The White House on Tuesday announced a six-week push ahead of the winter travel season to increase booster uptake in seniors, minority communities and rural areas – all of which have disproportionately suffered severe disease and death during the coronavirus pandemic.

Anthony Fauci, who serves as President Biden’s chief medical adviser, emphasized that vaccine effectiveness wanes over time and that the coronavirus is an unusual foe because of the emergence of new variants every few months. He pointed to data released Tuesday by the Centers for Disease Control and Prevention showing that the recently authorized omicron-specific boosters work better against new variants than previous vaccines.

“It is clear now, despite initial bit of confusion,” he said.

Fauci – in his last briefing before his planned retirement in December, after more than four decades in government fighting HIV/AIDS and Ebola and serving as the face of the scientific community during the coronavirus pandemic – urged the public to get their shots: “The final message I give you from this podium is that please, for your own safety, for that of your family, get your updated COVID-19 shot as soon as you’re eligible.”

The Biden administration has faced growing frustration over its bivalent booster rollout.

The United States spent nearly $5 billion to purchase 171 million bivalent boosters from Pfizer and its German partner BioNTech and from Moderna. But efforts to market the shots to the American public have been limited, and booster uptake has remained sluggish since it was approved on Aug. 31. The CDC estimates that only about 11 percent of people 5 and older have received a bivalent booster.

The new CDC study represents the first published estimates of efficacy based on real-world data for the bivalent booster shots. Previous studies have measured whether the bivalent vaccines increase the levels of virus-blocking antibodies in people’s blood.

It confirmed the importance of getting a new shot to boost immunity against the coronavirus as it continues to evolve and produce immune-evading variants but shows that the omicron-specific boosters appear to offer only a modest increase in protection against infection compared with previous shots.

Last winter’s omicron wave made the original monovalent mRNA vaccines less effective against symptomatic infection, prompting a race to create new vaccines that incorporated components from the omicron strains, BA.4 and BA.5.

“This is a tremendous volume of data in a short period of time,” said Francesca Beaudoin, a clinical epidemiologist at Brown. Overall, Beaudoin said, the study shows “there is a real but very modest benefit to the bivalent vaccine among people who have already been vaccinated, who are healthy and who show up” for shots.

But Beaudoin and others described the study as “underwhelming” because it does not shed light on such key questions as the ability of the new boosters to protect against severe disease.

That raises questions about the goal of the bivalent vaccines, according to Paul Offit, director of the vaccine education center and professor of pediatrics in the Division of Infectious Diseases at Children’s Hospital of Philadelphia.

“The only reasonable goal is to prevent serious illness – a goal hat has largely been reached with monovalent vaccines,” said Offit, who has previously expressed skepticism about booster shots. “We are still waiting for one shred of evidence that this bivalent vaccine or any bivalent is better than what we had.”

Meanwhile, the virus continues to evolve so fast that the BA.4 and BA.5 variants are no longer dominant.

“This should be a cautionary tale for what happens when you try to chase these variants,” he said.

Ashish Jha, the White House COVID-19 response coordinator, said in Tuesday’s news briefing that updated vaccines will probably be required at some point next year. A universal vaccine that targets all variants is necessary for long-term management of the pandemic, he said.

“We are at a point where this virus continues to evolve very, very rapidly,” Jha said.

He said that the United States is falling behind in some technologies related to viruses and that more resources are needed. “All I can say is Congress needs to do its job and step up and protect the American people,” Jha said.

The CDC study, which drew on data from the Increasing Community Access to Testing program – a program designed to make testing more available in areas with high social vulnerability – included more than 350,000 tests at almost 10,000 retail pharmacies between Sept. 14 and Nov. 11, among adults who reported symptoms consistent with COVID-19. At registration, participants reported their vaccination history, current symptoms and previous infection.

The CDC researchers then calculated vaccine effectiveness, the measure used to establish the reduction of disease among vaccinated people compared to those who remain unvaccinated.

The study found that bivalent boosters provided significant additional protection against symptomatic infection in people who had already received at least two shots of the monovalent vaccine. The benefit of the bivalent boosters increased with more time that passed since an individual’s last monovalent shot.

Celine Gounder, an infectious-disease specialist and senior fellow at the Kaiser Family Foundation, also noted that the study does not address the question that has shaped the Biden administration’s recent strategy.

“It doesn’t show the bivalents are better than the original boosters,” Gounder said.

But most people are asking a more basic question – “Should I get a booster?” – and the new study confirms the answer, Gounder said: “Get it.”