Coronavirus vaccines produced by Pfizer and partner BioNTech are being evaluated in a flexible, concurrent trial of multiple candidates on healthy young adults in the U.S. which researchers hope will reduce the development period for a COVID-19 antidote.

The US-based pharmaceutical company disclosed that the new vaccine, BNT162, has been dosed to the first human subjects in the United States. They started human trials of the prototype drug in Germany late last month.

The experiments are being carried out at the NYU Grossman School of Medicine and the School of Medicine at the University of Maryland, the pharmaceutical company announced Tuesday.

The short period of less than four months in which the company was able to switch from pre-clinical research to human trial is exceptional, Chief Executive Albert Bourla disclosed in a statement.

The two companies are actively developing a drug candidate based on a genetic material known as Messenger RNA, which holds the protein-making commands for the cells.

Through injecting a specially formulated messenger RNA into the body, the vaccine may essentially instruct the cells how to produce the coronavirus spike protein without causing a person to become ill.

Since this protein is normally used by the virus as a key to activating and capturing lung cells, the vaccine may enable a healthy immune system to generate antibodies to fight off an invasion.

The system also has the benefit of delivering faster and appears to be more durable than conventional therapies, which use weakened strains of viruses.

In the preliminary tests which started on Monday, there are several variants of the shot. The companies are going to advance models that seem safest and generate the strongest immune response.

Just as clinical trials begin, Pfizer plans to increase its manufacturing potential. The firm said it is the only way it can be certain that a product will be ready to combat the pandemic as soon as possible.

Pfizer pointed out that if it finds a drug candidate, it could develop millions of doses by end of the year, and could raise production to hundreds of millions by 2021.

There are no Food and Drug Administration-approved vaccines to treat Covid-19, and pharmaceutical firms are scrambling to develop a drug, which U.S. medical authorities said is estimated to take at least 12 to 18 months.

Shares of Pfizer were up 2.3 percent to $38.44 by mid-morning on Wall Street. They are down 2 percent so far this year.