What happened

Shares of three leading COVID-19 vaccine makers sank on Thursday. As of the market close, BioNTech (BNTX -2.55%) stock was down 8.8%. Shares of Moderna (MRNA 0.17%) and Novavax (NVAX -9.65%) fell 5.8% and 4.2%, respectively.

None of the companies announced any developments. However, there were a couple of stories that were likely related to the vaccine stocks sinking.

CNBC reported that some scientists are criticizing the U.S. government's decision to move forward with third booster doses of messenger RNA (mRNA) vaccines as premature. Also, results from a large U.K. study were published online. Those results showed that the effectiveness against the delta variant for the COVID-19 vaccines developed by Pfizer (PFE -0.97%) and BioNTech and by AstraZeneca falls off quickly.

Line chart trending down with a coronavirus and stock prices in the background.

Image source: Getty Images.

So what

Some investors could fret that the U.S. might reconsider its position on third booster doses if there's too much pushback from the scientific community. That could potentially impact BioNTech and Moderna. A rapid loss of efficacy against the delta variant could also make the Pfizer-BioNTech vaccine less attractive than other COVID-19 vaccines.

However, it seems highly unlikely that the Biden administration will change its mind on booster doses in light of the ongoing surge in COVID-19 cases. The U.K. study actually bolsters the argument that booster doses could be necessary.

Regardless, the U.S. has already agreed to buy enough doses of the Pfizer-BioNTech and Moderna vaccines to provide third booster doses to all fully vaccinated Americans. And Novavax arguably shouldn't be impacted all that much by any of this news. 

Now what

Many of the moves in these stocks on Thursday were likely more noise than anything significant. There are things to watch with each of the three companies, though, that could cause the stocks to move either higher or lower.

For BioNTech and Moderna, the key catalysts on the way are potential new supply deals for 2022 and beyond as well as the possibility of expanded U.S. authorizations for younger children. The main potential catalysts for Novavax are the anticipated authorizations over the next few months of its COVID-19 vaccine in several countries, including the U.S.