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A booster dose of mRNA vaccine resulted in improved effectiveness against severe disease from the Omicron variant, the White House COVID-19 Response Team said in a briefing on Friday.

CDC Director Rochelle Walensky, MD, walked through a series of reports that examined the effectiveness of boosters against infection, emergency department visits, hospitalizations, and even death, saying that the new data "add more evidence of the importance of being up-to-date on COVID vaccinations."

While the vaccine effectiveness of a two-dose series against emergency department visits, urgent care visits, and hospitalizations declined to 76% after 6 months, it rose back up to 94% after the third shot, said Walensky, referencing new data from the VISION Network published in an early edition of the

While vaccine effectiveness more than 180 days after a two-dose series declined to 38% against emergency department and urgent care visits during the predominance of Omicron, it rose back up to 82% 14 days or more after a booster dose, she added.

The numbers for hospitalizations were even more striking, with 90% effectiveness against COVID-associated hospitalizations after a third dose.

Walensky also pointed to a study published that found that a booster dose of Pfizer or Moderna provided "greater protection against symptomatic COVID disease versus the primary series."

Those who received a booster dose versus those who only received the two-dose primary series were less likely to test positive for Omicron (OR 0.34, 95% CI 0.32-0.36), according to data from the

Finally, she reviewed that examined both COVID-19 incidence and deaths during the Omicron surge, which found that, in December, unvaccinated adults had a five times higher risk of infection compared with adults who received a booster dose, relative to October-November.

Walensky also noted that showed that older adults derived the greatest benefit from boosters: adults ages 65 and up who received a booster were almost 50 times less likely to be hospitalized compared with unvaccinated older adults. Adults ages 50 to 64 were 44 times less likely to be hospitalized.

These data prove how protection against the Omicron variant is "highest for those up-to-date with vaccination," she said.

When a reporter asked why the CDC chose the phrase "up-to-date" rather than defining "fully vaccinated" as including a booster dose, Walensky pointed out that "up-to-date" is used in public health for other vaccinations, and noted that someone who recently got their second dose would be up-to-date, as they would not yet be eligible for a booster.

Redefining Protection

Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, walked reporters through a primer on the differences between antibodies, memory B cells, and CD4/CD8 T cells, as a way to explain why the vaccines provide protection against serious outcomes, but less protection against infection.

"Variants with extensive mutations more easily escape protection from infection," as protection by antibodies is "short-lived," Fauci said, while protection against severe disease is mediated by memory B cells and CD4/CD8 T cells.

That protection is "longer-lived and broadly active across variants," he added.

When asked by a reporter to provide the best-case and worst-case scenarios for the pandemic going forward, Fauci referenced the concept of "adequate control."

"The best-case scenario is a diminution of cases in many regions of the country to a baseline level, [where] vaccinated, boosted people and protection afforded by prior infection in the community" won't stress the healthcare system, he said.

"If you mount a good immune response over a period of several months, it is unlikely you will be reinfected by the same variant," he noted.

However, that was off the table with the worst-case scenario, he added, which consisted of "a variant so different that it evades the accumulation of immune protection that we got from infections and prior vaccination."

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