The new study, one of the largest and longest to track the effectiveness of a vaccine in Americans, found that the vaccine’s ability to protect against infection stood at 88% in its first month, then fell to 47% after just five months.

But even as the Delta variant became the predominant strain across the Southland, the vaccine’s effectiveness at preventing COVID-19 hospitalizations held steady at close to 90% for as long as six months. What’s more, it maintained that power across vaccine recipients of all age groups…

For starters, Tartof and her colleagues found that a fresh inoculation with the Pfizer-BioNTech vaccine protected just as well against an infection with the Delta variant as it did against infection with other versions of the coronavirus.

Second, the vaccine’s ability to keep vaccinated people out of the hospital remained high across a span of time when the Delta variant gained ground in Southern California.

And third, breakthrough infections were more closely linked to the amount of time that had lapsed since vaccination than they were to the particular viral variant involved.