The measles outbreak is growing with 59 confirmed cases in California—18 more than last week—and 42 of those are people who were exposed at Disneyland in December, California health officials said Wednesday.
Health experts are bracing more cases, in part because so many Americans are not fully immune to measles.
"We can expect to see many cases of this vaccine-preventable disease unless people take precautionary measures," Dr. Gil Chavez, California's state epidemiologist, told reporters.
People ranging in age from seven months to 70 years old have been infected. Infants under the age of 1 are at special risk because they're too young to have been vaccinated yet. Chavez's advice: Keep babies away from Disneyland until the outbreak is over.
"However, cases are now being identified in people who have no links to Disneyland or to these cases," he added.
Among the known cases, five people had received two or more measles vaccinations and one person had received at lease one dose of vaccine. At least 32 of those infected people are aged 20 or older, accounting for 63 percent of the outbreak, health officials said.
And among the five Disneyland employees diagnosed with measles to date, two were previously vaccinated.
Indeed, many adults who were vaccinated for measles decades ago as children are now highly susceptible to the virus—perhaps as many as one in 10 of those who were immunized, infectious disease experts say.
That's because as the years pass, people lose their original protections from childhood vaccines that come in the form of disease-battling antibodies and "memory cells" that attack infections if the body is ever again exposed, doctors say.
"With time, especially if you don't get natural boosting by being exposed to people with that same illness, your memory cells may tend to forget," said Dr. Marcelo Laufer, chief of the division of infectious diseases at Miami Children's Hospital.
That waning defense, years after childhood immunizations, holds true for measles, whooping cough and chicken pox, Laufer said.
Amid the expanding measles outbreak, previously immunized adults should consider getting measles booster shots, said Dr. James Cherry, who specializes in pediatric infectious diseases at Mattel Children's Hospital UCLA in Los Angeles.
"It may well be important in the future we should give additional doses to adults," Cherry told NBC News.
"You get the idea that we're now way below the level of immunization that we should have," Cherry added. "So we have about 1 in 10 people who are susceptible to measles.
"Now we have people … who are now 40 years out from vaccination and so some of these people will get measles because their protection has dropped."
Other recent measles outbreaks have offered hard evidence of such soft spots in infection defenses once thought to be lifelong following childhood immunizations.
Between Jan. 1 and April 18 last year, the California Department of Public Health received reports of 58 confirmed measles cases. That's the highest number reported for that period since 1995. But among that group, 11 people (or 19 percent) had documentation that showed, as kids, they had received two or more doses of the measles, mumps, and rubella (MMR) vaccine.
And last April, communities east of Vancouver, British Columbia became the epicenter of a measles outbreak that sickened more than 200 people, including several people who were previously immunized for measles as children.
The serious yet preventable virus causes a fever, dry cough, runny nose, watery eyes and a signature rash. People can spread the illness up to four days before the rash appears.
The latest figures provided by California health officials Wednesday show that measles now has spread to Oregon (one case) and is still appearing in Utah (three cases), Washington (two cases), Colorado (one case) and Mexico (one case). That raises to total count to 67 cases.
In the United States, the current recommended measles regimen is to give children a first dose of vaccine at 12 to 15 months of age, and a second dose at 4 to 6 years of age.
But that wasn't the case in the 1960s, 1970s and during much of the 1980s.
America's measles vaccination program was launched in 1963, Laufer said.
Before that watershed medical moment, virtually every American acquired measles before adulthood. Some 500,000 persons with measles were typically reported each year in the United States before the vaccine was developed, and about 500 of those people died annually, Laufer said. Another 48,000 people were hospitalized, on average, each year due to measles.
Initially, federal health officials recommended a single-dose of measles vaccination, and that cut the incidence of measles to between 22,000 and 75,000 cases per year, Laufer said. The largest decrease occurred among kids aged 10 and younger.
In 1989, federal officials changed their recommendation to two doses of measles-containing vaccine for kids. That followed a resurgence of the virus that infected more than 55,000 people, killing 123 Americans. By 1993, both epidemiological and laboratory evidence suggested that transmission of indigenous measles had been fully interrupted in the United States, Laufer said.
Americans who received one dose of measles vaccine have about a 90 percent protection against the virus, but they may still contract a more mild version of the illness, Laufer said.
The Miami doctor is one of those one-dosers. He was born in 1965 and was immunized in 1966, he said.
"I grew up in an era when many people were getting one dose. So probably many people then were having mild cases of measles. I was exposed to those people and probably got natural boosting from that," Laufer said.
Natural boosting from being close to a person with mild measles is a bit like having a vaccine for people who received that lone dose. That exposure creates antibodies and strengthens the body's innate ability to later fight off an exposure, Laufer said.