This week, the Biden administration strongly recommended booster shots for most vaccinated Americans, and the Centers for Disease Control and Prevention also released three studies that federal officials said provided evidence that booster shots of the Pfizer-BioNTech and Moderna coronavirus vaccines would be needed in the coming months.

Here’s what we know about booster shots — why Americans may need them and when they should get them:

  • Taken together, the C.D.C. studies show that although the vaccines remain highly effective against hospitalizations and deaths, the bulwark they provide against infection with the virus has weakened in the past few months.

  • The finding accords with early data from seven states, gathered this week by The New York Times, suggesting a rise in breakthrough infections and a smaller increase in hospitalizations among the vaccinated as the Delta variant spread in July.

  • The decline in effectiveness against infection may result from waning vaccine immunity, a lapse in precautions like wearing masks or the rise of the highly contagious Delta variant, experts said — or a combination of all three.

  • The new studies indicate overall that vaccines have an effectiveness of roughly 55 percent against all infections, 80 percent against symptomatic infection, and 90 percent or higher against hospitalization, noted Ellie Murray, an epidemiologist at Boston University.

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