Based on an observational study published Monday in the peer-reviewed journal The Lancet, the effectiveness of Pfizer and BioNTech's COVID-19 vaccine against severe sickness declines over time, falling from a peak of 88% a month after receiving the two-shot treatment to 47% six months later.

The vaccine's efficacy in avoiding hospitalization and mortality remained high for at least six months, the study shows. According to the researchers, the decline is attributable to diminishing effectiveness rather than more infectious variations.

The vaccination also offers effective protection against the highly infectious Delta form, scientists said. The vaccination was 93% effective against hospitalization from the Delta variant in a subset of patients who had samples of the pathogen sequenced, compared to 95% against other strains.

Scientists sequenced more than 5,000 samples of the virus. Overall, the Delta variant comprised around 28% of the samples taken, although it was the dominant variant in June and July.

The findings support earlier research from the Centers for Disease Control and Prevention and Israeli health officials, which revealed that infection control measures diminished over time but its efficacy in keeping patients out of the hospital remained stable.

Pfizer and Kaiser Permanente researchers analyzed the electronic health data of about 3.4 million Kaiser Southern California members between December 2020 and August 2021, when the vaccine first became accessible.

In remarks from The New York Times, Sara Tartof, an epidemiologist at Kaiser Southern California and the study's primary author, said "Protection against hospitalization stays strong over time, even when Delta predominates."

The findings come less than two weeks after U.S. health officials approved the distribution of Pfizer-BioNTech vaccine booster doses to a wide range of Americans, including the elderly and other people at high risk.

At this time, only a small percentage of patients who got Pfizer's shots are eligible for boosters. U.S.  President Joe Biden said the new policy would make third Pfizer jabs available to about 60 million individuals, with 20 million of them being instantly qualified.

Meanwhile, the authors noted that testing for variations is more likely to fail in vaccinated people, which might contribute to an underestimation of variant-specific efficacy in the research.