More than 30,000 volunteers have been enrolled in two major COVID-19 vaccine trials in the U.S., officials said Friday.
"We feel we're absolutely on track, if not a little bit ahead" of the goal of developing a safe, effective vaccine and producing tens of millions of doses by the end of the year, Paul Mango, deputy chief of staff for policy at the U.S. Department of Health and Human Services, said during a call with reporters.
The two vaccine manufacturers, Moderna and Pfizer, aim to enroll a total of 60,000 volunteers for their phase 3 trials. Mango said enrollment is about halfway complete, but he did not say how many people are in each trial. AstraZeneca, in partnership with the UK's University of Oxford, has also begun phase 3 trials on its COVID-19 vaccine, with study sites in the U.S.
Still, "there are no guarantees in science," Mango noted. It's always possible that the vaccines being tested will not be effective.
President Donald Trump promised quick work on a vaccine during his speech Thursday evening at the Republican National Convention, saying the nation "will produce a vaccine before the end of the year, or maybe even sooner."
Officials predicted the research would stay on track even if Trump loses re-election.
"The vast majority of folks who are working on Operation Warp Speed are not political appointees to the Trump administration," Mango said.
It's clear that one vaccine will not be enough to meet the anticipated demand. Some vaccines may turn out to be better suited for certain populations, such as older people or people with compromised immune systems, said Dr. Gregory Poland, an infectious diseases expert who directs the Mayo Clinic's Vaccine Research Group in Rochester, Minnesota.
"Young children are different than adults and different than elderly adults," Poland said, adding that groups such as pregnant women and people who are immunocompromised or have autoimmune diseases may also react differently.
The phase 3 trials should help answer those questions.
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Mango said officials feel "very good" about diversity in enrollment. "We have a very large number of folks over age 65. We have a lot of folks with multiple chronic issues. We have Hispanic participation, we have African American participation," he said.
Half of the study participants will receive the vaccine, and the other half will get a placebo. Who gets which version will be unknown to both the researchers and participants.
Mango predicted additional vaccines could enter larger trials, perhaps by next month.
In the meantime, Poland offered a reminder that tried-and-true protection measures still work: "If people would follow the simple guidance of wearing a mask, physical distancing and sanitizing your hands, you can be protected against this."
A version of this story originally appeared on NBCNews.com.