Largest COVID-19 Vaccine Study Finds Waning Effectiveness Over Time

A major new study from the University of Waterloo has shed light on the real-world effectiveness of COVID-19 vaccines over time. The observational study, conducted from 2024 to 2025, examined data on 1.8 million Nebraska residents to assess how the vaccines have been holding up against new virus variants.

The researchers found the latest COVID-19 vaccines to be highly effective, especially in preventing severe outcomes like hospitalization and death. Four weeks after vaccination, the vaccines provided 44.7% effectiveness against infection, 45.1% against emergency department visits, and 57.5% against hospitalization or death. However, this protection began to wane quickly, declining to 35% effectiveness against infection at 10 weeks and just 16.7% by 20 weeks.

"This is the largest study I know so far about this vaccine," said lead author Yangjianchen Xu, an assistant professor at Waterloo. "It's important to have that sort of large sample to accurately estimate the effectiveness of the vaccine." The study was conducted in collaboration with researchers from the University of North Carolina and the University of Nebraska Medical Center.

As the COVID-19 virus continues to evolve, the researchers expect new versions of the vaccine will be developed. They plan to continue the study to assess the effectiveness of these updated shots as they become available. Public health officials are urging people to stay up-to-date on their COVID-19 vaccinations, especially as colder weather approaches and virus transmission risks increase.