Biotech company Moderna has been making some pretty promising strides in developing and testing its covid-19 vaccine. The company just announced it was working with the US National Institutes of Health to launch what will be one of the largest covid-19 vaccine trials, a phase 3 study enrolling tens of thousands of American volunteers to assess whether the vaccine could truly protect people from infection. Here’s what you need to know. 

How does the vaccine work? Moderna’s vaccine works by introducing a viral messenger RNA (mRNA) into the body for human cells to pick up. The human cells use the mRNA to create a fragment of a SARS-CoV-2 viral particle that should not cause an infection, but is enough for the immune system to recognize as a foreign antigen. The immune system learns to recognize the fragment and respond to it so that if the real SARS-CoV-2 virus enters the body, the immune system can respond to it before a full-blown infection can occur. 

How promising is this vaccine? There has never before been a successful vaccine created out of mRNA. If this vaccine works, it will be the first of its kind. That’s not exactly super-encouraging news, but on the plus side, the vaccine has shown promising results in earlier trials, leading to strong antibody production with tolerable side effects. But while phase 1 and phase 2 trials are designed to check whether the vaccine is safe, phase 3 trials are the ones that actually look into how effective a vaccine is in preventing infection.  

The phase 3 trial: The study will enroll up to 30,000 participants, from 89 sites across the nation. Half will receive two shots of the vaccine 28 days apart, while the other half will receive two shots of a saltwater placebo. It will be a double-blind study, meaning neither the participants nor the medical staff handling injections will know who is getting the vaccine and who is getting the placebo. Researchers are interested in seeing not just how well the vaccine prevents infection, but also whether it can limit the severity of the illness when infections still do occur. 

Questions about covid-19 immunity: There’s still a lot we don’t know about how covid-19 immunity works and how long it lasts—and these questions will play a big role in how strongly we can rely on a vaccine to put an end to the pandemic. The phase 3 trial will be critical in shedding light on these questions.

If it does work, what can we expect? Moderna has said that should the vaccine prove effective, it would be able to deliver between 500 million doses a year, and possibly up to a billion doses a year starting in 2021. The company has said it will seek to sell the vaccine for profit.

What other vaccines are at this stage? According to the New York Times’ vaccine tracker, there are four other vaccines undergoing phase 3 trials—one spearheaded by researchers from the University of Oxford and AstraZeneca (which is testing people from several countries in the largest trial in the world), and three from Chinese groups. Pfizer and a German company called BioNTech are also working on an mRNA vaccine that is expected to start a phase 3 trial by the end of July.