The effectiveness of the Pfizer-BioNTech COVID-19 vaccine fell from 96 percent to 84 percent over six months, according to data released on Wednesday.
The preprint study funded by the companies determined that the vaccine’s effectiveness reached a high point of 96.2 percent within two months after the second dose.
But the efficacy “declined gradually” to 83.7 percent within six months, with an average decrease of about 6 percent every two months. But even with the slip in efficacy, the data indicates the vaccine offers protection six months later.
The ongoing study with more than 44,000 participants across the Americas and Europe determined the vaccine was overall 91.1 percent effective, after 81 cases emerged among the vaccinated and 873 among those who received the placebo.
The efficacy of the vaccine against severe disease including hospitalizations remained high, at 97 percent.
Researchers will continue to observe participants of the study up to two years and combined with “real-world” data “will determine whether a booster is likely to be beneficial after a longer interval.”
If the efficacy continued to decrease at the current rate, it could fall below 50 percent within 18 months, suggesting that booster shots could be needed.
Pfizer already has its eye on a booster vaccine, announcing earlier this month its plans to apply to the Food and Drug Administration (FDA) to authorize a third dose.
But the Biden administration pumped the brakes on the call for boosters, with the FDA and Centers for Disease Control and Prevention (CDC) declaring hours later that additional shots were not necessary at this time. U.S. health officials did note they may be needed later.
Wednesday’s study did not factor in the highly transmissible delta variant that has taken hold in the U.S. and has driven the CDC to reinstate mask recommendations for fully vaccinated people in high-risk areas.
Pfizer, however, also posted data ahead of its earnings call on Wednesday saying that a third dose of its vaccine “strongly” improves protection against the delta strain.