H3N2 Influenza outbreak among vaccinated sailors

Graphic depicting how influenza strains are named (Wikimedia Commons) Click to enlarge.

[T]his week’s MMWR (Morbidity and Mortality Weekly Report) from the CDC included a puzzling report concerning a US Naval minesweeper. In February of this year, 25 sailors from the 102 member crew aboard the USS Ardent, an Avenger class minesweeper, began to complain of fever, chills, and cough. Doctors conducted nasal swab PCR tests and diagnosed influenza, 20 of which were influenza A, and of those 20, 18 were H3N2. Here’s the other shoe: All but one of these men had been vaccinated against influenza A. Sequencing revealed that the strain infecting most of the men had a 99% similarity to A/Texas/50/2012, but 7 of the cases showed marked mutations with 5 amino acid substitutions in the HA1 protein.

So, what happened? Why did these men, nearly all of whom had been vaccinated about 3 months prior to the outbreak, become infected with influenza A? The report seems to imply that patient zero may have been a sailor who lived in San Diego, where the minesweeper was docked, who had developed vague symptoms in January. This sailor suffered from pyelonephritis (a kidney infection), so it had been assumed that his symptoms were somehow connected to his kidney ailment. In fact, a CT scan was performed of his lungs on the assumption that he had a pulmonary embolism! This CT scan showed nothing abnormal. The sailor in question had a roommate, who also served on the Ardent, so even though patient zero remained on shore due to his illness, the roommate reported for duty according to rotation, and it’s assumed that the disease was spread in common areas or through surface contamination (hand rails, etc.).

The sub was decontaminated thoroughly, we’re told, and all the sailors recovered nicely after a round of antivirals. The source of the vaccination failure is a mystery that may never be solved.

To read the entire MMWR report, go here.