U.S. health officials and drugmakers expect to start producing potential coronavirus vaccine doses by the end of the summer, a senior administration official said Monday.

The U.S. is aiming to deliver 300 million doses of a vaccine for Covid-19 by early 2021. The manufacturing process is already underway even though they aren’t sure which vaccine, if any, will work, a senior Trump administration officials told reporters on a conference call Monday. He said they are already buying equipment, securing the manufacturing sites and, in some cases, acquiring the raw materials.

“Exactly when the vaccine materials will be in production and manufacturing? It’s probably four to six weeks away,” the official said on the call, which was hosted by the Department of Health and Human Services. “But we will be actively manufacturing by the end of the summer.”

Because of the pandemic, U.S. health officials and researchers have been accelerating the development of vaccine candidates by investing in multiple stages of research even though doing so could be for naught if the vaccine ends up not being effective or safe.

U.S. health officials have previously said they are ramping up the manufacturing process to ensure they can immediately get a vaccine to market once they identify one that works.

The Trump administration has selected four potential vaccines as the most likely candidates, but the senior official said Monday that that list could grow. Among the list are vaccines from biotech firm Moderna and Johnson & Johnson. The two companies are expected to begin late-stage human trials for potential vaccines by the end of this month. It’s a record-breaking time frame to produce a vaccine — even as scientists say there is no guarantee the vaccines will be effective. 

President Donald Trump has repeatedly touted the “tremendous progress” of vaccine development, calling the project Operation Warp Speed, as coronavirus cases across the U.S. continue to surge. 

The U.S. has reported more than 3.3 million Covid-19 cases and at least 135,205 deaths as of Monday, according to data compiled by Johns Hopkins University. As of Sunday, cases are grew by 5% or more in 37 states and also Washington, D.C., according to CNBC’s analysis of the data. New cases hit a record daily average of 59,100 on Sunday, according to a seven-day average that smooths out fluctuations in day-to-day reporting gaps.  

Many state and local officials across the U.S. have criticized Trump and the administration, saying they lacked a strong, coordinated response. 

Last month, the Trump administration said the vaccine will be provided free of charge to Americans who can’t afford it.

Scientists are still learning about key aspects of the virus, including how immune systems respond once a person is exposed. The answers, they say, may have important implications for vaccine development, including how quickly it can be deployed to the public.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and a member of Trump’s coronavirus task force, has said he worries about the “durability” of a potential vaccine, saying there’s a chance it may not provide long-term immunity.

If Covid-19 acts like other coronaviruses, “it likely isn’t going to be a long duration of immunity,” Fauci said during an interview on June 2 with JAMA Editor Howard Bauchner.