Covid case numbers have been rising across the United States since mid-July. So the news that updated covid vaccines are finally available comes as a relief to many. The first updated covid-19 shots from Moderna and Pfizer, which target an omicron variant known as XBB, hit some pharmacies as early as yesterday, and many more doses will be administered in the coming days. 

On September 12, the US Centers for Disease Control and Prevention recommended that everyone six months or older get the new vaccine, predicting that following this advice could prevent 400,000 hospitalizations and 40,000 deaths over the next two years. The announcement has once again sparked a debate about who should get vaccinated, and who will benefit most from the new formulation.

“I would not recommend anything to others that I wouldn’t recommend for my own family,” wrote CDC director Mandy Cohen in an op-ed on Wednesday. “My 9- and 11-year-old daughters, my husband, my parents and I will all be rolling up our sleeves to get our updated Covid-19 vaccines along with our flu shots soon.” 

A universal recommendation has the benefit of being simple to communicate and implement, but some other medical experts have mixed feelings about the decision. “What I want to see is the ability of parents and physicians to choose to vaccinate across the age span,” says Gregory Poland, a vaccine expert at the Mayo Clinic in Rochester. “That’s different than saying, ‘I think everybody aged six months and older should receive this vaccine.’”

Jennifer Nuzzo, an epidemiologist at Brown University, agrees. “The data don’t say that the vaccines produce equal benefits for all age groups,” she says. “I think the recommendation should reflect that fact.”

For Kawsar Talaat, an infectious disease physician at Johns Hopkins School of Medicine, the benefits outweigh the risk for almost everyone: “The reality is [covid] can be severe in anybody.” That includes younger people who are otherwise healthy. The CDC presented data to its advisory panel on Tuesday showing that 54% of infants, children, and adolescents who ended up in the hospital with covid did not have an underlying condition.

So who will benefit most from a booster? On that question, experts agree. Older adults, people who are immunocompromised, and those with multiple underlying conditions that put them at risk for serious illness should get the updated vaccine. That group includes Poland, who is 68. Adults 65 and older have a far greater risk of hospitalization and death than any other age group.  Between January and the third week of July, more than 24,000 adults over 65 died of covid-19 related causes, nearly nine times the number of deaths reported in people aged 45 to 64. Pregnant people also have a higher risk of developing serious illness. 

The benefits for other groups aren’t quite as stark. Nearly all Americans have some existing immunity to covid through infection, vaccination, or both. “We still think that people who are not boosted have high levels of protection against severe illness, and we know that we’re using vaccines primarily to prevent severe illness,” Nuzzo says. A modeling analysis by the CDC’s covid-19 working group found that depending on whether covid case numbers were low or high, the shot averted an estimated 16 to 476 covid hospitalizations per million people vaccinated in the under-50 crowd.   

But the benefits go beyond preventing hospitalization. An updated shot could reduce the small risk of serious illness even further. It might also mean that if you do get infected, you don’t get quite as sick for quite as long, or that you are less likely to pass the virus on to others. Some emerging evidence even suggests a booster might also reduce the risk of long covid, although Nuzzo points out that the data are still preliminary. “I think that that is an interesting hypothesis,” she says. 

If you’re still expecting the vaccine to save you from infection, however, you might be disappointed. Flu vaccines do protect against infection, but there isn’t strong evidence that the same can be said for the covid-19 vaccines. In theory, a boost in circulating antibodies might be enough to fend off the virus, “but we don’t know how long that lasts,” says Monica Gandhi, an infectious disease expert and researcher at the University of California, San Francisco. Maybe months. Maybe only weeks. 

Still, Nuzzo doesn’t really see any downside to getting the updated shot. She isn’t concerned about side effects: “I would say get yourself boosted. I think it’s a good idea.” 

Other people, however, have expressed some safety concerns. Pablo Sanchez, a pediatrician and infectious disease expert at Children’s Hospital in Ohio, was the lone dissenting voice among the CDC advisors who voted to issue a universal recommendation. “I am very much pro-vaccination,” he said. But Sanchez pointed out that we have extremely limited data on children and infants. “I also think that in certain circumstances, we still have to be concerned about potential side effects, especially in young adults and young adult males,” he said. The data suggest that adolescents and young adults, especially males, who receive the Moderna and Pfizer covid-19 vaccines have a slightly higher incidence of myocarditis, an inflammation of the heart muscle. But “the risk of myocarditis from the virus is far greater than the risk of myocarditis from the vaccine,” Talaat says. If your child has had myocarditis in the past or is at an increased risk, you should talk to a physician. 

Other countries are taking a more targeted approach. In the United Kingdom, for example, the vaccine is only available to people 65 and over, those with underlying health conditions, and a couple of other select groups. Gandhi is concerned that a universal recommendation will diffuse the message that older adults really need to get the updated vaccine. 

And who will actually get the vaccine remains to be seen. Uptake of the bivalent booster was low, even in the highest-risk groups. Nuzzo was stunned by just how low—less than 50% in people over the age of 65. That’s a problem. The number suggests “that we need to double down our efforts to keep people out of the hospital and prevent them from dying,” she says. “If we don’t do all we can to prevent deaths, then we basically have failed.”