Nancy Ellen Messonnier (née Rosenstein; born November 1965) is an American physician, currently serving as dean of the Gillings School of Global Public Health at the University of North Carolina at Chapel Hill since September 2022.

She previously served as director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention from 2016 to 2021. She worked on the CDC's response to the COVID-19 pandemic in the United States.

Messonnier was born in Philadelphia, Pennsylvania, to Robert Rosenstein, who ran a small business, and Gerri Rosenstein, a bookkeeper and local school board president. She grew up in Lower Moreland Township, Pennsylvania, with her brother Rod Rosenstein.

Messonnier graduated from Lower Moreland High School in Lower Moreland Township. She received her Bachelor of Arts degree from the University of Pennsylvania in 1987. She then attended the University of Chicago School of Medicine, where she received her Doctor of Medicine in 1992. She then returned to the University of Pennsylvania for her residency training in internal medicine between 1992 and 1995.^{[citation needed]}

Messonnier began her career in public health in 1995 as an epidemic intelligence service officer in the National Center for Infectious Diseases, a program of the Centers for Disease Control and Prevention (CDC). During her tenure at the CDC, she led the Meningitis and Vaccine Preventable Diseases Branch in NCIRD's Division of Bacterial Diseases from 2007 to 2012. She also served as the deputy director of NCIRD from 2014 to 2016 before becoming director of the center on April 4, 2016.

During the course of her career, Messonnier notably worked on the 2001 anthrax attacks response, serving as co-leader of the anthrax management team and vaccine working group. She also played a leading role developing a low-cost meningococcal meningitis vaccine to prevent an emerging epidemic in Africa, known as MenAfriVac, in 2010. She has also worked on communications strategies to promote the use of vaccines to prevent the emergence of disease outbreaks.

Starting January 2020, Messonnier helped lead the CDC efforts to address and combat the emerging threat of COVID-19, the disease caused by the novel coronavirus (SARS-CoV-2). When 195 Americans were evacuated out of Wuhan because of the virus, the CDC moved to quarantine all of them, with Messonnier noting: "While we realize this is an unprecedented action, this is an unprecedented threat."

In a February 25 press briefing at the White House, Messonnier warned of the impending community spread of the virus in the United States, stating: "Disruption to everyday life might be severe." Following her comments during the February White House press briefing, she did not appear again at the briefing, and there was speculation that Messonnier had been "silenced" for her comments stressing the growing urgency of the COVID-19 pandemic in the United States. On February 28, she said that the U.S. "acted incredibly quickly before most other countries. Aggressively controlled our borders and we were able to slow the spread into the United States. ...We have been testing aggressively." While Messonnier no longer appeared in White House briefings, she continued giving regular CDC briefings, which were broadcast to the public, until April 2020, and she made public appearances in All Things Considered on NPR.