Pfizer (PFE 0.56%) stated earlier this month that it expects its COVID-19 vaccine will generate sales of close to $26 billion this year. More revenue is on the way beyond 2021. For example, Pfizer and its partner BioNTech (BNTX -1.47%) recently announced a new deal with the European Union to supply up to 1.8 billion additional doses through 2023.

However, the dynamics of the COVID-19 vaccine market are constantly changing. Last week, researchers in Spain reported results from a clinical study that included Pfizer's COVID-19 vaccine. Could these results be bad news for future sales of Pfizer's vaccine?

Vaccine vials lined up to form a dollar sign.

Image source: Getty Images.

A mix-and-match milestone

Scientists have wondered what would happen if the initial dose of one COVID-19 vaccine was followed by a second dose from another vaccine. A few so-called "mix-and-match" clinical studies have been undertaken to find the answer. On May 18, the first results from one of those studies were announced.

Researchers at the Carlos III Health Institute in Spain recruited participants for the CombiVacS clinical study who had received only a single dose of AstraZeneca's (AZN -1.48%) vaccine within the previous eight weeks. Instead of administering the second dose of AstraZeneca's vaccine, the Spanish team gave the participants Pfizer's and BioNTech's COVID-19 vaccine instead.

What happened? Participants receiving a first dose of the AstraZeneca vaccine followed by a second dose of the Pfizer vaccine experienced immune system responses that were "greatly enhanced." The researchers reported that antibody levels for this group were at least 40 times higher than for individuals who only received a single dose of AstraZeneca's vaccine. These antibody levels were also higher than those reported for individuals who received two doses of the Pfizer vaccine.

Side effects were similar to those observed in the past with the Pfizer vaccine. There were no adverse effects that required special medical attention or hospitalization. The most common side effects were headache, malaise, nausea, cough, and fever, most of which were mild cases.

Bad for Pfizer?

It's easy to see how these mix-and-match results might negatively impact the sales of Pfizer's vaccine. If a combination of one dose of the AstraZeneca vaccine with a second dose of the Pfizer vaccine is more effective than two doses of the Pfizer vaccine, governments could want to take this approach. In theory, that could slash Pfizer's sales in half.

But don't put the cart before the horse: The Spanish clinical study hasn't been peer-reviewed yet. Also, the study itself was relatively small -- only 673 people in total, with 443 receiving the mix-and-match doses and 232 only receiving the first AstraZeneca dose.

The European Union isn't likely to want to go with a mix of AstraZeneca and Pfizer vaccines, however. The EU is suing AstraZeneca over allegedly failing to meet its supply commitments. This dispute played a major role in Pfizer and BioNTech winning a big supply deal that covers the next couple of years.

Even if the findings made by the Spanish researchers are eventually confirmed, it could still be too late to make much of a difference. There's a good chance that in future, only single-dose annual vaccinations will be required. If that's the case, any advantages offered by a two-dose regimen, including one combining AstraZeneca's and Pfizer's vaccines, could be irrelevant.

What really matters

Results from the mix-and-match studies probably won't present major problems for Pfizer (or for any of the other leading vaccine makers, for that matter). What really matters for the company is how frequently booster doses will be needed.

The answer to that question probably won't be answered until sometime this fall. Meanwhile, investors appear to be discounting the likelihood that Pfizer will be able to count on huge recurring revenue for years to come: The big pharma stock trades at only around 12 times expected earnings. Should COVID-19 vaccines be needed at least once per year from here on out, Pfizer could be viewed, in retrospect, as a bargain at its current price.