DEAR DR. ROACH: I just read an article that vaccinations are on the decline in many states. My question is about children who already have vaccinations. Do unvaccinated children who catch a disease pose any risk of infecting vaccinated children? Who is actually at risk here: Is it only the kids who don’t receive vaccinations who are vulnerable to spread disease among themselves? My thought is that parents who don’t vaccinate their kids take responsibility for any disease that they catch, while parents who vaccinate their kids have nothing to worry about. — R.S.

ANSWER: This is a commonly asked question, and one that is sometimes used to justify not vaccinating. However, there are two reasons why not vaccinating children is bad, not only for them, but for society.

The first is that no vaccine is perfect. Take measles, for example, where two doses of the vaccine, properly stored and correctly given, is about 97 percent effective at preventing measles. That is not 100 percent: No medical procedure, test or drug is perfect. Having most of the people in the population immune to measles prevents large-scale outbreaks and protects those in whom the vaccine has not worked, a concept known as “herd immunity.” However, the person who was vaccinated but it did not work is at risk (without knowing it) from a person infectious with measles, an extremely contagious disease.

Secondly, there are people who are unable to get the vaccine. This includes children with cancer receiving chemotherapy as well as those with primary or acquired deficiencies in their immune system. Children under 1 are too young to effectively respond to vaccines and are at risk. Older people may lose immunity, and some people born between 1963 and 1967 may have received an inactive vaccine. Again, society can protect them by ensuring high compliance with vaccination in the entire population.

Measles is so contagious that 95 percent of the population needs to be immune (through previous illness or effective vaccination) in order to have the protection of herd immunity. When that number drops, the risk of a large outbreak increases, which unfortunately has been the case in several recent small outbreaks in the U.S. and larger outbreaks in Europe.

Measles is not a benign condition. About 1 person per thousand with measles will die from it. Another 1-2 will have serious complications. Measles also causes significant damage to the entire immune system, which takes years to recover from, and which can make another disease more likely to be fatal.

DEAR DR. ROACH: Why are MRI machines so loud? Is the loud noise really necessary? Also, how often does this noise cause tinnitus or hearing problems? — K.C.H.

ANSWER: MRI scanners make noise because the moving of electrical coils inside the machine, which create the magnetic fields, causes a vibration that can be very loud. Some MRI machines can be as loud as a jet engine! This absolutely can cause short-term tinnitus and hearing loss, and should prompt ear protection.

Manufacturers of MRI scanners are using new technology with much, much quieter machines. These will likely gradually replace the loud machines.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

Dr. Roach