Pfizer and German biotech firm BioNTech announced Saturday that they have submitted a proposal to the U.S. Food and Drug Administration to expand the phase-three trial of its coronavirus vaccine to include up to 44,000 participants, a significant increase from its previous target of 30,000. 

The companies, which are developing the vaccine together, said in a statement that the trial is proceeding as planned and they expect to have enrolled 30,000 participants by next week. 

“The proposed expansion would allow the companies to further increase trial population diversity, and include adolescents as young as 16 years of age and people with chronic, stable HIV (human immunodeficiency viruses), Hepatitis C, or Hepatitis B infection, as well as provide additional safety and efficacy data,” the companies said. 

Representatives of the Food and Drug Administration and of the Department of Health and Human Services did not immediately return CNBC’s request for comment.

Public health specialists have emphasized the need for participants in vaccine trials to reflect a diverse population, including the elderly, communities of color and people with underlying health conditions, all of whom have been disproportionately impacted by the virus. Including diverse participants helps ensure that the vaccine candidate is truly safe and effective across a variety of populations. 

Moderna, another company developing a Covid-19 vaccine, told CNBC earlier this month that they were slowing enrollment slightly in its trial to ensure it has sufficient representation of minorities most at risk for the disease.

Pfizer CEO Albert Bourla said earlier this month that the pharmaceutical company could have results from its late-stage trial as soon as October. On Saturday, the companies reiterated the possibility that they could have data reflecting whether the vaccine is effective or not by the end of October. However, it could take longer to collect enough data to determine whether the vaccine is safe, because it will take months of follow-up work to determine what kind of side effects, if any, the vaccine may cause.

“The pivotal trial is event-based and there are many variables that will ultimately impact read-out timing,” the companies said. “As stated previously, based on current infection rates, the companies continue to expect that a conclusive readout on efficacy is likely by the end of October.”

Through Operation Warp Speed, the Trump administration’s effort to quickly bring a Covid-19 vaccine and therapeutics to market, the U.S. has now invested more than $10 billion in six candidates, including Pfizer’s. The U.S. announced in July that it agreed to pay nearly $2 billion to Pfizer and BioNTech for at least 100 million doses. 

In July, Pfizer announced that early-stage clinical trial showed the vaccine produced some neutralizing antibodies, which researchers believe is necessary to build immunity to the virus. 

—CNBC’s contributed to this report.

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