As tens of millions who are eligible in the United States consider signing up for a Covid-19 booster shot, a growing body of early global research shows that the vaccines authorized in the United States remain highly protective against the disease’s worst outcomes over time, with some exceptions among older people and those with weakened immune systems.
While the vaccines’ effectiveness against severe disease and hospitalization has mostly held steady, even through the summer surge of the highly transmissible Delta variant, a number of published studies show that their protection against infection, with or without symptoms, has fallen.
Public health experts say this decline does not mean that the vaccines are not working.
In fact, many studies show that the vaccines remain more than 50 percent effective at preventing infection, the level that all Covid vaccines had to meet or exceed to be authorized by the Food and Drug Administration back in 2020. But the significance of these declines in effectiveness — and whether they suggest all adults should be eligible for a booster shot — is still up for debate.
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