The Centers for Disease Control and Prevention revealed on Monday that 25 percent of U.S. adults have been fully vaccinated against COVID-19.

NBC News reports that the major milestone shows the country is on track to get plenty more Americans vaccinated before the summer. That means, as of this week, over 64 million people in the country have been completely vaccinated. The CDC also revealed that 40 percent of U.S. adults have received at least one dose of the vaccine, while 75 percent of seniors have received their first dose. The milestone comes ahead of President Joe Biden’s targeted date for universal U.S. vaccine eligibility, April 19.

Vaccination efforts in the U.S. kicked off in December, following the authorization of a vaccine from Pfizer and BioNTech. With the addition of vaccines from the likes of Moderna and Johnson & Johnson, these efforts continued to accelerate. Various states including Alaska, South Dakota, and New Mexico have now fully vaccinated over 30 percent of their adult populations.

Despite the success of vaccination efforts for the most part, there’s still no immediate end date in sight for the global pandemic. Some states such as California are waiting to reopen based on how vaccination rates go, while others have already mostly reopened. The New York Times reports states like Florida and Georgia have mostly kept businesses open and not issued face-covering mandates.

Worryingly, a recent poll from NPR/Marist found that one in four Americans are hesitant to get the vaccine. NPR reports that refusal to take the vaccine could put herd immunity at risk. “Vaccine hesitancy is a big problem for all of us," said Ali Mokdad from the Institute for Health Metrics and Evaluation at the University of Washington. He also suggested that by May, we “will have more vaccines than people willing to take the vaccine."