Staff volunteers queue to receive a fourth dose of the Pfizer COVID-19 vaccine at Sheba Medical Center in Israel on Dec. 27, 2021, as the hospital conducted a trial of a fourth jab of the vaccine. Jack Guez/AFP via Getty Images hide caption

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STORIES OF LIFE IN A CHANGING WORLDHealth
SARS-CoV-2, the coronavirus that causes the disease COVID-19, is a big viral family with many variants. Just like with a human family tree, the distance between two variants shows roughly how related they are. This illustration is inspired by the SARS-CoV-2 phylogenetic tree generated by Emma Hodcroft of Nextstrain.org and the University of Bern. Her tree is reproduced within the story below. Kat Hubbs for NPR hide caption
Fever is a possible side effect of a COVID19 vaccine. Catie Dull/NPR hide caption
International visitors who fly into the U.S. will have a new set of rules and requirements regarding COVID-19 vaccines, starting Nov. 8. Angus Mordant/Bloomberg via Getty Images hide caption
Dr. KJ Seung, a professor at Harvard Medical School, speaks at a September rally outside the home of the CEO of Moderna, maker of one of the two mRNA vaccines that prevent COVID-19. The fake bones are meant to represent lives lost unnecessarily to the coronavirus. Seung was part of a group of doctors demanding Moderna share vaccines — and its recipe — with low-resource countries. Jasper G. Goodman/The Harvard Crimson hide caption
Rochelle dos Santos helps her daughter with physical therapy. Many children with congenital Zika disorder are physically disabled. Ueslie Marcilino/Undark Magazine hide caption
Dr. Saleema Rehman stands outside Holy Family Hospital in Rawalpindi, Pakistan. The Afghan refugee of Turkmen origin has won UNHCR's Nansen Award for her work helping refugee moms and babies in Pakistan. Betsy Joles for NPR hide caption
COVID-19 vaccines from COVAX, the international vaccine-sharing program, arrive in Khartoum, Sudan, on Aug. 5. In a letter to President Biden, health experts are asking him to take action to manufacture and distribute vaccines to the entire world. Ebrahim Hamid/AFP via Getty Images hide caption
Nurses feed newborn babies rescued and brought to Ataturk National Children's Hospital in Kabul, Afghanistan, on May 15, 2020, after their mothers were killed in an attack on a maternity ward operated by Doctors Without Borders. The health-care nonprofit runs clinics and hospitals in various parts of the country. Wakil Kohsar/AFP via Getty Images hide caption
Afghanistan's Health Care Is In 'Limbo' Following Taliban Takeover, Says MSF Rep
A mask is still an important protective measure for both the unvaccinated and the vaccinated. A new study looks at the best ways to encourage folks to mask up. Ian Morton/NPR hide caption
COVID-19 patients receive treatment at the St. Luke Foundation for Haiti hospital in Port-au-Prince, Haiti, on July 16. The country received its first shipment of COVID-19 vaccine doses that month. Ricardo Arduengo/Reuters hide caption
A Bright Spot Amid Haiti's Woes: Its 1st Mass Rollout Of COVID Vaccines
Performing last rites before the cremation of a family member who died of COVID-19. Anupam Nath/AP hide caption
India's Pandemic Death Toll Estimated At About 4 Million: 10 Times The Official Count
Staffers distribute the required personal protective gear before they transfer a COVID-positive patient to a hospital. They'll wear a gown, gloves, mask and face shield. Ashley Westerman/NPR hide caption
Shakuntala Thilsted, winner of the 2021 World Food Prize, is one of the world's leading experts on the nutritional benefits of small fish. Finn Thilsted hide caption
Dr. Eugene Richardson in a helicopter during the Ebola outbreak in Sierra Leone, where he worked as a clinical lead for Partner in Health's response to the outbreak. He is the author of the new book Epidemic Illusions: On the Coloniality of Global Public Health. Katie Barron hide caption
When it's cold outside, is it safe to bring a socially distanced gathering into your house if the windows are open for a short bit and then shut so indoor temperatures don't plummet? We answer this question below. Tessa Bunney/Corbis via Getty Images hide caption
Air passengers wearing practically full-body coveralls at the Netaji Subhas Chandra Bose International Airport in India. Dibyangshu Sarkar/AFP via Getty Images hide caption
Joan Garcia became pregnant at 14 and gave birth at 15. She and her child travel by raft between the two shacks where they live in Navotas fish port on Manila Bay. Hannah Reyes Morales for NPR hide caption
A fan wears a neck gaiter as he watches the Los Angeles Dodgers play at home against the San Francisco Giants last week. Gina Ferazzi/Los Angeles Times via Getty Images hide caption
In 2014, Martha Phillips, an American nurse, traveled to West Africa during the Ebola crisis to provide medical care. Treating Ebola not only taught her how to stay safe around a deadly virus but also how to manage the stress and sadness of working during a disease outbreak. She's now drawing on that experience to help other nurses cope with the challenges of coronavirus. Martha Phillips hide caption
People with protective masks walk in front of the Eiffel Tower in Paris on March 9. The new coronavirus has had an impact on international travel. Mehdi Taamallah/NurPhoto via Getty Images hide caption
An office worker is screened with an infrared thermometer as he enters a building in New Delhi, India. Prashanth Vishwanathan/Bloomberg via Getty Images hide caption