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The Los Angeles Times reports that research conducted in Southern California has confirmed the dramatic erosion of the Pfizer-BioNTech COVID-19 vaccine’s protection against “breakthrough” coronavirus infections.

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.

The study, funded by Pfizer and published Monday in the journal Lancet, also provides strong new evidence that the waning immunity against infection probably would have been seen with or without the arrival of the Delta variant.

Researchers 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.

By showing that waning immunity, not the Delta variant, was the likely reason for the rise in breakthrough infections, the study suggests it may not be necessary to reformulate a Pfizer-BioNTech booster that specifically targets Delta. For now, at least, a third shot identical to the first two would probably extend the vaccine’s early record of protection against all strains, including Delta.

The protection provided by the Pfizer vaccine beyond six months has been an open question, hinted at only by Israeli studies that suggest COVID-19 hospitalization rates rise in those above 60 years of age.

In another recent study, researchers from Emory University and Stanford found that six months after being inoculated with the Pfizer vaccine, roughly half of 56 young and middle-aged adults had no detectable neutralizing antibodies against the SARS-CoV-2 virus. The reduced immunity was particularly dramatic against the coronavirus variants Delta, Beta and Mu.

That study was posted last week on BioRXiv, a site where researchers share preliminary work before it has been peer-reviewed. But its findings of “a substantial waning of antibody responses” – as well as a drop in the immunity provided by T cells – suggest that a third booster immunization “might be warranted,” its authors wrote.