otherwise healthy individuals against hospitalization or death,” Dr. Mike Ryan, a director at the W.H.O., said at a news conference on Wednesday.
He and other scientists have said that the unchecked spread of the coronavirus through largely unvaccinated populations, like those in Africa, is likely to give rise to variants like Omicron.
Not all experts are lining up in support of booster shots.
The push for extra doses is predicated on the idea that antibodies are the central aspect of immunity, a false perspective that overlooks the importance of other parts of the immune system in preventing severe illness and death, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and an adviser to the Food and Drug Administration.
He said he would be more worried if vaccinated people infected with the Omicron variant were hospitalized in droves. But limited evidence to date suggests that the vaccines still prevent severe illness, he said.
“That’s always been true — it’s been true for the first three variants, and it’s likely to be true here,” he said. “If you’re setting the goal as protection against mild illness, then we’re going to be boosting until the day we die.”
Even if Omicron turns out to be resistant to vaccines, an extra shot of the original vaccines may not be the best solution, Dr. Offit said: “I just think that’s a detour from what is really going to be the way to get on top of this pandemic, which is to vaccinate the unvaccinated.”
But waiting may not be an option.
If laboratory tests indicate that Omicron sidesteps the vaccines, manufacturers say that they are poised to tailor new versions. That process will take at least a few months, and booster shots of current vaccines may be needed to help keep the variant in check until then.
Even if the antibodies stimulated by those shots are not quite as effective at fending off Omicron as they were against prior variants, the rise in quantity alone could compensate, Dr. Gounder said.
“You’re able to override some of that lower affinity by having the higher numbers,” she said.
If necessary, multiple booster doses — first with the current vaccines, then with Omicron-specific versions — would need to be exquisitely timed, because stimulating immunity too frequently can backfire, Dr. Moore said. Certain immune cells may stop responding to the vaccines.
“This is where it all gets complicated — certainly, nobody should be sitting on dogma here,” he said. “We’re reacting in a low-information environment where the consequences are potentially quite serious.”
