Investors have been anxious this week, creating stock market volatility. That hurt the Nasdaq Composite (^IXIC 2.48%), which was down about 0.3% as of 11:15 a.m. EDT on Friday.

Yet the biggest news on the Nasdaq came from a big winner in the biotech field. Many investors have looked at Moderna (MRNA 11.09%) as the ultimate COVID-19 vaccine success story from an investment perspective, as its vaccines have already helped tens of millions of people around the world. On Friday, Moderna's stock got a boost from a move that has nothing to do with its fundamental business -- and it would never have happened had it not been for the decision of one of its biggest rivals on the vaccine stock front.

Medical professional gives a shot to a patient.

Image source: Getty Images.

S&P makes its latest move

Moderna's stock was up almost 9% late Friday morning. The move came amid news that the biotech will join one of the world's most-followed stock benchmarks.

S&P Dow Jones Indices announced that Moderna will be added to the S&P 500 Index effective before the beginning of trading on Wednesday, July 21. The addition makes sense intuitively, given that the vaccine maker's market capitalization has shot well above $100 billion after its recent success.

Interestingly, it was another COVID vaccine maker that set the stage for Moderna's invitation into the elite index. It will replace Alexion Pharmaceuticals (ALXN), whose $40 billion market capitalization stems largely from its Soliris treatment for generalized myasthenia gravis.

The replacement isn't happening because S&P has any problem with Alexion. It's because AstraZeneca (AZN 0.21%) is set to close on its long-planned acquisition of Alexion in the near future. S&P routinely makes additions and deletions to its index products when mergers and acquisitions are imminent.

Old vs. new

The irony, of course, is that AstraZeneca is also a key provider of a COVID vaccine. It hasn't been high on the radar of U.S. investors because the company hasn't gotten Emergency Use Authorization (EUA) from the Food and Drug Administration for its COVID vaccine. Indeed, AstraZeneca is now contemplating skipping the EUA process entirely and simply requesting full approval for its vaccine from the FDA.

That didn't stop the U.S. government from committing to buy hundreds of millions of doses of AstraZeneca's vaccine. But without approval to use it in the U.S., the government has shipped it to other countries, sometimes as a donation for those in need. Moreover, troubling side effects have raised controversies in other areas where the AstraZeneca vaccine has seen wider use.

It's important for investors to remember, though, that AstraZeneca has a lot more going on than just its COVID vaccine development. The drugmaker already has a lot of unrelated treatments on the market, including blockbusters like oncology drug Tagrisso and asthma/COPD fighter Symbicort.

By contrast, Moderna's COVID vaccine has been its introduction to the market. It has a pipeline of other candidates and a promising future, but Moderna hasn't proved itself the same way that AstraZeneca has.

What's next for Moderna?

Unlike with some high-profile additions to the S&P, investors aren't going to have to wait a month for Moderna to be added to the index. That should minimize the artificial impact of the addition on the stock price. The company can expect to see its stock keep trading on the prospects for its vaccine and future products.