The ideal coronavirus vaccine will be a single shot that is effective, easy to ship, stable at room temperature and have low side effects, especially in vulnerable populations. But no single candidate has checked all of those boxes despite strong efficacy data coming from the two front runners.

Pfizer Inc., along with partner BioNTech SE, and Moderna Inc. have each found their vaccines to be around 95% effective. While both have cold storage requirements, Pfizer's candidate must be stored in ultra-low freezers that reach -70 degrees Celsius for long-term storage. Moderna's vaccine is stable for longer at higher temperatures and can be stored in a regular refrigerator. Both require two doses.

Another leading vaccine candidate from healthcare giant Johnson & Johnson is being tested as a single dose, but has fallen behind in the race after a phase 3 trial was paused following an "unexplained illness" in a trial participant.

"We just don't know who's going to win," said Louise Chen, managing director at Cantor Fitzgerald. "One of the things that people ask me is, should we just ignore all these smaller players now that we've got 90% efficacy? I don't think so. Because you just don't know who's going to come up with a vaccine that's ideal for sort of mass vaccination that can have the durability to last over the longer term."

Chen does not anticipate J&J's vaccine will reach 90% efficacy based on results from a phase 1/2 trial.

One dose versus first out

Even with the high effectiveness of Pfizer-BioNTech and Moderna's candidates, some experts worry the double dose will discourage people from getting the vaccine because of the inconvenience.

Like J&J, Merck & Co. Inc. has been conducting a trial for a single-dose vaccine, V590, as well as comparing one and two doses of its other vaccine candidate V591. The company has also been testing an oral drug for the treatment of COVID-19 with Ridgeback Biotherapeutics LP.

A single shot may have the advantage of being more convenient, but David Kaplan, director of healthcare corporate ratings at S&P Global Ratings, still sees a lot of value in being the first available. S&P Global Ratings and S&P Global Market Intelligence are owned by S&P Global Inc.

"When you're first, you get the resources. You can lock up manufacturing capacity," Kaplan said in an interview. "Second, once you have approved vaccines, why would somebody want to go into a trial that gives them a 50% chance of getting a vaccine that may or may not work and 50% chance of getting a placebo if there's already a viable product that's available?"

Kaplan also thinks the 90% efficacy is a strong selling point for the front runners and may be more important than some of the other factors.

"I think efficacy is probably the number one priority. I can imagine among the elderly, where they're really at risk of being hospitalized or of this disease being fatal, the efficacy comes before everything," Kaplan said.

While the early results from Pfizer and Moderna have certainly been positive, experts have pointed out that crucial safety data and detailed efficacy in at-risk populations and in children is still missing.

Rolling up your sleeves

Two or three years down the road, one vaccine is likely to emerge as the winner in the COVID-19 vaccine market as the initial need for millions of doses subsides, according to Chen. Due to the ultra-cold storage requirements, Chen doubts Pfizer's BNT162b2 will be that winner.

Jeylan Mammadova, global sector lead at investment research firm Third Bridge, thinks that Pfizer's vaccine could find a niche in larger cities because of its complex storage requirements, whereas Moderna becomes the vaccine of choice in more rural areas through community drugstores and pharmacies.

Other COVID-19 vaccine candidates like CureVac NV's mRNA vaccine CVnCoV and Inovio Pharmaceuticals Inc.'s DNA vaccine INO-4800 are stable at more convenient temperatures and for longer, even though they are further behind in clinical testing. Pfizer is also working on a powdered version of the vaccine that can be stored at different temperatures. BioNTech executives said in a third-quarter earnings call that the companies are continuing to test the current vaccine's stability with the next batch of data expected in mid-December.

Vaccines with lower efficacy could find their way into the market in other ways, with younger populations, patients who are uncomfortable with multiple vaccinations or the newer mRNA technology that Pfizer and Moderna are using, according to Kaplan.

The science is also still unclear on how long the vaccines will last, Kaplan noted. If a vaccine that is 70% effective provides immunity for 10 years while a 90% effective vaccine requires redosing every year, people could be swayed toward the less effective vaccine.

Mammadova pointed to the race between Merck & Co. and GlaxoSmithKline PLC to develop a vaccine for the shingles virus. Zostavax, which was licensed by the U.S. Food and Drug Administration in 2006, had higher local reactions with fever and muscle pain. Shingrix easily overtook the competitor once approved in 2017 because it had fewer side effects. As of Nov. 18, 2020, Zostavax is no longer available for use in the U.S., while Shingrix brought in £1.8 billion for GSK in 2019.

Before clear safety and efficacy data was released on Pfizer and Moderna's vaccines, a Gallup poll found that 58% of Americans are willing to get a COVID-19 vaccine.

"If you look at what we've all been through over the last eight, nine months, a lot of people are ready to roll up their sleeves, even if it means a little bit higher price or other things," Kaplan said. "There's going to be a lot of demand for this for very understandable reasons."