Stanford’s Michelle Mello on Latest Measles Outbreak

Measles is once again in the news with the most recent outbreak in Washington state infecting 37 people and officials expecting the number to climb. In the discussion that follows, Stanford Law Professor Michelle Mello, an expert in health law, discusses the disease, vaccination, and the law.

How effective is the measles vaccination? And is it safe?

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Stanford Law School Professor Michelle Mello

The measles vaccine is considered very safe and effective.  Side effects are rare and usually minor.  Two doses of vaccine are 97% effective in preventing measles.  Compare that to the flu shot, which is typically 40-60% effective.

Can parents choose to forgo vaccinating their children in most states? How many states legally mandate vaccinations?

All states require immunizations as a condition of school entry, but they vary in the reasons that will be accepted for granting an exemption to those requirements.  California, Mississippi, and West Virginia grant exemptions only for medical reasons.  All other states grant religious exemptions, and 18 states also grant “personal belief” exemptions to people who are philosophically opposed to vaccination.

So, the law allows some people to choose not to get a vaccination. But doesn’t that put people with compromised immune systems, such as the very young or old or the ill, at risk?

The most serious risk is to children who haven’t yet received all of their vaccines, and others who can’t be vaccinated for medical reasons (for example, because their immune system is suppressed due to another illness).  The more people start opting out, the broader the risk becomes.  We need most people in the population to be vaccinated in order to maintain our “herd immunity.”  That’s our collective ability to protect ourselves from disease by stopping it from passing from person to person. The percentage needed to maintain herd immunity varies depending on how contagious the disease is.  Measles is highly contagious, so we lose herd immunity when about 5% of the population is unvaccinated.  That’s not a lot!  During the last measles outbreak, more than a quarter of the kindergartens in California fell short of the mark for herd immunity.

After the 2015 measles outbreak in Disneyland, California passed a law outlining strict exemptions for vaccinations.  Was that effective? 

Yes.  Before the law went into effect, 92.9% of California kindergartners received all the required vaccines.  The year after it was implemented, that rate rose to 95.6%.  There is some concern that certain healthcare providers are essentially selling medical exemption certifications of dubious merit, but even with that problem, the law has made a difference.

Is there a model law for this?

I think California’s strong approach is the model to emulate.  It extends to private as well as public schools and is administered by a very effective public health department, with plenty of transparency.

Is there anything else you’d like to add?

Parents of young children today are lucky enough to have been raised in the age of vaccines, and most of us have no personal experience with measles.  That makes it easy to overlook how serious the disease is. I personally was very moved by author Roald Dahl’s essay about the death of his daughter, Olivia.  Measles is staggeringly contagious—there is one case of a man contracting it simply by walking by an infected child in an airport lounge.  As a parent, I sleep better knowing my kids are immunized.

One of the most frustrating aspects of this issue for health scholars is the persistence of mistaken beliefs about vaccine risks, which are often traceable to a discredited and retracted study about vaccines and autism. Concerned parents can get helpful information from their pediatrician and from

Michelle Mello, Professor of Law and Professor of Health Research and Policy (School of Medicine), is a leading empirical health scholar and the author of nearly 200 articles, including research on the effects of vaccination policies.