What happened

Shares of BioNTech (BNTX 0.58%) were 9.1% lower as of 11:51 a.m. EDT on Thursday. The German biotech didn't report any news, so why did its stock slide? You can probably blame Moderna (MRNA 1.69%).

Moderna announced its third-quarter results before the market opened on Thursday, and revenue and earnings were well below Wall Street's expectations. The company also lowered its full-year 2021 revenue outlook.

This disappointing report stemmed in large part from Moderna shifting deliveries of its COVID-19 vaccine from the second half of 2021 into 2022. The company's woes weighed on other vaccine stocks as well.

A healthcare professional giving a shot to a person.

Image source: Getty Images.

So what

From an objective point of view, there's no reason BioNTech's shares should have dropped because of Moderna's problems. BioNTech's partner, Pfizer (PFE 0.55%), increased the 2021 sales guidance for their COVID vaccine, Comirnaty, in its third-quarter update earlier this week. 

But stocks that share common denominators sometimes move in sync. These sympathy plays don't always make sense, though. Today's decline for BioNTech is one of those cases.

The good news for investors is that these irrational moves can often present buying opportunities. BioNTech's shares trade at only a little over six times expected earnings. If you think that sales for Comirnaty will remain robust for years to come, today's sell-off makes the stock even more attractively valued.

Now what

Instead of focusing on Moderna's third-quarter results, investors are better off waiting to hear what BioNTech has to say in its own third-quarter update, scheduled for Nov. 9.