Moderna reigned supreme this month in what was a solid August for stocks ahead of the expected September doldrums.

The best-performing stock in the S&P 500 for August was Moderna, with shares of the biopharmaceutical company surging 158%.

Shares nearly tripled on Aug. 19 after Moderna and Merck reported positive trial results for their personalized cancer vaccine, intismeran autogene. The surge marked Moderna's largest single-day gain ever and the biggest move for any stock in the benchmark index over the past 25 years. The breakthrough comes at a crucial time for Moderna as it works to prove to investors it can build a sustainable business beyond respiratory vaccines.

The other four stocks rounding out the five best-performing S&P 500 names for the month were Palantir Technologies, Salesforce, Veeva Systems, and Paramount Skydance.

As for the worst-performing stock? That was California-based utility Edison International.

Shares of Edison International declined 27% in August, with the bulk of that loss coming Monday. The stock fell 23% on Monday, putting it on pace for its worst trading session in more than 25 years, according to Dow Jones Market Data.

The severe investor reaction followed the California State Assembly's decision over the weekend to amend a key wildfire bill and not include liability protection for utilities. Analysts noted that the legislation doesn't protect investors and that there's "significant downside" potential for California utilities "in the event of multiple catastrophic wildfires."

Rounding out the five worst-performing S&P 500 stocks were DaVita, Trade Desk, Honeywell Aerospace, and PG&E.