Pfizer's COVID-19 vaccine was 90 percent effective in preventing hospitalization and death for at least six months, according to a study published Oct. 4 in The Lancet.
From Dec. 14 to Aug. 8, researchers studied electronic health records for more than 3.4 million people who were members of Kaiser Permanente Southern California. The median age for those who were part of the study was 45.
Although the study found Pfizer's vaccine provides strong protection against hospitalization and death for at least six months, it also found its effectiveness in preventing COVID-19 infection decreased from 88 percent during the first month after full vaccination to 47 percent after five months.
The vaccine's effectiveness against delta variant infection was 93 percent in the first month after full vaccination, but it decreased to 53 percent after four months. For other coronavirus variants, efficacy decreased from 97 percent to 67 percent.
"Reduction in vaccine effectiveness against SARS-CoV-2 infections over time is probably primarily due to waning immunity with time rather than the delta variant escaping vaccine protection," the researchers wrote.
A potential limitation of the study is its lack of data on whether participants were adhering to masking guidelines and social distancing protocols.