More evidence highlighting the benefit, and limitations, of covid-19 vaccines.
After the Centers for Disease Control and Prevention shifted its coronavirus vaccine guidance from a near-universal recommendation to a more limited approach, many readers asked what has changed in the science to warrant the change. Some wondered how effective boosters really are and whether they should continue receiving them once a year.
A new study published in JAMA Internal Medicine helps answer these questions. It found that last season’s updated vaccine provides significant protection against covid-19, particularly against severe illness and death. The results reaffirm the benefit of staying current with covid vaccination.
Researchers analyzed data from approximately 1.8 million Americans between August 2024 and April 2025, of whom only about 13 percent received the 2024–2025 coronavirus shot. They evaluated three clinical outcomes related to the disease: infections, emergency department visits and hospitalization or death.
The vaccine reduced the chances of infection by about 45 percent at four weeks after vaccination. That declined to roughly 36 percent at 10 weeks and about 17 percent at 20.
Protection against emergency visits followed a similar pattern, though it waned less: It was about 45 percent at four weeks, 43 percent at 10 and 39 percent at 20.
The strongest protection was against hospitalization and death. Effectiveness was about 57 percent at four weeks, 50 percent at 10, and 34 percent at 20.
Importantly, the benefits of vaccination persisted even after adjusting for differences in age, socioeconomic status, time since prior vaccination, and medical conditions including immunocompromising disorders. The vaccines were also effective across multiple subvariants that circulated during the study period.
These results are consistent with data collected in prior seasons. We’ve known that the most important benefit of coronavirus vaccines is reducing severe disease. This matters most for older adults, who accounted for nearly 80 percent of hospitalizations in this study and stand to benefit most from booster doses.
We’ve also known that effectiveness against infection wanes quickly. Even at its peak, it only cuts the chances that someone will catch the virus by half. These data should inform expectations: The goal is not to prevent every infection but to prevent serious illness and death. Breakthrough cases do not mean the vaccine isn’t working; rather, they show the virus can still cause mild infections even as vaccines blunt its worst effects.
For those seeking optimal protection before a high exposure event, getting vaccinated two to four weeks in advance remains a sound strategy. It is not, however, a guarantee against infection. People who want to minimize their risk further should also consider wearing a high-quality N95 mask and limit the time they spend in crowded indoor settings.
What about safety? A separate study published in JAMA Network Open analyzed data from more than 1.5 million people in Denmark. It found no increase in adverse outcomes among those who received the 2024-2025 booster compared with those who did not. Another study in the New England Journal of Medicine supported the safety of not only coronavirus vaccines but also shots against influenza and respiratory syncytial viruses.
Together, these studies make a strong case for vaccination. There really is little reason for eligible people to skip the updated booster. As Robert Califf, former commissioner of the Food and Drug Administration, wrote in a JAMA editorial, “It is difficult to understand why the use of messenger RNA technology, which has been shown capable of teaching our immune systems to prepare to combat a virulent threat and reduce the risk of death and serious illness with relatively few detectable serious adverse outcomes, has produced such a broad-based hesitation.”
If anything, this latest research raises a different question: Should those most vulnerable to severe disease continue to receive two doses a year rather than one? Judging by these results, the answer seems to be yes.
The CDC has yet to weigh in on the question of a spring booster. It should recognize the mounting evidence in favor of it and make sure anyone who wants additional protection can get it.
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