Recently, New York City suffered flooding, fires, power outages, and many deaths after a superstorm that cycled into the city from the Atlantic. Throughout history, cities particularly suffer from singular natural or man-made events because of their population density. Crowded conditions also make cities rife ground for the transmission of contagious disease – yellow fever, cholera, and influenza swept New York City during the last two centuries. But unlike hurricanes and other natural disasters against which we continue to be largely powerless, medicine has gifted us with the tools to help prevent and contain infectious disease outbreaks. One of the most important public health measures to thwart epidemics has been vaccination. Diseases such as smallpox and polio have become historical footnotes, and illnesses such as pertussis, tetanus, measles, mumps, and rubella are rare in the United States. Still, sometimes these diseases make startling appearances. This week’s NEJM reports on a large outbreak of mumps in New York City between 2009 and 2010. What’s surprising is this outbreak occurred in mostly vaccinated individuals. Is this new fuel for the anti-vaccinationists? After all, people who have received the mumps vaccine were still getting sick. However, as the details of this study reveal, the vaccine continues to limit the spread of mumps ever since it was introduced in 1967.
In the days before the vaccine was widely available, mumps usually struck young children, commonly presenting as swelling of the parotid glands, and frequently causing viral meningitis or leaving the child with permanent unilateral hearing loss. Following the introduction of the mumps vaccine in the 1960s, cases of mumps in the United States dropped a staggering 99%. During the start of the 21st century, a second dose of the vaccine was recommended in children to further increase efficacy. This two-dose vaccination strategy increased the efficacy of the vaccine from 80% to 90%.
The outbreak in New York described by Barskey et al this week, began in 2009 when an eleven year old Orthodox Jewish boy returned to the United States after being exposed to a mumps outbreak in the United Kingdom. He had been appropriately vaccinated with the two-dose regimen. While attending a Orthodox camp in upstate New York, he developed parotitis. Within 2 months, 25 cases of mumps were reported among other boys and adults who attended the same camp. When these participants returned to their homes, the virus subsequently spread throughout Orthodox communities in Brooklyn and surrounding counties. Almost every case occurred and predominated in Orthodox neighborhoods. The majority of affected patients (76%) had received the 2 recommended doses of the vaccine.
How could children who are doubly immunized still be developing mumps? The study’s lead author Barskey argues: “Children who contracted the disease participated in intense, close face-to-face contact in the yeshiva,” he says, referring to the practice of “chavrusa” or the close discussion of religious texts across a narrow table. Such high-intensity exposure can allow for the transmission of the mumps virion through respiratory droplets at such high loads that this can overwhelm the capacity of the immune system – even among vaccinated individuals. That the disease did not spread outside of Orthodox circles suggests that routine vaccination contained the disease within this high-exposure community; without immunization, the outbreak would have spread to casual contacts, family members, and the crowded streets of the metropolis. Other outbreaks of mumps reported in the past decade are similar to the New York outbreak, occuring in settings of high exposure with no significant spread to external communities. “That this did not happen underlines the safety and efficacy of the mumps vaccine,” Barskey says.
Like a sudden autumn hurricane, a disease outbreak has a lot of moving parts, making it difficult to predict when it will occur and whom it will affect. What’s clear is that vaccination has been doing its job. The rates of mumps in the United States are at record low levels, with only 370 cases last year. Now, we only notice clusters of disease, and when they do occur, vaccination is an effective barricade that restricts the disease only to populations with intense exposure, preventing wider transmission. Targeted public health measures at such pockets of disease and continued population-based vaccination strategies are imperative to keep the incidence of mumps and other contagious diseases at very low levels. “I’ve never even seen a case of mumps,” I tell Barskey, who knows it wasn’t always so.