What happened

Shares of defense and aerospace giant Boeing (BA 1.51%) closed 3.7% lower on Monday, a day that saw shares of the commercial plane-builder's customers such as airlines American and United fall as much as 6%.

Covid was the reason. Lately, it seems Covid is almost always the reason.

Three red airplanes leaving contrails pointing down.

Image source: Getty Images.

So what

Over the weekend, Presidential Chief Medical Advisor Dr. Anthony Fauci warned Americans about the COVID-19 Omicron variant's ability to evade immunity conferred by existing COVID-19 vaccines, even as medical researchers in Israel reported their own findings that "three shots [of the] Pfizer COVID [vaccine were] 4x less effective against Omicron than Delta."  

Rounding out the bad news, on Monday the World Health Organization (WHO) cautioned that the variant poses a "very high" risk not just in America but all around the world.  

Now what

Commenting on the falling airline stocks this morning, my fellow Fool and airlines specialist Lou Whiteman warned that even in the face of "post-pandemic pent-up demand ... we are still looking at years before the industry fully recovers from the crisis."

Faced with this prospect, it would make sense for airlines to postpone planned purchases of airplanes from Boeing and Airbus, and step down their pace of placing new orders for new airplanes as well. That would of course be bad news for Boeing stock; the company's commercial airplanes division has already racked up two straight years of negative earnings and is en route to a third.  

Although analysts remain optimistic that Boeing's business will be back in the black in 2022, Omicron still holds the potential to upset Boeing's apple cart for a fourth year in a row.