AstraZeneca (NASDAQ:AZN) recently became the third company in as many weeks to report positive interim results from a big randomized study with an experimental coronavirus vaccine.

How does AstraZeneca's candidate compare to experimental vaccines from Pfizer (NYSE:PFE) and Moderna (NASDAQ:MRNA) that are just a few steps ahead? As you might suspect, the different candidates have their own strengths and weaknesses. Here's how they stack up to each other based on what we've learned so far.

Person preparing a vaccine.

Images source: Getty Images.

Safety profile

The current vaccine candidates have similar safety levels.

AstraZeneca's candidate, AZD1222, uses a weakened virus derived from a common cold known to only infect chimpanzees. The nonreplicating viral vector delivers genetic instructions for spike proteins found on the surface of SARS-CoV-2, the virus responsible for COVID-19. Moderna's mRNA-1273 and Pfizer and its collaboration partner BioNTech's (NASDAQ:BNTX) BNT162b2 both deliver strands of messenger-RNA (mRNA) that instruct human cells to produce copies of the same spike protein. 

These vaccine candidates haven't given us reasons to worry about potential side effects, but just a handful of treatments using viral vectors have been approved by the Food and Drug Administration, and not one of those was a vaccine. Drugs that employ stands of mRNA are even less well understood because BNT162b2 and mRNA-1273 were the first to show clear signs of efficacy in a phase 3 study.

Efficacy

AstraZeneca performed its interim analysis after 131 positive cases of COVID-19 emerged among the first volunteers to join ongoing phase 3 studies in the U.K. and Brazil. Just 30 positive COVID-19 readings occurred among patients randomized to receive the vaccine, none severe enough to require hospitalization.

A group of around 2,700 volunteers who initially received half a dose of AZD1222 followed by a full-sized booster shot were 90% less likely to test positive than patients given a placebo. Unfortunately, the interim analysis included around 8,900 volunteers in the U.K. and Brazil who received two full doses.

For mysterious reasons, the vaccine only appeared 62% effective among patients given two full doses of AZD1222. A complete lack of COVID-19 hospitalizations among volunteers randomized to receive AZD1222 is an important achievement, but AstraZeneca still hasn't told us how many COVID-19 cases in the placebo group became severe.

The mRNA-based candidates are leading AstraZeneca's candidate in the efficacy department so far. Among the first 170 volunteers in Pfizer's phase 3 trial to test positive, just eight had been randomized to receive BNT162b2. Nine cases in the placebo group turned severe, compared to just one person given the actual vaccine candidate.

Among the first 95 volunteers to test positive in Moderna's pivotal coronavirus vaccine trial, just five were in the placebo group. This suggests mRNA-1273 is 94.5% effective at preventing people from testing positive for COVID-19, which is slightly less than the 95% efficacy score that Pfizer and BioNTech presented.

Before calling Pfizer's candidate the most effective, it's important to remember that one volunteer randomized to receive BNT162b2 developed a severe case of COVID-19. This didn't happen to any volunteers given mRNA-1273 or AZD1222.

Person receiving a vaccination.

Image source: Getty Images.

Logistics

There are clear logistical distinctions between the three most advanced coronavirus vaccine candidates. With strands of genetic information wrapped up in a viral vector, AstraZeneca's candidate is much easier to store and transport than vaccine candidates from Pfizer and Moderna. AZD1222 can be stored, transported, and handled for at least six months at normal refrigerator temperatures between 36 degrees and 46 degrees Fahrenheit.

Since BNT162b2 and mRNA-1273 are encapsulated in relatively unstable layers of lipid nanoparticles, they need to be kept much colder than AZD1222. Pfizer's mRNA-based vaccine candidate must be kept at an ultrafrigid minus 94 degrees Fahrenheit. Once pulled out of its deep freeze, BNT162b2 must be discarded if it sits at regular refrigerator temperatures for longer than 24 hours.

In the logistics department, Moderna's vaccine candidate falls in between AZD1222 and BNT162b2 with a six-month storage temperature requirement of minus 4 degrees Fahrenheit or less. The lipids encapsulating mRNA-1273 will remain stable at standard refrigerator temperatures for up to 30 days once pulled out of the freezer.

Still a tight race

Any time you compare clinical trial results from different populations, you're also comparing results from different populations. Without head-to-head studies between the top vaccine candidates, the efficacy differences need to be taken with a grain of salt.

We can't be certain about which vaccine candidate will become the most popular, but we know it's going to have plenty of competition. Keep this in mind if you're considering purchasing biotech stocks driven by their coronavirus vaccine programs.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.