The eight-month-long study is comprised of about 800,000 veterans — about 500,000 of whom were vaccinated.
"I think that it is a little bit concerning that we've seen some decrease in effectiveness over time, but I wouldn't say it was completely unexpected," said Allina Health's Vice President of Medical Operations for Primary Care Dr. Kevin Best.
The study says immune systems and their levels of defense can wane over time. Plus, the highly transmissible delta variant may contribute to the vaccine's falling effectiveness.