Anthony Fauci, director of the National Institute for Allergies and Infectious Diseases at the NIH, speaks about the daily press conference at the White House in Washington on January 21, 2021.

Jonathan Ernst | Reuters

The White House chief physician, Dr. Anthony Fauci on Friday urged Americans to make sure they get their second dose of the Covid-19 vaccines, saying the second shot offers “dramatic” benefits.

Pfizer and Moderna’s Covid vaccines require two doses three to four weeks apart. Both vaccines are about 95% effective against the virus, but that strong protection doesn’t kick in until two weeks after the second dose, officials say.

Fauci said Friday that about 8% of Americans who received a dose of the Pfizer or Moderna Covid vaccines have not returned for their second shot. However, skipping the second dose could cause problems for these Americans, as a single shot of the vaccine triggers a weaker immune response than two.

Fauci cited numerous scientific studies, including a report published Wednesday by the Centers for Disease Control and Prevention, which found Pfizer-BioNTech and Moderna Covid-19 vaccines to be 64% effective at reducing hospitalizations in the elderly after one Prevent shot, but 94% after two doses. The study examined 417 adults in 14 states in hospital from January through March.

“If you are on two-dose therapy make sure you get that second dose,” he said during a White House briefing about the pandemic.

Early on, public health officials and experts said they feared it would be difficult for some Americans, especially workers who cannot easily take time off, to come back for a second dose. Even so, officials have said the second shot pickup is better than expected.

On Friday, Fauci also urged health care providers to ensure that canceled visits for second admissions are rescheduled.

Fauci’s remarks come because the US is seeing its first real slowdown in daily vaccination rates after months of steady growth. According to CDC data, the country had an average of 2.6 million reported vaccinations per day over the past week, up from a high of 3.4 million reported shots per day on April 13.

His comments also come as the US pursues highly contagious new varieties of the virus. Fauci previously said that two doses of Pfizer or Moderna’s Covid vaccines are better than one to protect against variants.

Earlier this month, Fauci gave advice to those diagnosed with Covid after the first vaccination and before the second vaccination.

He said that people who contract the coronavirus between Covid-19 vaccinations can get their second dose after recovering from the disease and are no longer considered contagious.

–CNBC’s Nate Rattner contributed to this report.