New studies released from the Centers for Disease Control and Prevention (CDC) on Friday showed that a booster shot of COVID-19 vaccine provides robust protection against hospitalization and severe disease.
The studies from scientists at the CDC are some of the first based on real world data, and include both the delta and omicron variants.
The findings, while promising, come as the U.S. is seeing a massive spike in infections due to the omicron variant, which is overwhelming hospitals throughout the country.
While there was a brief surge in the number of people receiving boosters when omicron first began spreading in the U.S., demand for additional doses has been lagging. During a media briefing Friday, CDC Director Rochelle Walensky urged everyone eligible to get a booster.
"There are still millions of people who are eligible for booster dose and have not yet received one, as we continue to face the omicron variant representing over 99 percent of infections in the United States today. I urge all who are eligible to get their booster shot to get it as soon as possible," Walensky said.
One CDC analysis examined hundreds of thousands of visits to emergency departments and urgent care centers, and tens of thousands of hospitalizations, between August 2021 and Jan. 5, 2022.
The study found that getting a third dose of an mRNA vaccine was at least 90 percent effective at preventing COVID-19-associated hospitalization, both during the delta and omicron periods.
A third shot reduced a person's risk of an emergency department and urgent care visit by 94 percent during delta and 82 percent during omicron.
In comparison, two doses were only 57 percent effective in reducing hospitalization for omicron when it had been at least six months since the second shot was administered.
A second CDC study concluded that people with three shots had the highest protection against COVID-19 infection, at least during the early days of omicron.
Looking at data from 25 state and local health departments, CDC researchers found that among those who were boosted, there were 149 cases per 100,000 people on average each week.
For those who had only two doses of the vaccine, it was 255 cases per 100,000 people.
A third study published Friday in JAMA by CDC scientists found that a third dose of an mRNA vaccine provides significant protection against symptomatic disease for both the delta and omicron variants, but greater protection against delta.
The conclusions of all three studies raise questions about what it means to be "fully vaccinated."
The CDC defines "fully vaccinated" to mean that a person has completed just their primary series of vaccinations, though the agency recently said a person is only "up to date" if they have also received a booster dose.
Walensky said the agency is not changing the definition of fully vaccinated, but will "pivot the language."
"We really want to make sure people are up to date. That means if you recently got your second dose, you're not eligible for a booster, you're up to date. If you are eligible for a booster and you haven't gotten it, you're not up to date and you need to get your booster in order to be up to date," Walensky said.
Only about 39 percent of the people who are fully vaccinated have received a booster dose to date, according to the CDC. Federal health officials have been urging boosters for months, though experts think early conflicting messages about who is eligible likely contributed to lower uptake.
But the people at greatest risk remain the unvaccinated. Walensky noted that when omicron first emerged in December, unvaccinated adults had a five times higher risk of infection compared to adults who had received a booster dose.
Nearly 20 percent of the people eligible - any American over five years old - remain unvaccinated, according to CDC.
Updated at 2:30 p.m.