As several COVID-19 vaccines reach the homestretch of research, News 12 is taking a deeper look at phase three and how the process works.

Phase three is the most important part of clinical trials because this is the trial done on a large scale, showing researchers whether or not the vaccine is effective in protecting transmission in the community.

At least five pharmaceutical companies, including Pfizer in Rockland County, are in phase three trials.

As News 12 reported, Pfizer is now looking to expand its phase 3 trials to include an additional 14,000 volunteers.

The trials happening are not looking to prevent infection. Instead, researchers say they're looking at what difference the vaccine makes if people catch the virus.

"We don't think we can actually prevent infection with these vaccines because it's such an infectious respiratory agent, so what we're doing is these trials have clinical endpoints. What that means is we're seeing if the vaccine makes a difference in people getting sick, getting pneumonia, getting hospitalized," says Dr. Chris Beyrer, a professor of epidemiology at Johns Hopkins Bloomberg School of Public Health.

Dr. Beyrer is part of a COVID-19 vaccine prevention network launched by the National Institutes of Health, which is focused on phase three trial research, which includes Moderna and AstraZeneca.

MORE: Extended interview with Dr. Beyrer

Vaccines that are ineffective are a wash - that's why Pfizer is proposing to increase its final trials by 50% so it can study a more diverse population.

If all three phases are successful with no significant safety or effectiveness concerns, it heads for FDA approval among a team of chemists and microbiologists who make a risk benefit assessment while the facility where the vaccine is manufactured undergoes inspection.

Even after approval, the agency claims it oversees production with periodic inspections.