Four countries have either paused or discouraged the use of Moderna's (MRNA -2.53%) Spikevax COVID-19 vaccine in some age groups. Sweden won't give the messenger RNA (mRNA) vaccine to anyone under 30. Finland is suspending the use of Moderna's vaccine to men under 30. Denmark won't give Spikevax to anyone under 18. Meanwhile, Norway is discouraging anyone under 30 from receiving Moderna's vaccine.

The Nordic countries have made these moves because of potential safety issues found in a study conducted by Sweden's Public Health Agency. The agency determined that Moderna's COVID-19 vaccine could cause inflammation of the heart muscle or pericardium (the membrane enclosing the heart) in rare cases. 

There is another mRNA vaccine on the market in all four countries -- Pfizer's (PFE -2.09%) Comirnaty. How could Pfizer potentially be affected by Moderna's vaccine safety issues?

A masked healthcare professional giving a shot to a masked patient.

Image source: Getty Images.

The go-to alternative

Finland healthcare authorities plan to offer Comirnaty to young men under the age 30 instead of Spikevax. Norway is strongly encouraging anyone under age 30 to get the vaccine developed by Pfizer and its partner BioNTech (BNTX -0.48%) instead of Moderna's vaccine.

Comirnaty has basically become the go-to alternative to Moderna's vaccine for the affected demographic groups. But why is that the case since both Comirnaty and Spikevax use similar approaches?

To be sure, there have been some reported cases of heart inflammation with the Pfizer-BioNTech vaccine also. However, those reports are less frequent than with Moderna's vaccine. Two recent studies conducted in Israel found that the risk of heart inflammation in younger men from Comirnaty is very low, with one study suggesting a probability of around one in 50,000. 

Perhaps the most important difference between the two mRNA vaccines is the dosage. Moderna's 100-microgram dose is more than triple that of Pfizer's 30-microgram dose. 

Minimal financial impact

The decisions by several countries to suspend or discourage the use of Spikevax in certain cases took a toll on Moderna stock, with shares dropping last week on the news. However, the negative financial impact on Moderna and the positive financial impact on Pfizer will probably be minimal.

Both companies have already locked in supply deals with the European Commission for European Union countries (including Denmark, Finland, and Sweden). They also have smaller separate supply agreements with Norway, which isn't a member of the EU. 

Moderna has received orders from the European Commission for 460 million doses of Spikevax so far. At least 150 million of those doses are scheduled for delivery in 2022. Pfizer and BioNTech are supplying 600 million doses of Comirnaty to the European Union this year with another 900 million doses to be delivered in 2022 and 2023. 

Data from Sweden's study have already been sent to the European Medicines Agency to review. It's possible that other larger European countries could make similar moves as the Nordic countries have with Moderna's vaccine. But the supply agreements already in place won't be affected by those decisions.

The bigger picture

Could the potential safety issues with Moderna's vaccine make a difference in the fortunes of Moderna and Pfizer over the longer term? It's at least possible that countries could choose to order more doses of Comirnaty and less of Spikevax after current supply deals are fulfilled.

However, these reported safety issues are very uncommon. And safety is only one consideration for future supply agreements. Efficacy is also important. So far, the efficacy of Moderna's COVID-19 vaccine appears to be holding up more over a longer duration than that of the Pfizer-BioNTech vaccine.

Also, the dynamics of the COVID-19 vaccine market could change significantly. Both Pfizer and Moderna are developing newer vaccines that specifically target variants. Questions remain to how frequently boosters will be needed beyond 2022, if at all.

Probably the most likely scenario for these vaccine stocks is that the safety issues prompting responses by Nordic countries won't matter very much in the short term or the long term.