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.

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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.”

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RNC 2020

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Vice President Mike Pence arrives with his wife Karen Pence to speak on the third day of the Republican National Convention at Fort McHenry National Monument and Historic Shrine in Baltimore, Wednesday, Aug. 26, 2020. (AP Photo/Andrew Harnik)

Donald Trump Jr., speaks as he tapes his speech for the first day of the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Monday, Aug. 24, 2020. (AP Photo/Susan Walsh)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. President Donald Trump speaks with frontline workers in a pre-recorded video broadcasted during the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

WASHINGTON, DC – AUGUST 24: Kimberly Guilfoyle pre-records her address to the Republican National Convention at the Mellon Auditorium August 24, 2020 in Washington, DC. The novel coronavirus pandemic has forced the Republican Party to move away from an in-person convention to a televised format, similar to the Democratic Party’s convention a week earlier. (Photo by Chip Somodevilla/Getty Images)

Tiffany Trump walks on stage to tape her her speech for the second day of the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Tuesday, Aug. 25, 2020. (AP Photo/Susan Walsh)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, Patricia and Mark McCloskey, a couple from St. Louis who pointed guns at Black Lives Matter protesters, addresses the virtual convention in a pre-recorded video broadcasted on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

Former U.N. Ambassador Nikki Haley speaks during the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Monday, Aug. 24, 2020. (AP Photo/Susan Walsh)

Republican National Committee Chairwoman, Ronna McDaniel, speaks during the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Monday, Aug. 24, 2020. (AP Photo/Susan Walsh)

Donald Trump Jr., speaks as he tapes his speech for the first day of the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Monday, Aug. 24, 2020. (AP Photo/Susan Walsh)

Delegates are seated for the first day of the Republican National Convention, Monday, Aug. 24, 2020, in Charlotte, N.C. (AP Photo/Chris Carlson, pool)

The room is set and delegates begin to arrive for the first day of the Republican National Convention, Monday, Aug. 24, 2020, in Charlotte, N.C. (Travis Dove/The New York Times via AP, Pool)

President Donald Trump speaks the Republican National Committee convention site, Monday, Aug. 24, 2020, in Charlotte. (AP Photo/Evan Vucci)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. President Donald Trump (C) speaks with freed hostages (L-R) Pastor Andrew Brunson, Sam Goodwin, Michael White, Joshua and Tamara Holt and Pastor Bryan Nerran in a pre-recorded video broadcasted during the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. President Donald Trump (C) speaks with freed hostages (L-R) Pastor Andrew Brunson, Sam Goodwin, Michael White, Joshua and Tamara Holt and Pastor Bryan Nerran in a pre-recorded video broadcasted during the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, Pennsylvania congressional nominee Sean Parnell addresses the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

WASHINGTON, DC – AUGUST 24: Wearing a face mask to reduce the risk of transmission of the novel coronavirus, Donald Trump Jr. (R) watches his girlfriend Kimberly Guilfoyle as she pre-records her address to the Republican National Convention at the Mellon Auditorium August 24, 2020 in Washington, DC. The COVID-19 pandemic has forced the Republican Party to move away from an in-person convention to a televised format, similar to the Democratic Party’s convention a week earlier. (Photo by Chip Somodevilla/Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, Natalie Harp, campaign advisory board member for U.S. President Donald Trump, addresses the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. Rep. Vernon Jones (D-GA) addresses the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. President Donald Trump speaks with frontline workers in a pre-recorded video broadcasted during the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. Rep. Jim Jordan (R-OH) addresses the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, nurse practitioner Amy Johnson Ford addresses the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 24: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, surgeon Dr. G.E. Ghali addresses the virtual convention on August 24, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

First lady Melania Trump speaks on the second day of the Republican National Convention from the Rose Garden of the White House, Tuesday, Aug. 25, 2020, in Washington. (AP Photo/Evan Vucci)

First lady Melania Trump arrives to speak on the second night of the Republican National Convention from the Rose Garden of the White House, Tuesday, Aug. 25, 2020, in Washington. (AP Photo/Evan Vucci)

First lady Melania Trump arrives to speak on the second night of the Republican National Convention from the Rose Garden of the White House, Tuesday, Aug. 25, 2020, in Washington. (AP Photo/Evan Vucci)

WASHINGTON, DC – AUGUST 25: Eric Trump, son of U.S. President Donald Trump, pre-records his address to the Republican National Convention at the Mellon Auditorium on August 25, 2020 in Washington, DC. The coronavirus pandemic has forced the Republican Party to move away from an in-person convention to a televised format, similar to the Democratic Party’s convention a week earlier. (Photo by Drew Angerer/Getty Images)

WASHINGTON, DC – AUGUST 25: A stage manager adjusts the tie of Eric Trump, son of U.S. President Donald Trump, before he pre-records his address to the Republican National Convention at the Mellon Auditorium on August 25, 2020 in Washington, DC. The coronavirus pandemic has forced the Republican Party to move away from an in-person convention to a televised format, similar to the Democratic Party’s convention a week earlier. (Photo by Drew Angerer/Getty Images)

Tiffany Trump speaks as she tapes her speech for the second day of the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Tuesday, Aug. 25, 2020. (AP Photo/Susan Walsh)

President Donald Trump reaches to hold hands with first lady Melania Trump after her speech to the 2020 Republican National Convention from the Rose Garden of the White House, Tuesday, Aug. 25, 2020, in Washington. (AP Photo/Evan Vucci)

White House counselor Kellyanne Conway walks onto the stage to tape her speech for the third day of the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Wednesday, Aug. 26, 2020. (AP Photo/Susan Walsh)

CHARLOTTE, NC – AUGUST 26: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, U.S. Sen. Joni Ernst (R-IA) addresses the virtual convention on August 26, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 26: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, Chinese human rights activist Chen Guangcheng addresses the virtual convention on August 26, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 26: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, North Carolina congressional nominee Madison Cawthorn addresses the virtual convention on August 26, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 26: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, Former South Carolina Gamecocks football coach Lou Holtz addresses the virtual convention on August 26, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

CHARLOTTE, NC – AUGUST 26: (EDITORIAL USE ONLY) In this screenshot from the RNC’s livestream of the 2020 Republican National Convention, Sister Dede Byrne, surgeon, military veteran and member of the Little Workers of the Sacred Hearts of Jesus and Mary, addresses the virtual convention on August 26, 2020. The convention is being held virtually due to the coronavirus pandemic but will include speeches from various locations including Charlotte, North Carolina and Washington, DC. (Photo Courtesy of the Committee on Arrangements for the 2020 Republican National Committee via Getty Images)

First lady Melania Trump speaks on the second night of the Republican National Convention from the Rose Garden of the White House, Tuesday, Aug. 25, 2020, in Washington. (AP Photo/Evan Vucci)

Vice President Mike Pence speaks on the third day of the Republican National Convention at Fort McHenry National Monument and Historic Shrine in Baltimore, Wednesday, Aug. 26, 2020. (AP Photo/Andrew Harnik)

Supports wait to hear Vice President Mike Pence speak on the third day of the Republican National Convention at Fort McHenry National Monument and Historic Shrine in Baltimore, Wednesday, Aug. 26, 2020. (AP Photo/Andrew Harnik)

President Donald Trump and first lady Melania Trump arrive on South Lawn of the White House on the fourth day of the Republican National Convention, Thursday, Aug. 27, 2020, in Washington. (AP Photo/Evan Vucci)

President Donald Trump and first lady Melania Trump arrive for his acceptance speech to the Republican National Committee Convention on the South Lawn of the White House, Thursday, Aug. 27, 2020, in Washington. (AP Photo/Evan Vucci)

President Donald Trump speaks from the South Lawn of the White House on the fourth day of the Republican National Convention, Thursday, Aug. 27, 2020, in Washington. (AP Photo/Alex Brandon)

President Donald Trump speaks from the South Lawn of the White House on the fourth day of the Republican National Convention, Thursday, Aug. 27, 2020, in Washington. (AP Photo/Evan Vucci)

Ivanka Trump speaks to introduce President Donald Trump from the South Lawn of the White House on the fourth day of the Republican National Convention, Thursday, Aug. 27, 2020, in Washington. (AP Photo/Evan Vucci)

Housing and Urban Development Secretary Ben Carson speaks on the fourth day of the Republican National Convention from the Andrew W. Mellon Auditorium in Washington, Thursday, Aug. 27, 2020. (AP Photo/Susan Walsh)

(L-R) Second Lady Karen Pence, Vice President Mike Pence, Barron Trump and US First Lady Melania Trump listen to President Trump deliver his acceptance speech for the Republican Party nomination for reelection during the final day of the Republican National Convention from the South Lawn of the White House on August 27, 2020 in Washington, DC. (Photo by Brendan Smialowski / AFP) (Photo by BRENDAN SMIALOWSKI/AFP via Getty Images)

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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.

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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.

Related: “We were told to get this answer as fast as we possibly can,” one researcher said.

The phase 3 trials should help answer those questions.

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.”

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