The delta variant of the coronavirus continues to worry investors. The Dow Jones Industrial Average (DJINDICES:^DJI) is down on Aug. 9, falling from Friday's record high even as many companies continue to report strong earnings for the second quarter. Dow stocks that are down today include American Express (NYSE:AXP), which is the second-best-performing Dow stock so far in 2021, up 41.6% at this writing.

Fellow consumer credit and electronic payments giant Visa (NYSE:V) is also down today, along with oil giant Chevron (NYSE:CVX) on another crude oil sell-off tied to worries that the coronavirus surge will impact demand more sharply than expected. Goldman Sachs (NYSE:GS) shares are bucking the trend, up 1.3% today. But these gains from the best Dow stock of 2021 (up 52%) aren't enough to offset a down day for most of the index. 

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Despite strong earnings, economic indicators, delta variant keeping investors on edge

By and large, this earnings season has been great. Visa reported revenue was up 27% in the second quarter in late July, and U.S. payments volume had surpassed 2019 (i.e., pre-pandemic) levels. American Express reported similarly strong results, with revenue and transaction volumes -- with the notable exceptions of travel and entertainment -- largely back to pre-pandemic levels in many of its markets. 

Economic indicators have also continued to point in a positive direction. We saw this firsthand on Friday when the Dow moved to a new record high following a great July jobs report. The prior day's weekly unemployment report was similarly positive, indicating a continued trend in the right direction: 

US Initial Claims for Unemployment Insurance Chart

US Initial Claims for Unemployment Insurance data by YCharts

But the numbers investors seem to be paying more attention to right now are the ones related to the surge in coronavirus cases. In recent days, the number of confirmed U.S. cases has surged to more than 100,000 per day, levels we have not seen since before wide-scale availability of vaccines near the beginning of the year. Hospitalizations are also on the rise:

US Coronavirus Cases Per Day Chart

US Coronavirus Cases Per Day data by YCharts

Falling oil prices, mixed stocks underpin investor worries

The rising number of coronavirus cases, along with the surge in hospitalizations -- which has lagged the surge in cases by a couple of weeks -- is a significant source of concern. Crude oil prices are down another 3% today as China and other countries ponder potential lockdowns on travel to slow the spread of the delta variant. The crude oil sell-off is weighing on Chevron today, as investors ponder whether oil prices will continue to decline. Crude prices have fallen all but one trading day in August. 

In parts of the U.S., particularly areas with lower rates of coronavirus vaccination, healthcare systems are already feeling the strain of the surge. Hospitalizations tend to lag coronavirus diagnoses, so it's likely that the next few weeks will see even more people hospitalized, pressuring officials to act to throttle the spread. And that's weighing on what would otherwise be a positive earnings season. 

As a result, investors are caught in between. Will the solid results keep coming in, or will the delta variant derail the recovery and cause stocks to crash? That's the question investors want answered the most. 

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.