Data released on Monday (October 4) suggests that the effectiveness of Pfizer and BioNTech's COVID-19 vaccine drops by nearly half six months after the second dose.
The data was published in the Lancet medical journal, and had previously been considered by U.S. health agencies deciding on the need for booster shots.
But the data suggests the vaccine still remains 90% effective at preventing hospitalization and death, even against the Delta variant, for at least six months.
Researchers from Pfizer and Kaiser studied the health records of over 3 million people over the span of eight months.
The analysis suggested Pfizer's vaccine effectiveness against the Delta variant was 93% after the first month, plunging to 53% after four months.
Against other variants, efficacy declined from 97% to 67%.
Researchers say the drop in effectiveness is due to waning efficacy, rather than more contagious variants, and that the data suggests Delta isn't evading virus protection.
Meanwhile, there was no data on the occupations of study participants, nor their adherence to mask-wearing guidelines, either of which could have affected their likelihood of virus exposure.
The U.S. Food and Drug Administration has authorized Pfizer booster shots for older adults and Americans at high-risk of getting infected.
Scientists have called for more data on whether boosters should be recommended for all.