Amid worldwide omicron surges, many scientists who were not initially convinced of the necessity of booster shots for all adults are reconsidering their policy stances—even as a new study indicates that any combination of vaccine brands provides notable additional protection.

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Could booster shots offer additional protection from omicron?

The omicron variant contains around 50 mutations, about 30 of which relate to the amino acids on omicron's spike protein—specifically, the area of the spike protein that attaches to human cell receptors, Science reports.

The omicron variant: The 'good,' 'bad,' and 'ugly' scenarios

And while much is still unknown, vaccine experts are at odds over how well current vaccines—at their current dosages—will provide protection against it.

NIH Director Francis Collins said he believed that since Covid-19 vaccines have been effective against other coronavirus variants, they're likely to be effective against omicron as well.

"Given that history, we expect that most likely the current vaccines will be sufficient to provide protection," Collins said. "And especially the boosters will give that additional layer of protection because there's something about the booster that causes your immune system to really expand its capacity against all kinds of different spike proteins, even ones it hasn't seen before."

However, according to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, "the profile of the mutations strongly suggest that … it might evade immune protection that you would get, for example, from a monoclonal antibody or from the convalescent serum after a person's been infected and possibly even against some of the vaccine-induced antibodies."

Amid this uncertainty, many experts who were initially opposed to a boosters-for-all approach now believe that the additional doses could provide the best defense against the new variant, the New York Times reports.

For instance, Celine Gounder, an infectious disease specialist at Bellevue Hospital Center initially opposed the Biden administration's push for widespread booster shots—but she revaluated her stance with the emergence of omicron. "Based on what we know about the potential for immune evasion, I would err on the side of giving the booster," she said.

Separately, John Moore, a virologist at Weill Cornell Medicine, said, "If it is highly resistant to antibodies, which seems likely but unproven, then additional doses are appropriate." He continued, "I'd like to see more data, but it won't do anybody any harm to have additional protection."

Similarly, Camille Kotton, an infectious disease physician at Massachusetts General Hospital and CDC advisor, initially voiced concern over the lack of clarity on whether the benefits of booster shots outweighed the risks. However, additional data has since eased her concerns. "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."

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"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.

However, there is still hesitation from some health leaders. For example, the World Health Organization (WHO) has not yet changed its position on booster shots. "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," said Mike Ryan, a WHO director.

Separately, Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia and FDA adviser, highlighted the fact that omicron infections seem to be milder so far. "If you're setting the goal as protection against mild illness, then we're going to be boosting until the day we die," 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," he added.

At the very least, if an omicron-specific formulation of the vaccine becomes necessary, booster shots would likely slow the spread—giving vaccine makers time to develop and test new formulations, the Times reports.

New research suggests a 'mix-and-match' combination is safe and effective

According to a new study published Thursday in The Lancet, a "mix and match" booster strategy that combined different Covid-19 vaccine brands safely and effectively boosted immune response following initial vaccinations, MedPage Today reports. FDA authorized Americans to switch vaccine brands for their booster in October.

The U.K. study’s 2,878 participants initially received two shots of either the AstraZeneca or Pfizer-BioNTech's Covid-19 vaccines. The researchers then tested seven different vaccines as boosters: AstraZeneca, Pfizer-BioNTech, Moderna, Johnson & Johnson, Novavax, Valneva, and CureVac.

While there was a wide variation in recorded antibody levels among the different recipients four weeks post-booster, most of the boosters used in the study raised antibodies to a level that would be equivalent to at least 90% protection against infection.

According to Saul Faust of the NHS Foundation Trust, "[t]he side effect data show[s] all seven vaccines are safe to use as third doses, with acceptable levels of inflammatory side effects like injection site pain, muscle soreness, fatigue." 

According to the Times, the Pfizer-BioNTech and Moderna mRNA vaccines produced much higher antibody levels than the other vaccines did. But Faust cautioned, "If your country or region of the world only has one of the vaccines that we've shown can boost, that will be fine to use as a booster and safe to do it … It's not all about mRNA.”

Notably, the study did not measure the boosters against the new omicron variant. (Mandavilli, New York Times, 12/1; Zimmer, New York Times, 12/2; Walker, MedPage Today, 12/2)