Multiple drugmakers are in phase 3 clinical trials for their coronavirus vaccines -- the last stage before potential approval. Meanwhile, Merck (MRK -1.04%) is just now starting the first human study of its coronavirus vaccine, V591.

Being so far behind, the company didn't even bother announcing the start of the phase 1/2 clinical trial. The Wall Street Journal noticed the new clinical trial in the U.S. government's database, which was updated on Sept. 9.

While speed is important in a pandemic, Merck deserves get a bit of a break for its relatively slow start. The company only got its hands on the vaccine technology after acquiring a privately held Austrian company called Themis Bioscience in May.

Themis' technology, which was licensed from the French research nonprofit Institut Pasteur, uses a modified measles virus to deliver antigens to the immune system. In the case of V591, the antigen would help the patient develop antibodies against the novel coronavirus that causes COVID-19.

Nurse showing a syringe to a child

Image source: Getty Images.

Merck's clinical trial, which is being run in Belgium, will enroll up to 260 participants who will be treated with different dosing regimens, which could include a single shot as well as two-dose regimens with booster shots on day 57 or day 169.

The booster regimen of V591 is substantially prolonged compared to Pfizer (PFE -0.34%) and BioNTech's (BNTX -0.67%) 21-day booster and Moderna's (MRNA -1.02%) 28-day booster.

The extended time to a booster will delay clinical trial results but could also be a benefit Merck if the patient is fully protected in the time between the first dose and the booster. Better yet, there's the possibility that a single dose will be enough to fully protect patients from COVID-19.