I am glad that you are asking this question now. Yes, it is safe for your family to go to Disneyland, but your children should receive their second dose of measles vaccine as soon as possible to protect them from measles virus.

Measles is probably one of the most contagious viruses; it is usually spread by direct contact with infectious droplets from those who are sick with measles. An infected person can be contagious for 1-2 days before they become sick, and 3-5 days after onset of the typical measles rash.

We are the only host for this virus, which means that measles doesn’t infect any other animal at all. When there is an outbreak of measles, we look for sick human contact instead of any other source.

Before the discovery of measles vaccine, measles infection happened mostly in late winter and spring, especially in preschool and young school-aged children. By the time a person reached adulthood, he almost certainly had been infected and therefore not susceptible to another measles infection.

In early 1960s, measles vaccine became available in North America. Within a few years, the number of children infected by measles declined rapidly. Along with that decline was the number of complications and deaths associated with measles infection. Many children suffered brain damage as a result of encephalitis, a serious inflammation of their brain.

Because of this decline, many parents have never heard of the danger of measles infection, and even wonder why they should vaccinate their children against a virus that they don’t hear about. Many young doctors have never seen the severe rash and dangerous complications of measles infections.

The reality is that we are very lucky in North America; the rest of the world is not nearly as fortunate. As recent as 2014, there was a large epidemic of measles in Philippines. Over 57,000 people were infected, with 110 deaths, mostly in children. In United States, 25 travellers returning from Philippines contracted measles during their visit, most of them never received any measles vaccine.

When measles vaccine was discovered, research showed that it was very effective. The incidence of measles declined very quickly. However, when most of the children were already immunized, there were still small epidemics across Canada and United States. Further research showed that about 5 percent of children did not respond adequately to the vaccine and was still susceptible to measles infection when exposed to the virus. If these children received two doses of measles vaccine at least 28 days apart, virtually everyone developed long-lasting immunity.

Therefore, in 1990s, the recommendation of measles vaccination was changed to two doses for all children. The first dose is usually given shortly after 12 months of age. The second dose can be given at 15 months of age, or before a child enters elementary school.

Since then, measles has virtually disappeared in North America. Small outbreaks still occur from time to time, and all of them can be traced to unvaccinated or partially vaccinated individuals contracting measles while travelling in other countries.

This is the same situation in the current Disneyland outbreak. Although the original source of infection has not been identified, the measles virus that caused the infection was isolated and tested genetically, and found to be identical to the measles virus in Philippines last year. The outbreak has spread to multiple states as well as Canada. All those who contracted measles were either unvaccinated or inadequately vaccinated.

Anyone born after 1970s should receive two doses of measles vaccine. Your two children have received their first dose of vaccine, I assume, after they turned one year of age. They should receive their second dose as soon as possible. This will ensure that they are fully vaccinated, and unlikely to develop measles, even if they are exposed to the virus while they travel.

There is a very good chance that you may have received only one dose of measles vaccine when you were young. You should try to locate your immunization record from your doctor or from the public health department. If it is not certain whether you have received two doses of measles vaccine, it would be wise for you to get another dose before you travel.

Once all of you are fully immunized, you can relax and enjoy your trip to California and Disneyland, knowing that you won’t have to worry about the measles outbreak there.