Concept image of COVID-19 cells (variants Gamma, Delta, and Omicron). For a long time, scientists couldn't figure out where Omicron had come from. Now, studies appear to point to one specific group. Matt Anderson Photography/Getty Images hide caption
A box of Evusheld, an antibody therapy developed by pharmaceutical company AstraZeneca for the prevention of COVID-19 in immunocompromised patients, is seen in February at the AstraZeneca facility for biological medicines in Sweden Jonathan Nackstrand/AFP via Getty Images hide caption
Brothers Chase Miller (left), 10, and Carson Miller, 11, in November 2021. The two brothers have a rare genetic disorder and are immunocompromised. Their family has to practice extreme caution to prevent coronavirus exposures. Danny Miller hide caption
Having a compromised immune system puts you at higher risk of severe illness and death from COVID-19. Studies show that the initial vaccine doses are less effective for people with weakened immune systems. A third shot can boost protection. Christiana Botic/Boston Globe via Getty Images hide caption
Even after full vaccination against COVID-19, people who have had organ transplants are urged by their doctors to keep wearing masks and taking extra precautions. Research shows the strong drugs they must take to prevent organ rejection can significantly blunt their body's response to the vaccine. DigiPub/Getty Images hide caption
An illustration of the variant found in the United Kingdom. To infect a cell, the virus's spike protein (red) has to bind to a receptor on the cell's surface (blue). Mutations help the virus bind more tightly. Science Source hide caption
Candace Palmerlee (right), with her wife Toddy Fineberg and their kids Ellery and Cyrus Brownlee. Sarah Deragon hide caption