About 2.7 percent of U.S. adults, according to the CDC, or about 7 million people, are immunocompromised. Some, like transplant patients, take immune-suppressing drugs to prevent organ rejection. Others have blood cancers and other illnesses that damage the immune system. Still others are on cancer chemotherapy. Making such people eligible for an extra shot, doctors say, is preferable to having worried patients seek out additional inoculations on their own. Not all people with impaired immune systems will need an additional dose as of now, experts say.