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Pfizer to Study Vaccine Breakthroughs to See if Boosters Needed

June 14, 2021, 10:32 PM

Pfizer Inc. will examine cases of fully vaccinated people who contracted Covid-19 to determine whether and when a booster shot is necessary, a company official said Monday.

“We will be looking at real world data to help us understand when we might see a change in vaccine effectiveness,” David Swerdlow, clinical epidemiology lead for Pfizer Vaccines, said at the Precision Medicine World Conference.

“We’re going to be monitoring this closely and using immunological data, clinical data, and real world data to help us think about when a booster might be needed,” he said.

More than 10,200 Covid-19 vaccine breakthrough cases have been reported in the U.S. as of April 30, although it’s rare for fully vaccinated people to get infected. About 43% of the U.S. population is fully vaccinated, with 311 million doses administered so far, according to Bloomberg’s vaccine tracker.

Booster, or follow-up, shots are for patients who have received their full dosing of their initial vaccine—one or two doses, depending on the brand—and whose immune systems may need an additional nudge down the road to combat variants or bolster dwindling antibodies.

“We’re working on understanding the impact of booster doses. We will be getting data from continued monitoring of our clinical trials to see how long immune markers last,” Swerdlow said.

There are still “unanswered questions,” said Julie Louise Gerberding, executive vice president at Merck & Co. and former director of the Centers for Disease Control and Prevention. “Even with the astonishingly wonderful efficacy of the vaccines that are currently authorized, we don’t know the durability of protection long-term,” she said.

“We need to understand breakthroughs and how we might be able to predict them and who’s at risk for them,” Gerberding said.

A real-world study called HERO-Together, led by the Duke Clinical Research Institute, is designed to address health outcomes and long-term effects of the Covid-19 vaccines, but it’s limited to health-care workers. Pfizer indicated its researchers might expand beyond that population.

“There’s a lot of science that needs to be monitored and assessed as we go forward, we’re certainly not out of the woods,” Gerberding said. “I think now is a moment to step back and say thank you to the people who have been able to move their vaccines so quickly.”

To contact the reporter on this story: Lesley Torres in California at

To contact the editors responsible for this story: Fawn Johnson at; Alexis Kramer at