A health worker administers a measles vaccine during a vaccination drive, prompted by a measles outbreak, in Navi Mumbai, India, in December 2022. A new UNICEF report finds that India has the world's largest number of children with zero doses of childhood vaccines: 2.7 million Bachchan Kumar/Hindustan Times via Getty Images hide caption
Children in Venezuela get toys as a reward for taking vaccines for polio, rubella and influenza. Worldwide vaccination rates have been on a downward plunge. Ariana Cubillos/AP Photo hide caption
Dr. Atul Gawande delivers a speech in 2015. In January 2022, he became the head of the U.S. Agency for International Development's work in global health. Lisa Lake/Getty Images hide caption
Pope Francis delivers the Angelus noon prayer in St.Peter's Square, at the Vatican on Sunday. Gregorio Borgia/AP hide caption
A syringe is filled with a dose of the Pfizer-BioNTech COVID-19 vaccine. While the vaccine has now been authorized for children between ages 5 and 11, it may take several weeks for shots to become widely available. Patrick T. Fallon/AFP via Getty Images hide caption
Travelers watch a JetBlue aircraft taxi away from a gate at Ronald Reagan Washington National Airport on May 25. Patrick Semansky/AP hide caption
A student arrives for the first day of class at Brooklyn's PS 245 elementary school in New York on Sept. 13. Mark Lennihan/AP hide caption
A woman is vaccinated at a COVID-19 vaccination center in Tel Aviv, Israel, on Jan. 5. Israel will offer a coronavirus booster to people over 60 who have already been vaccinated. Oded Balilty/AP hide caption
To mask or not to mask: That is the question for vaccinated people as the delta variant surges. The answer may depend on the situation, experts say. Here, these roller coaster riders mask up at Knott's Berry Farm in Buena Park, Calif. Allen J. Schaben/Los Angeles Times via Getty Images hide caption
A 16-year-old gets a Pfizer-BioNTech COVID-19 vaccine in Anaheim, Calif., on April 28. Advisers to the Centers for Disease Control and Prevention now say it's not necessary for adolescents to wait two weeks after a COVID shot to receive routine immunizations. Paul Bersebach/MediaNews Group via Getty Images hide caption
Tokyo Games Delivery Officer Hidemasa Nakamura holds a sample of an updated version of the playbook during a news briefing on Wednesday. Franck Robichon/Getty Images hide caption
A nurse administers a shot at a COVID-19 mass vaccination site at Martinsville speedway in Ridgeway, Va., on March 12. Ashish Jha, a public health policy researcher, noted Sunday that "despite phenomenal vaccination rates, variants pulled ahead this week." Andrew Caballero-Reynolds/AFP via Getty Images hide caption
Edith Arangoitia receives a COVID-19 vaccination in Chelsea, Mass., a heavily Hispanic community, on Feb. 16. Joseph Prezioso/AFP via Getty Images hide caption
Amelia Hunt (left), principal of Stevens Early Learning Center in Washington, D.C., and Dr. Craig DeWolfe, a hospitalist at Children's National. Lulu Garcia-Navarro/NPR hide caption
Navajo Nation President Jonathan Nez receives the Pfizer-BioNTech COVID-19 vaccine in late December at Gallup Indian Medical Center in Gallup, N.M. Noel Lyn Smith/USA Today Network via Reuters hide caption
On Dec. 14, West Virginia Gov. Jim Justice was one of the first U.S. elected officials to get immunized against COVID-19. The state has since completed a first round of shots in all long-term care facilities as well as with front-line health workers. State of West Virginia/Via AP hide caption
Mothers and their babies in Nigeria wait at a health center that provides vaccinations against polio. Vaccination rates lag in the middle-income country. Hannibal Hanschke/Picture Alliance/Getty Images hide caption
Registered Nurse Miriam Nampil vaccinates 1-month-old Joy Nowai, as she is held by her mother, Julie Nowai. The island nation of Vanuatu has launched a drone-delivered vaccination program to reach children in remote areas. UNICEF hide caption
A Pakistani policeman guards a team of polio vaccinators during an immunization drive in Karachi on January 22. Officials have stepped up protection in the wake of the January 18 attack. Rizwan Tabassum/AFP/Getty Images hide caption
People walk down a flooded street as they evacuate their homes in Houston on Monday. Joe Raedle/Getty Images hide caption
Demonstrators in Rome protest June 11 against imposition of vaccine requirements. A new law requires childhood vaccination against 12 diseases. Marco Ravagli/Barcroft Images/Barcroft Media via Getty Images hide caption
Abraham Vidaurre, 12, checks his arm after receiving an HPV shot in Corpus Christi, Texas. The vaccine is recommended for 11- and 12-year-old boys and girls. Matthew Busch/The Washington Post/Getty Images hide caption
Ukrainian deputy minister of health Igor Pereginets holds a 2-year-old getting a polio vaccination in Kiev on October 21 — part of a campaign launched after two cases were reported in September. Genya Savilov/AFP/Getty Images hide caption