A health care employee prepares to inject a patient with a dose of the Moderna coronavirus vaccine in November. Eric Lee for The Washington Post

Coronavirus boosters are sparking confusion and questions again as the United States confronts the growth of a new variant adept at evading immunity, while federal officials consider switching to an annual shot model.

The XBB.1.5 variant is a highly transmissible descendant of omicron that is now estimated to cause about half of new infections in the country. Early lab studies showed that it was especially effective at evading virus-fighting antibodies, prompting concerns that it might more readily slip past the updated boosters.

But a new study released Wednesday by the Centers for Disease Control and Prevention offers reassurance that the updated booster shots, which rolled out in the fall, are still protecting people in the real world.

The new booster shots are bivalent, meaning they are designed to protect against both the original strain and the BA.5 omicron subvariant that caused most infections over the summer. The latest data shows that the booster protects people from getting sick with XBB.1.5 about as well as it did against BA.5.

The CDC data arrived around the same time that the Food and Drug Administration is considering switching the country’s vaccine strategy to mimic the one used for the flu, with people getting annual shots targeting whichever strain is predicted to dominate during the fall and winter.

Amid a swirl of new information, The Washington Post received hundreds of questions about booster shots for a recent reader live chat with health reporters and CDC Director Rochelle Walensky. Here’s what we know and don’t know about boosters.

How effective is the bivalent booster against XBB.1.5 infections?

CDC analysts used real-world data gathered from a national testing program available at pharmacies to see how well the updated vaccine protected people who had received the shot up to three months earlier compared with people who had been previously vaccinated but hadn’t received a bivalent booster.

Based on data collected from Dec. 1 to Jan. 13, researchers found that the updated boosters cut the risk of symptomatic infection by about half for most adults, and by more than one-third for people 65 and older. Symptomatic infection was defined as having one or more of the common symptoms of covid-19, such as cough or fever.

Syringes with Moderna vaccine doses are ready for use. Eric Lee for The Washington Post

“Bottom line: We did not see reduced vaccine protection against symptomatic illness for XBB/XBB.1.5 compared with recent BA.5-related variants, which is reassuring,” said Brendan Jackson, who heads CDC’s coronavirus response, during a briefing Wednesday. “This was true regardless of which boosters people had received, but people who received the updated, or bivalent, coronavirus vaccines had higher levels of protection than those who received only the previous vaccines.”

Earlier lab studies had raised concerns, because the bivalent vaccine produced lower levels of virus-fighting antibodies against the XBB-related subvariants. But that does not seem to be the case with the latest real-world data, CDC officials and experts said.

“It doesn’t provide perfect protection, but it does reduce the risk of becoming ill by roughly 45 percent if you already received two or more doses of the vaccine,” Albert Ko, an epidemiologist and infectious-disease physician at Yale University, said in an email.

How effective is the bivalent booster against severe outcomes from XBB.1.5?

Officials said there isn’t enough data yet to know how well the updated boosters protect against more severe disease, hospitalization and death. But they expect that the updated boosters will provide higher protection against these outcomes.

“What we know from past experience is generally that the vaccines protect better against more severe disease,” said the CDC’s Ruth Link-Gelles, who leads the agency’s team on coronavirus vaccine effectiveness.

She added that the latest study did not take into account previous coronavirus infections, which also provide some protection. People who have previously had covid-19 start from a slightly more protected place than those without prior vaccination or infection, Link-Gelles said. “So, if anything, these estimates that we’re publishing are probably an underestimate” of the vaccine’s protection.

Data released in December shows that adults who received the updated boosters cut their risk of having to visit an emergency room or being hospitalized with covid-19 by 50 percent or more at a time when the BA.5 variant was still dominant. The boosters provided even higher protection – 70 to 80 percent – for those 65 and older.

If XBB.1.5. evades antibodies, does that mean I have no immunity?

This is a really important question, and the good news is that, as long as you’ve been vaccinated or have had a previous infection, you still have immunity.

Even though XBB.1.5 and other omicron subvariants can slip past the initial defenses and infect you, your body is still trained to keep fighting the coronavirus from previous vaccination and infection. It’s like invaders breaking through to the castle gate, only to be defeated by soldiers in the courtyard.

Fighting severe outcomes from an infection involves more than just antibodies. Some of the worst health effects of covid-19 in the dark early days of the pandemic were from the immune system overreacting to an unfamiliar virus.

“You can see in someone’s blood that their antibodies have waned, but they still have protection against serious disease,” Link-Gelles said. Other parts of the immune system response are “many orders of magnitude harder to study, so that’s why we often rely on antibodies as kind of our first understanding.”

When can I get my next booster?

If you are up-to-date on your shots, you would not qualify for another booster until the fall under the new strategy proposed by the FDA. But the agency also said medically vulnerable people, including older people and those who are immunocompromised, may need two shots a year. It’s unclear when they would qualify for a second bivalent shot as regulators weigh different approaches, such as two shots in the fall given one month apart.

Post reporter Laurie McGinley writes that the latest proposal would entail determining “in the spring which strain will pose the greatest threat the following winter. A vaccine targeting that strain would then be administered in the fall, in the hopes that a streamlined effort would encourage vaccine uptake.”

The plan drew mixed reactions and will the subject of a Thursday meeting of FDA advisers. You can follow news from that meeting and other future booster developments here or by signing up for our coronavirus newsletter.

How long does bivalent booster protection last?

We don’t yet know. Since booster campaigns started, research has shown that boosters offer short-lived protection against infection and longer-term protection against severe outcomes.

The newly released CDC data covers only three months so far, and it will take even longer to determine how protective boosters are against people falling severely ill with XBB.1.5. Link-Gelles said officials are still examining the question, and she cautioned that people who were infected last spring or summer but who have not gotten a bivalent booster probably “don’t have a lot of protection left over from that previous infection.”

Experts recognize that this can be frustrating for high-risk people trying to account for the risk of the coronavirus while making plans for the year, such as booking international summer vacations.

“We should learn in the next several months how long the booster provides protection and what the FDA and CDC recommend,” Ko said. “If you are significantly concerned about your risk about getting covid-19, you may want to wait before making plans.”

Should I worry if my last shot was months ago?

Given that protection against infections is short-lived, scores of Post readers who fall into high-risk groups asked during the live chat whether they should be worried that their protection has worn off.

While we await data on the effectiveness of bivalent boosters against severe illness and hospitalization for XBB.1.5, experts note that they have yet to see evidence of this variant causing more severe illness. Hospitals in the Northeast, where that variant became dominant in December, have not been raising alarms about patients coming in sicker, which happened in areas where the delta variant surged in 2021.

Eric Topol, a professor of molecular medicine at Scripps Research who has championed the bivalent booster, said it’s too early to say whether a second shot is necessary, especially for those who had previously been infected and received shots. He is skeptical of an annual shot model for older people and other high-risk individuals and said they need better guidance from the federal government.

“The chance of getting covid throughout the year is unlike the flu,” Topol said. “The chance of having full protection for a year isn’t backed up by data. The best data we have is four to six months.”

Should I get boosted again each time infections spike?

Sometimes it can feel as if vaccines are pointless in avoiding infections when you are hearing about vaccinated friends and loved ones getting infected. But the goal of the U.S. vaccine strategy for covid-19 has been to reduce severe outcomes, not infections, and the latest CDC data is a reminder that vaccines still offer some protection against mild infections. Although boosters can temporarily shore up protection against infection, data suggests that they have less of a benefit when the last shot was recent.

In short, it’s not realistic to constantly get a booster every time coronavirus cases are rising to avoid getting sick. That’s when you’ll have to tap the other tools available for avoiding infections, such as wearing high-quality masks, testing before gatherings and avoiding crowded indoor settings during periods of heightened transmission.

Paul Offit, a vaccine adviser to the FDA and pediatrics professor at the Children’s Hospital of Philadelphia, said people who are very old should recognize that, even with boosters, they may struggle to mount a strong immune response to the coronavirus. He said they should consider heightened precautions to avoid infections during waves and prioritize early antiviral treatment if they do get sick.

“We keep talking about boosting and boosting and boosting,” said Offit, who has been critical of frequently boosting the general population. “But antivirals save people’s lives.”

The Washington Post’s Laurie McGinley contributed to this report.