Only three coronavirus vaccine candidates with the potential to be marketed in the U.S. are currently in late-stage testing. AstraZeneca (AZN 0.34%) and Moderna (MRNA -1.92%) claim two of the three.

If you think that these two stocks are beating the market so far in 2020, you're right. Moderna has been a much bigger winner than AstraZeneca year to date. But which of these coronavirus stocks is the better long-term pick? 

Close-up of gloved hand holding vaccine bottle labeled COVID-19 vaccine

Image source: Getty Images.

The case for AstraZeneca

Probably the best argument for buying AstraZeneca instead of Moderna is that it's less risky. If Moderna's COVID-19 vaccine candidate runs into problems in late-stage testing, its shares will almost certainly plunge. AstraZeneca, on the other hand, can more readily absorb any setbacks because of its diversified portfolio.

We've already seen this happen. AstraZeneca announced on Sept. 9 that it was temporarily pausing its phase 3 testing of coronavirus vaccine candidate AZD1222 after a participant experienced an unexplained illness. Its shares slipped only 3% and quickly bounced back.

Investors shouldn't worry too much about AZD1222. AstraZeneca already received the green light to resume late-stage clinical trials in the U.K. Scientists with the University of Oxford, which initially developed AZD1222, think that the patient illness that resulted in the clinical trial pause probably wasn't related to the vaccine. 

AstraZeneca should still have a pretty good chance at achieving commercial success with AZD1222. More importantly, the company has many other potential growth drivers. Three of AstraZeneca's cancer drugs (Tagrisso, Imfinzi, and Lynparza) delivered year-over-year sales growth of at least 45% in the first half of 2020. Four of the company's other drugs generated double-digit percentage growth.

In addition, AstraZeneca's new drugs are picking up solid momentum, especially blood cancer drug Calquence. The company's pipeline includes 166 programs, nine of which are new candidates in late-stage testing.

AstraZeneca's shares trade at 22 times expected earnings. That valuation isn't all that high, though, considering that analysts expect the company to grow its earnings by an average of 19% annually over the next five years.

The case for Moderna

Moderna is admittedly riskier than AstraZeneca. But that higher level of risk also means that the biotech stock should deliver much higher gains if COVID-19 vaccine candidate mRNA-1273 is successful in late-stage testing.

The chances of mRNA-1273 achieving this success appear to be pretty good at this point. Phase 1 results for the messenger RNA vaccine were promising: mRNA-1273 produced strong immune responses in participants with no severe adverse reactions.

Moderna slowed down enrollment in its phase 3 study of mRNA-1273 to increase the diversity of participants. That slowdown has paid off, though. As of Sept. 16, around 28% of participants in the study were from diverse communities. It's possible that Moderna could secure FDA emergency use authorization for mRNA-1273 before the end of this year. 

Tremendous growth prospects are already baked into Moderna's share price. However, the market opportunity for mRNA-1273 alone gives the stock room to run. And if mRNA-1273 wins regulatory approvals, investors will likely see a lot more value in the rest of Moderna's pipeline.

That pipeline includes six other prophylactic mRNA vaccine candidates in clinical testing. Moderna also has five mRNA clinical-stage programs targeting cancer plus an antibody against chikungunya virus in phase 1 testing. The company has even partnered with AstraZeneca to develop one of its experimental mRNA cancer therapies along with a therapy targeting coronary artery disease.

Better coronavirus stock?

My view is that the better coronavirus stock to buy between AstraZeneca and Moderna depends on your investing style. If you're a more conservative investor, AstraZeneca is definitely the better pick. It's less risky than Moderna and offers solid growth prospects plus an attractive dividend.

On the other hand, aggressive investors will probably prefer Moderna. The biotech could become much larger over the next decade and beyond if its mRNA programs deliver on their potential.