1 / 9 The effectiveness of the Pfizer Inc (PFE.N)/BioNTech SE vaccine in preventing infection by the coronavirus dropped to 47% from 88% six months after the second dose, according to data published on Monday that U.S. health agencies considered when deciding on the need for booster shots2 / 9The data, which was published in the Lancet medical journal, had been previously released in August ahead of peer review.3 / 9The analysis showed that the vaccine's effectiveness in preventing hospitalization and death remained high at 90% for at least six months, even against the highly contagious Delta variant of the coronavirus.4 / 9The data suggests that the drop is due to waning efficacy, rather than more contagious variants, researchers said.5 / 9Researchers from Pfizer and Kaiser Permanente studied electronic health records of roughly 3.4 million people who were members of Kaiser Permanente Southern California between December 2020 - when the vaccine first became available - and August of 2021.6 / 9'Our variant-specific analysis clearly shows that the (Pfizer/BioNTech) vaccine is effective against all current variants of concern, including Delta,' said Luis Jodar, senior vice president and chief medical officer at Pfizer vaccines.7 / 9A potential limitation of the study was a lack of data on adherence to masking guidelines and occupations in the study population, which could have affected frequency of testing and likelihood of exposure to the virus.8 / 9Vaccine effectiveness against the Delta variant was 93% after the first month, declining to 53% after four months. Against other coronavirus variants, efficacy declined to 67% from 97%.9 / 9'To us, that suggests Delta is not an escape variant that is completely evading vaccine protection,' said study leader Sara Tartof with Kaiser Permanente Southern California's Department of Research & Evaluation. The U.S. Food and Drug Administration has authorized the use of a booster dose of the Pfizer/BioNTech vaccine for older adults and some Americans at high-risk of getting infected. Scientists have called for more data on whether boosters should be recommended for all.