Moderna (MRNA -0.87%) plans to start a phase 3 clinical trial testing its mRNA-1273 coronavirus vaccine candidate in less than two weeks. But don't expect a quick turnaround of full study results. Moderna plans to follow patients for 759 days.

That's over two years.

Add in a month or two (or three or four) to enroll all 30,000 patients and a couple of months to crunch all the numbers after the study is completed, and it could be 2023 before we'll know if mRNA-1273 actually protects people from getting COVID-19.

Last month, the Food and Drug Administration published guidance making it clear the agency won't allow drugmakers to cut any corners in their development of coronavirus vaccines, so the two-year data will likely be required to gain a full approval for mRNA-1273.

Gloved hands giving an injection into a child's shoulder

Image source: Getty Images.

Fortunately there's a parallel regulatory pathway -- an emergency use authorization -- which could allow Moderna to start selling the vaccine much earlier with data that's substantially easier to obtain.

While the primary efficacy measurement of Moderna's clinical trial is the number of participants who develop COVID-19, the study will also take blood samples on day 1, 29, 57, 209, 394, and 759 to determine whether patients develop antibodies that bind and neutralize SARS-CoV-2, the novel coronavirus that causes COVID-19. Patients will receive the vaccine or placebo on day 1 and a booster on day 29, so the day 57 results should give an indication of whether the vaccine is creating antibodies at levels similar to what was seen in the phase 1 clinical trial.

Again, assuming a couple of months to enroll the clinical trial, the day 57 antibody levels should be available around Thanksgiving, a goal management has previously disclosed.