Costco Wholesale (NASDAQ:COST) capitalized on rapidly changing consumer habits to post impressive sales growth during 2020. The leading warehouse club's low prices and its broad selection of essentials and discretionary items helped it gain market share during the COVID-19 pandemic.

The rollout of coronavirus vaccines has already started to tame the pandemic. As a result, some investors worry that Costco could abruptly run out of momentum this year. Yet despite increasingly difficult year-over-year comparisons, the retail titan continues to grow sales at a double-digit pace, suggesting that the bears' concerns are overblown.

A year of stellar sales growth

Costco's sales first accelerated because of the COVID-19 pandemic in February 2020, as consumers began stocking up on food and other essentials. Adjusted comparable sales (excluding the impact of changes in gasoline prices and foreign currency fluctuations) jumped 11.7% in that month, compared with a 5.3% increase in January 2020.

Indeed, whereas Costco's adjusted comparable sales rose 6.1% between September 2019 and January 2020 -- equivalent to the first five months of its 2020 fiscal year -- Costco averaged double-digit comp sales growth for the remainder of fiscal 2020, which ended last August. As a result, it ended the fiscal year with a full-year adjusted comp sales gain of 9.2%.

The entrance to a Costco warehouse

Image source: Costco Wholesale.

Costco's sales momentum accelerated last fall. In the first five months of fiscal 2021, adjusted comparable sales surged 14.7%. However, during this period -- from September 2020 to January 2021 -- Costco faced relatively easy year-over-year comparisons.

The retail giant finally started to lap last year's pandemic-driven acceleration in sales during February. Nevertheless, Costco has continued to report impressive sales results.

Sales growth holds up as comparisons get tougher

Costco faced its first difficult year-over-year comparison in February. The company took it in stride, delivering a 12.3% adjusted comp sales increase. Sales growth did slow for food and sundries: the items that consumers were most likely to stock up on a year ago. However, sales of fresh foods and non-food items remained buoyant, jumping more than 20% year over year.

Last week, Costco reported that comparable sales growth for fresh foods slowed to a single-digit pace in March. Moreover, sales of food and sundries decreased. This reflected extremely tough comparisons against March 2020, when people rushed to fill their pantries as the pandemic spiraled out of control.

Nevertheless, Costco's adjusted comparable sales rose 11.1% in March, thanks to a 50%-plus surge in sales of non-food merchandise, which consumers had shunned a year ago. Management also noted that the timing of Easter negatively impacted sales last month. On an apples-to-apples basis, adjusted comp sales would have increased nearly 13%.

A great sign for investors

Costco's ability to continue posting double-digit comparable sales growth even as it laps stronger sales results from 2020 bodes well for the future. The company is on track to report stellar sales growth this month, as adjusted comparable sales declined slightly in the year-ago period. And while Costco will face its toughest year-over-year comparisons in the summer and fall, its strong February and March sales results should give investors confidence that the retailer will keep posting positive comps throughout the year ahead.

After a strong run in the second half of last year, Costco stock faltered in early 2021. Investors seemed to worry that Costco would suffer as economies reopen. The company's earnings also fell short of analysts' expectations last quarter (largely due to short-term pressure on the profitability of its gasoline business).

COST Chart

However, Costco stock has bounced back in recent weeks. The stock is somewhat pricey, trading for more than 36 times the company's projected earnings for fiscal 2021. Still, Costco's robust sales momentum, deep moat, and ample expansion opportunities should allow it to post strong earnings growth for many years to come. That translates to plenty of upside for long-term investors.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.