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Saturday September 24, 2016

What is "vaccine shedding" and is it a threat?

Christmas

Responding to the various fears, myths and half-truths about vaccination can sometimes feel like a lively game of Whac-A-Mole. Successfully point out the strong body of science that shows no connection between vaccines and autism, and an argument might pop up that children need their childhood diseases to build a strong immune system (despite the potential dangers of diseases like measles or chickenpox).

In the wake of the Disneyland measles outbreak of 2014-2015, some vaccine opponents suggested that the outbreak was caused not by an unvaccinated person who had become infected with measles, but a vaccinated person through a process known as “vaccine shedding.” But is that even possible? 

We asked expert Dr. Susan H. Wootton, associate professor at the University of Texas’ McGovern Medical School and a specialist in pediatric infectious disease, to help shed light on the rumors. 

"Shedding" is nothing more or less than the way any contagious disease is spread, through pathogen-filled sneezes, coughs and the like. And because certain vaccines use weakened versions of the live virus, the philosophy goes, those diseases are just as contagious among the vaccinated.

That’s not really how things work, said Wootton.

Only three vaccines in the routinely recommended childhood vaccination schedule are made using weakened, but live, viruses. In some instances, the virus can be detected for a short period of time in the vaccinated patient and that can sometimes lead to others being infected with the vaccine version of the virus. But those instances are extraordinarily rare.

With the chickenpox vaccine, for example, 11 people are known to have caught the disease through exposure to a vaccinated person—but that’s out of more than 50 million people who have been vaccinated. And even then, the vaccine is shed only in the case of people who develop a chickenpox-like rash following vaccination. That is why if a person recently vaccinated against chickenpox does develop a rash, they are advised to avoid contact with individuals with weakened immune systems.  

Other than that, however, there’s no evidence of harm to others, Wootton said. The measles and mumps vaccines do not shed at all. Signs of the third virus in the MMR trio, rubella, can be found from 7 to 28 days after immunization, Wootton said, but at levels far too low for anyone to catch it.

And rotavirus is found for a while in the stool of some babies who have received the vaccine, but the chances of transmission to others are small, and the benefits of the vaccine far outweigh the small risks involved. Getting the vaccine is important because without it almost every child in the country will be infected with rotavirus by age 5, and that’s contagious as well.

In some cases, Wootton said, vaccine shedding can actually be a public health benefit.

“If you look at the history of oral polio in developing countries, they specifically used a live vaccine,” she said. (The live-virus polio vaccine is no longer used in the United States.) Vaccinated children shed virus and in areas with poor sanitation, that often means other children are exposed through open sewage and thus protected.

“That’s a good thing,” Wootton said. It serves as a sort of indirect inoculation, immunizing children who weren’t reached with vaccination campaigns.

The tiny bits of truth about vaccine shedding have unfortunately been used to stoke unjustified fears that vaccinated children are dangerous to their friends in school, Wootton said. It’s just not so.

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