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Recent Spike in US Mumps Cases Linked to Vaccine's Waning Protection

New strains of the mumps virus are not to blame for the recent resurgence of cases, say researchers in a new study, instead pinning the recent outbreaks on declining protective effects of the mumps vaccine over time.

The number of annual cases of mumps in the United States has spiked in recent years, and a new study suggests that recent outbreaks may be due to waning vaccine-induced immunity.

Mumps is a contagious viral disease that causes swollen salivary glands, leading to the telltale symptoms of puffy cheeks and swollen jaws. Other symptoms include fever, headache, muscle aches, tiredness, and loss of appetite, and may take 12 to 25 days to appear. The virus spreads through the saliva or mucous of an infected individual, from coughing, sneezing, food, shared utensils, or contaminated surfaces. Prior to the start of the US mumps vaccination program in 1967, the country saw about 186,000 reported mumps cases each year. With the introduction of a mumps vaccine, today part of the measles, mumps, and rubella (MMR) vaccine, the number of cases has fallen to fewer than 1,000 cases in recent decades.

However, in 2016, the United States saw 6,366 cases of mumps and more than 5,500 cases in 2017, marking the biggest outbreaks of the virus in a decade. The outbreaks have notably impacted universities, grade schools, and even professional sports teams, despite high rates of coverage for the recommended 2 doses of the MMR vaccine. In a recent recommendation from the Advisory Committee on Immunization Practices published in the Center for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report, the committee called for a third dose of the MMR vaccine for individuals who have been previously vaccinated with 2 doses and are at risk of acquiring mumps due to an outbreak. On the heels of that recommendation, a study by Harvard researchers that was recently published in the journal Science Translational Medicine found that waning vaccine protection may be causing the recent resurgence of mumps in the United States.

The study comes amid questions on whether the resurgence has been due to new strains of the mumps virus evading the protection of the vaccine, or from a wearing off of vaccine-induced immunity. Synthesizing data from 6 US and European studies on the effectiveness of the mumps vaccine, the study’s authors found no evidence that the vaccine has become less effective due to new strains of the mumps virus. Instead, they found that protection induced by vaccination wanes after 27 years on average. Overall, 25% of individuals vaccinated against mumps in the United States. may lose protection within 7.9 years, 50% within 19 years, and 75% within 38 years, according to the authors.

“This analysis helps address a persistent question surrounding the recent mumps outbreaks, pointing to the key role played by waning vaccine-induced immunity, and helps frame the research and policy questions on how best to control mumps,” said the study’s co-author, Yonatan Grad, MD, PhD, in a recent statement.

With their findings, the authors write that individuals who have received 2 doses of the mumps vaccine during childhood may be able to extend the vaccine’s protection with a third dose at 18 years of age. “Vaccination is the centerpiece of current public health strategy against mumps,” said co-author Joseph Lewnard, PhD. “Knowing that protection wanes in the long term can help inform how we deploy vaccines to prevent or contain future outbreaks.”

As of February 24, 2018, the CDC says that 32 states and the District of Columbia have reported a total of 304 mumps infections since the start of the year.