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Pfizer Vaccine Not as Long-Lasting as Previously Believed, Study Finds


A pharmacist makes an injection of the Moderna COVID-19 vaccine at a pharmacy in Paris, on June, 4, 2021. STEPHANE DE SAKUTIN/AFP via Getty ImagesIt has been more than six months since the first FDA-authorized COVID-19 vaccine was administered in the U.S. Many scientists expect coronavirus vaccination to eventually evolve into an annual requirement like flu shots. But with dangerous viral mutations complicating the ongoing pandemic, we may need a booster dose sooner than 12 months.

A study pre-print uploaded in medRxiv Thursday suggests a “gradual decline” in efficacy over six months after two doses of the Pfizer/BioNTech vaccine in more than 45,000 people worldwide.

Efficacy of preventing symptomatic infection peaked at 96 percent a few weeks after injection and fell an average 6 percent every two months after. At the end of the six-month period, the vaccine was only 84 percent effective.

The ongoing study, which has yet to be peer-reviewed, shows that the Pfizer/BioNTech vaccine was overall 91 percent effective at preventing COVID-19 infection over the course of six months. Protection against severe illness and hospitalization was 97 percent.

These new findings suggest that the Pfizer shot may not be as long-lasting as scientists previously believed. “If the vaccine’s efficacy continues to decline at the rate observed in the paper, it would fall below the 50 percent threshold—a benchmark for vaccine utility—within 18 months of vaccination,” the health news site STAT estimates.

For comparison, Moderna’s COVID-19 vaccine, which is also mRNA-based, was 90 percent effective against symptomatic infection (95 percent effective against severe cases) after six months, the company said in April. Johnson & Johnson hasn’t disclosed six-month efficacy data for its single-shot vaccine.

Experts say the decline in efficacy observed in the Pfizer study could be in part a result of the onset of the delta variant in recent weeks—which was not a material factor when Moderna conducted its six-month efficacy study.

“Either the vaccine’s efficacy against mild disease is decreasing slightly, or the emergence of new viral variants made it look less effective,” STAT reported, citing Natalie Dean, a biostatistician at Emory University.

The finding could support Pfizer’s case of adding a third booster shot to its existing vaccine. (It would require amending the FDA emergency use authorization or the vaccine gaining full approval.)

Multiple studies published last week found that authorized COVID-19 vaccines were less effective against the delta variant than previous strains of the coronavirus.

The good news is, new data from Pfizer show that a third booster dose increases neutralizing antibody levels against the delta variant by more than five times, compared to levels after a second dose, in people ages 18 to 55 years old, the company said in its second-quarter earnings report Wednesday.

The data came from a small-scale study of just 11 people in the 18-55 age group and 12 people older than 65. But “these preliminary data are very encouraging as Delta continues to spread,” Mikael Dolsten, Pfizer’s chief scientific officer said during an earnings call Wednesday. Pfizer said the data has been submitted for publication in a medical journal.