On February 10, 2014, the USS Ardent, a U.S. Navy minesweeper, was moored in San Diego, California, while conducting training.
Over the course of 3 days, 25 of 102 crew members sought medical care because of influenza-like illness (ILI).
The HA gene sequence of an outbreak isolate was 99% identical to strains circulating during the 2013–14 influenza season and antigenically similar to the H3N2 component of the 2013–14 influenza vaccine.
At the time of the outbreak, 99% of the crew had received influenza vaccine.
This outbreak highlights the risk for an H3N2 influenza outbreak among vaccinated and otherwise healthy young persons.
Of the 25 crew members with ILI symptoms, 16 were vaccinated via intradermal injection, eight via intranasal mist, and one had not received vaccination.
The current U.S. Department of Defense influenza vaccination policy mandates that all uniformed personnel receive seasonal influenza vaccination, unless medically exempt, or face punishment under the Uniform Code of Military Justice.
The policy specifically directs all Navy operational units to be at least 90% vaccinated.
However, despite vaccination measures, influenza outbreaks can still occur in highly vaccinated military populations.