The stricken ship has been quarantined at sea for months now as doctors work to control and understand the outbreak. The first case of the infection was spotted December 22, not long after the ship left Mayport Naval Station in Florida for its deployment in the Persian Gulf. It has avoided port calls since early January.
The Navy says that none of the infections were serious or life-threatening, and 26 of the 27 service members have recovered and returned to duty.
According to Vaxopedia, mumps is one of the infectious diseases that members of the US armed forces are vaccinated against when they join a branch of service. The Navy, in an abundance of caution, also gave the more than 700 service members on board booster vaccinations for measles, mumps and rubella, the common triple-vaccine combination. The Fifth Fleet, however, noted that the mumps portion of that vaccine is the least effective, "providing 88 percent effectiveness after completion of the two dose series," it said in its statement to Business Insider.
How this outbreak happened remains a mystery. "The point of origin has not yet been determined," the Fifth Fleet told the outlet. The Navy and Marines have launched an epidemiologic investigation into the outbreak; the investigation is still ongoing. The close quarters of Navy ships, of course, provide ample opportunities for infections to spread.
How long the ship will remain at sea is also unknown, as the service will not consider the outbreak over until two incubation cycles have passed without a new infection. The mumps incubation cycle is 25 days, so it will be nearly two months at the very earliest before the USS Fort McHenry returns to shore.