What happened

Shares of coronavirus vaccine hero Moderna (MRNA 3.06%) won a new round of applause this morning, rising 10.4% through 10:10 a.m. EDT.

The reason: Moderna stock is joining the S&P 500.

Arrow angles up on a green stock chart

Image source: Getty Images.

So what

The news broke this morning. With Alexion Pharmaceuticals (ALXN) due to be absorbed into AstraZeneca (AZN 0.69%) soon, a slot is opening up in the S&P 500 index of America's largest publicly traded companies. Moderna has been tapped to fill that spot, and will become part of the index just before trading begins on Wednesday, July 21.  

When this happens, mutual funds and ETFs that track the performance of the S&P 500 will need to buy shares of Moderna in order to accurately reflect the index's composition. Knowing that these companies will be forced to buy Moderna, other investors are front-running that need today, buying the stock in anticipation that the price will get bid up next week.

Now what

Well and good. Traders gonna trade, and if they want to buy Moderna today in hopes of earning a quick profit next week, that's certainly their right. (Although one wonders how much profit remains to be made -- how big a bump will the stock really get next week, after it has already jumped today).

Longer-term investors, meanwhile, have a different, more substantive, and potentially even more lucrative catalyst to look forward to. On Thursday, Aug. 5, Moderna will report its earnings for the fiscal second quarter of 2021. Analysts are forecasting that the vaccine maker will reverse a year-ago loss to record $6.04 EPS on sales of nearly $4.3 billion this quarter -- revenue growth of more than 6,000%!