About a quarter of sailors immunized with the influenza vaccine on board the U.S. Navy minesweeper,USS Ardent contracted the flu, primarily H3N2 influenza, according to a study published in the latest MMWR published today.

USS Ardent Image/Photographer's Mate 1st class Robert R. McRill
USS Ardent
Image/Photographer’s Mate 1st class Robert R. McRill

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).

Diagnostic testing revealed that 20 specimens were influenza A, of which 18 were subtype H3N2, the two others were unable to be typed.

ILI was defined as illness with two or more of the following symptoms: fever >100.4°F (>38.0°C), chills, sore throat, cough, shortness of breath, congestion, headache, body aches, and nausea. Twenty crew members reported sick on February 10, one on February 11 and four more on February 12.

All 25 crew members with ILI symptoms were otherwise healthy men aged 21–44 years. No recent travel history was noted.

In an effort to reduce spread and impact of disease, oseltamivir (75 mg twice a day for 5 days) was prescribed to each ILI patient who reported that symptoms had developed within 48 hours of their medical visit, regardless of their vaccination status and rapid influenza testing results. In addition to antiviral medication, rapid identification of the influenza outbreak, and immediate isolation of affected persons (crew members with ILI symptoms were sent off ship to their homes for 48 hours), additional steps to control the outbreak were taken: thorough cleaning of spaces throughout the ship by the crew and use of the ship’s public address system to instruct personnel to wash hands frequently, use hand sanitizer, cover their mouths when coughing, and report for medical evaluation if they were experiencing ILI symptoms.

Interviews revealed a possible source of the outbreak to be an Ardent crew member (patient A), aged 26 years, who had been evaluated at a local emergency room for fever and cough on January 30, 11 days before the first ILI case was diagnosed.


The researcher’s conclusion is as follows:

Since the 1950s, a policy of mandatory annual vaccination against influenza for active duty personnel has been largely successful in limiting influenza epidemics in the military. 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.

Related: Flu season 2014-2015: Flu vaccine composition, dosage and availability