The advent of COVID-19, with its stay-at-home orders keeping workers from eating out and its social distancing recommendations making them leery of running to the grocery store, is jump-starting the online grocery shopping business.

Some retailers are struggling to adjust to the sea change, with (AMZN -0.17%) apparently overwhelmed by the sudden popularity of its Amazon Fresh and Whole Foods online ordering services, and unable to keep up with demand. Still, analysts forecast that as much as $20 billion in online grocery business could be up for grabs for the companies that manage to figure this out.

Bag full of groceries sitting on a laptop keyboard

Image source: Getty Images.

Walmart (WMT 0.83%) could be the biggest beneficiary.

According to a report just out from App Annie, the self-described "leading global provider of mobile market data," the Walmart Grocery app is now the No. 1 most downloaded app for ordering groceries online.  

From March 29 to April 4, 2020, reports the analyst, consumers downloaded 106 million shopping apps, up 15% from the equivalent period in January 2020, before the coronavirus appeared on the scene.

Of those downloads, 14.4 million happened in the U.S. (up 20% from January), and Walmart Grocery downloads in particular shot up 460%. Downloads of the Me@Walmart app, used by in-store workers, were also up significantly -- 220%.

In contrast, Amazon's shopping app, the second most downloaded shopping app which began the period with "an incredibly high existing install base," lagged Walmart's pace of downloads by 20 percentage points.