Amazon's (NASDAQ:AMZN) Prime Day, which started in 2015 as a sale commemorating the website's 20th anniversary, subsequently became an annual shopping holiday like Black Friday.

The first Prime Day blindsided Amazon's rivals, but Walmart (NYSE:WMT), Best Buy (NYSE:BBY), eBay (NASDAQ:EBAY), and others now launch "Anti-Prime Day" sales to counter the retail juggernaut. However, Edison Trends, which analyzed 60,000 transactions at Amazon and its rivals on July 15 and 16, recently reported that Amazon still accounted for 86.7% of all online spending during the Prime Day window.

Online spending by retailer during Prime Day 2019.

Data source: Edison Trends. Chart by author.

Edison Trends also reviewed 120,000 Prime Day transactions and found that the average hourly spend of U.S. Amazon customers rose 35% this year. Amazon's extension of Prime Day from a 36-hour event in 2018 to a 48-hour one this year likely boosted those engagement rates.

That all sounds like dire news for Amazon's rivals. However, research firm Captify found that product searches on Walmart, eBay, and Best Buy surged 130%, 72%, and 255%, respectively, during the first 24 hours of Prime Day. Amazon's still the clear winner here, but Prime Day could be creating a halo effect for retailers across the board.