Protection against severe illness and death was, in fact, the original goal of vaccines. When I spoke with vaccine experts as the trials were under way last summer, they universally told me to temper expectations. Vaccines against respiratory viruses rarely protect against full infection because they are better at inducing immunity in the lungs than in the nose, where respiratory viruses gain their first foothold. (Consider: The flu shot is 10 to 60 percent effective depending on the year.) But “the extraordinary efficacy” from the initial clinical trials raised expectations, Ruth Karron, the director of the Center for Immunization Research at Johns Hopkins University, told me. With the Pfizer and Moderna vaccines 95 percent effective against symptomatic infection, eliminating COVID-19 locally, like measles or mumps in the U.S., suddenly seemed possible.

Then came the less pleasant surprise: new variants, like Beta, Gamma, and now Delta, that erode some protection from vaccines. “We now are where we thought we would be a year ago,” Karron said. The vaccines still protect against serious illness very well, as expected, but herd immunity again seems out of reach. The virus will continue to circulate, but fewer people will get sick enough to be hospitalized or die.

The Atlantic