Majority of mumps cases are among the vaccinated, CDC finds

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Cases of mumps in the US continue to be widespread among vaccinated people, including children.

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Once a common childhood illness, cases of mumps declined by more than 99 percent in the US in 1967 after a vaccine against the highly contagious respiratory infection was developed. Cases dropped to 231 in 2003, from over 152,000 in 1968. But cases began to climb again in 2006, when 6,584 were reported, most of them in vaccinated people.

According to a report from the Centers for Disease Control and Prevention, one-third of mumps cases in the US occurred in children and adolescents from 2007 to 2019. Of those who contracted the disease, 94 percent were vaccinated.


“Previously, large outbreaks of mumps was not common in fully vaccinated people, including vaccinated children,” said Mariel Marlowe, an epidemiologist at the CDC who led the new study. “But the symptoms of the disease are usually mild and complications are less common in people who are vaccinated.”

Experts are not sure why vaccinated people get mumps, but several factors are affecting immunity in vaccinated people, including lack of prior exposure to the virus, decreased immunity, and circulation of genotypes that do not contain the vaccine. .

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The mumps virus is spread by direct contact with saliva or respiratory droplets from the mouth, nose or throat of an infected person. An infected person can spread the virus through close contact activities such as coughing, sneezing, talking or sharing drinks or sports. Around 91 percent The US population has received at least one dose of the measles, mumps and rubella, or MMR, vaccine, given between 12 months and 6 years old and is 88 percent effective against the disease.

Cases in recent years have been largely driven by localized outbreaks, although a peak in 2016 and 2017 included more than 150 outbreaks in 37 states and nearly 9,000 reported cases in Washington, DC. Mumps cases decreased last year compared to the previous six years, but the disease continued to spread in the US despite distance, lockdowns and masking. From April 1, 2020 to the end of the year, 32 health departments reported 142 mumps cases.

The disruption from the COVID-19 pandemic resulted in many children missing out on well-child visits and routinely recommended vaccines, including the MMR, which may contribute to an increase in cases or outbreaks in the future.

Mariel Marlowe, CDC epidemiologist

The numbers are still low, and they are not reason to believe that vaccines are no longer effective, said joseph levnard, assistant professor of epidemiology in the School of Public Health at the University of California, Berkeley.

“We’re talking about an infection that almost every child in America will get before the age of 20. Compared to the pre-vaccine era, children who get the disease are more likely to get it. MMR vaccine remain extremely safe from mumps,” Leeward said.

decisive cases

in some people, Antibodies from mumps vaccination Decrease over time, reducing protection. Leeward said older adolescents are most at risk during outbreaks among young people because they are more likely to have lower immunity than younger children due to reduced protection from the vaccine.

“The protection is still high, but there will be some who lose protection even within a decade or less after vaccination,” he said.

Marlow said most people aren’t regularly exposed to mumps, so there’s also less immunological growth — when people are exposed to mumps that boosts their immunity but doesn’t make them sick. Because mumps continues to spread globally during epidemics, she expects national-level mumps cases and outbreaks that could be worsened by a large uninfected population to continue.

“We know that disruptions from the COVID-19 pandemic have resulted in many children missing well-child visits and routinely recommended vaccines, including MMR, which may contribute to an increase in cases or outbreaks in the future. are,” she said.

Third dose?

Dr. Amesh Adalja, a senior scholar in the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health, said US mumps vaccines contain the genotype A strain, which is no longer circulating in the US, but does not appear to be. Vaccines less effective.

“It’s one of the mysteries to understand because even when you give a genotype A vaccine during an outbreak, it still works,” he said. “We’ve seen that during outbreaks on college campuses, a third dose of MMR is enough to stop it.”

Adalja said combating new outbreaks could be as simple as changing the MMR vaccine schedule to two to three doses. Adjusting Schedules Isn’t New: CDC’s Advisory Committee on Immunization Practices Initially recommended A single dose of mumps vaccine was introduced for routine use in 1977 and was increased to two doses in 1989.

In 2017, the panel suggested that a Third dose of MMR vaccine May be given to people at high risk of catching mumps during large outbreaks.

“Maybe we will need to update the vaccine to suit the strain we are seeing, but this may not be necessary. The current vaccine still works very well, and when it is doesn’t work, the third dose works,” Adalja said.

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