Novavax's (NVAX -4.82%) long wait is over. After multiple delays, the U.S. Food and Drug Administration (FDA) finally granted Emergency Use Authorization (EUA) to the company's COVID-19 vaccine on Wednesday. 

You might think that the good news would have caused Novavax's shares to skyrocket. Instead, the vaccine stock sank like a brick on Thursday. What happened? Here are five reasons why Novavax stock crashed after winning U.S. authorization.

1. News about potential side effects

A Reuters article published on Thursday was probably the biggest cause of Novavax's big slide. This report revealed that the European Medicines Agency (EMA) determined that severe allergic reaction and "unusual or decreased feeling in the skin" are potential side effects of Nuvaxovid, the brand name under which Novavax markets its COVID-19 vaccine in Europe.

Should investors have panicked over this news? Not really. Severe allergic reactions can be side effects of many medicines and vaccines.

Also, the cases of abnormal sensation or decrease of sensation in the skin have been rare. The EMA stated that only 256 reports of these side effects have been made worldwide so far out of more than 1.5 million vaccine doses distributed. 

2. Slow uptake in Europe

Investors also might have been rattled by the very slow uptake of Nuvaxovid in Europe so far. Reuters noted on Thursday that European Centre for Disease Prevention and Control data shows that only 250,000 doses of Novavax's vaccine have been administered in Europe since December 2021. 

There's perhaps a simple reason for this underwhelming number, though. Nuvaxovid hasn't received authorization as a booster in the European Union (EU) yet and only recently won authorization as a primary vaccine in adolescents. Nearly 73% of individuals in EU/European Economic Area countries have already received their primary course of vaccination.

3. "Buy the rumor, sell the news"

Most investors have heard the old adage, "Buy the rumor, sell the news." The idea is that stocks rise before any anticipated good news is reported but fall afterward. That could be happening with Novavax at least to some extent.

The stock had been sizzling hot in recent weeks, partially due to other positive developments in the EU and other countries. Although there was still some uncertainty about the FDA's EUA decision, most investors were probably expecting a thumbs-up after the FDA's advisory committee unanimously recommended the authorization of Novavax's COVID-19 vaccine.

4. Other hurdles to jump over

To be sure, receiving EUA in the U.S. was good news for Novavax. However, savvy investors realized that there are still other hurdles the company must overcome to achieve significant success in the U.S. market.

The FDA authorized Novavax's COVID-19 vaccine only for adults ages 18 and older and only for the primary two-dose series. But nearly 77% of Americans in that age range have already received their primary doses. Many of those who haven't could still be reluctant to receive any COVID-19 vaccine -- even one that's based on the tried-and-true protein sub-unit approach used by Novavax.

5. A major down day for the overall market

Thursday was also a horrible day for the overall stock market. All of the major market indexes were down quite a bit. Investors worried about the prospects of more aggressive interest rate hikes after Wednesday's report that inflation reached a 40-year high in June.

It's difficult for any stock to perform well when the market is sinking. Granted, this is probably the least important factor behind Novavax's crash. However, investors might not have reacted nearly as negatively to the Reuters report if the overall market surged on Thursday.

Novavax could swim against the stream if it wins additional authorizations for boosters around the world (and especially in the U.S.). However, for now, the company and its shareholders must still wait.