A study appeared in the Lancet this week confirming that vaccine effectiveness against infection is fading fast. The study involved 3,436,957 people over the age of 12 who are members of the healthcare organisation Kaiser Permanente Southern California. It sought to assess the effectiveness of the Pfizer vaccine against SARS-CoV-2 infections and COVID-19-related hospital admissions for up to six months, with a study period covering December 14th, 2020, to August 8th, 2021. Comparing fully vaccinated to unvaccinated, and controlling for confounders such as prior infection, the researchers found that effectiveness against infection plummeted from 88% (95% confidence interval 86-89%) during the first month after double-vaccination to 47% (43-51%) after five months. The variation by age (depicted above) was largely within the margins of error. Among sequenced infections, the researchers found vaccine effectiveness against Delta infection was 93% (85-97%) during the first month after double-vaccination but dropped to 53% (39-65%) after four months. Effectiveness against infection from other variants the first month after double-vaccination was 97% (95-99%), but declined to 67% (45-80%) at 4-5 months.
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