In this April 2, 2020 file photo, an adult and a child, both wearing face masks amid the coronavirus outbreak, carry bags in the Chinatown neighborhood of Los Angeles. (DAMIAN DOVARGANES / AP)

After watching the number of confirmed coronavirus cases in the Los Angeles area escalate and braving a few trips to grocery stores, Cindy Wu decided to schedule food delivery online, but that was no easy task.

"There was just no availability for food delivery on the websites of a lot of the major grocery outlets," said Wu, of Alhambra, California. She is a member of Amazon Prime and has been trying for a week to book a delivery slot on Amazon Fresh. By the time she got a slot, half of the items in her online cart were no longer available.

Amazon Fresh had this message displayed on its front page: "Inventory and delivery may be temporarily unavailable due to increased demand. New windows are released during the day"

The demand for grocery delivery is facing a crunch as cities and states across the United States mandate or recommend people stay at home, and more consumers choose to have groceries delivered to their doors instead of risking exposure in public.

A quick search recently on the grocery delivery service Instacart revealed that no delivery options were available for the next three days for an address in Arcadia, California, near Alhambra. A message on Shipt asked the customer to "check back later" as no more delivery windows were available on Friday to the same address.

On the website of Vons, a Southern California supermarket chain with some locations in Nevada, delivery options for the next three days have all been unavailable.

Amazon Fresh had this message displayed on its front page: "Inventory and delivery may be temporarily unavailable due to increased demand. New windows are released during the day."

Jay Carney, senior vice-president of global corporate affairs at Amazon, told CNN in a recent interview: "It feels like we are essential to so many Americans－millions and millions around the country who are depending on Amazon and other services like it to deliver things that they might otherwise have felt uncomfortable getting at a physical store.

"Amazon's a big e-commerce company, but e-commerce is still only about 10 or 11 percent of retail. During this crisis, I think people are relying on home delivery, getting things at their doorsteps, and that's why we have to call on our employees to just do heroic things, to work really hard, to add hours and then to hire more, just get Americans what they need."

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Record surge in demand

San Francisco-based Instacart said it has seen the highest customer demand in its history.

"In the last few weeks, the company has seen order volume grow by more than 150 percent year-over-year, with average customer basket size also increasing by 15 percent," the company said in a statement.

"Based on the current surge in demand, customers can expect to see delivery availability vary across stores during the busiest request windows."

The company encouraged customers to check their app frequently for available delivery times.

Instacart recently announced plans to hire 300,000 more full-service employees over the next three months in response to increased demand. The company said it also has increased paid sick leave to 14 days for workers in quarantine and raised bonus payments.

Instacart is hiring 30,000 new employees, including delivery drivers and personal shoppers, to help meet the demand.

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A spokeswoman for Walmart, the world's largest retailer, said that the US multinational company is "seeing an increase in the number of orders coming in for our pickup and delivery services along with customers in our stores".

According to the marketing research firm Nielsen, only 4 percent of grocery sales in the country came from online purchases last year. As of March 15, the number of downloads of the Instacart app increased 218 percent over last year, and Walmart's grocery app was up 160 percent. Data show that many new online subscribers are aged over 60, researchers said.