Dr. Francis Collins said Tuesday he is "guardedly optimistic" about the possibility of a long-term inoculation to replace the one "you'd have to renew every year," USA Today reported.
A universal vaccine "seemed completely out of reach only a few years ago," Collins said, because flu viruses mutate yearly, quickly rendering each year's vaccines developed for specific flu viruses obsolete.
However, researchers have discovered "there are parts [of the virus] that don't change …. If you designed a vaccine to go after the constant part of the virus, you'd be protected against all strains," Collins said.
Such a vaccine is "not a question of whether, but when," Arnold Monto of the University of Michigan said. "I think five years is a bit ambitious, given where we are now."
In February, researchers in the United Kingdom announced some preliminary success in creating a universal flu vaccine for humans, USA Today reported.
Notable deaths of 2014 [PHOTOS]
CDC: Get your flu vaccine